B.Sc.II Semester IV Paper VII Notes

B. Sc. Part II Semester- IV

ZOOLOGY   Paper-VII

DSC-(REPRODUCTIVE BIOLOGY)

Theory: 30 hrs. (37.5 lectures of 48 minutes)

Marks-50 (Credits: 02)

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Unit 1

Functional anatomy of female reproduction

Outline and histological structure of female reproductive system in rat and human; Ovary: folliculogenesis, ovulation, corpus luteum formation and regression; Steroidogenesis and secretion of ovarian hormones; Reproductive cycles in human and their regulation, changes in the female tract; Ovum transport in the fallopian tubes; Sperm transport in the female tract, fertilization; Hormonal control of implantation; pregnancy diagnosis Hormonal regulation of gestation, , Lactation and its regulation.

Unit 2

Functional anatomy of male reproduction

Outline and histology of male reproductive system in human; Testis: Cellular functions, germ cell; Spermatogenesis: hormonal regulation, Epididymal function and sperm maturation; Accessory glands functions; Sperm transportation in male tract.

Unit 3

Reproductive Health

Infertility in male and female: causes, diagnosis and management; Assisted Reproductive Technology: sex selection, sperm banks, frozen embryos, in vitro fertilization, ET, EFT, IUT, ZIFT, GIFT, ICSI, PROST; Modern contraceptive technologies.

A) FUNCTIONAL ANATOMY OF FEMALE REPRODUCTION

Outline and histological structure of female reproductive system in human; Ovary: folliculogenesis, ovulation, corpus luteum formation and regression; Steroidogenesis and secretion of ovarian hormones; Reproductive cycles in human and their regulation, changes in the female tract; Ovum transport in the fallopian tubes; Sperm transport in the female tract, fertilization; Hormonal control of implantation; pregnancy diagnosis Hormonal regulation of gestation, Mechanism of parturition and its hormonal regulation; Lactation and its regulation.

1.                  OUTLINE AND HISTOLOGICAL STRUCTURE OF FEMALE REPRODUCTIVE SYSTEM IN HUMAN.

Following parts constitute the female reproductive system, a pair of ovaries, fallopian tube, uterus, cervix, vagina and accessory genital glands and a pair of mammary glands.

a.                  Pair of Ovaries

Each ovary is of the shape of unshelled almond and the size is 3.5 cm long, 2 cm wide and 1 cm thick. It is placed in the abdominal cavity. Ovary is attached to the uterus by ovarian ligament. Ovary is suspended from the abdominal wall by a mesentery called mesovarium. Each ovary is lined by cuboidal germinal epithelium and is solid. Underneath the germinal epithelium lies a layer of connective tissue called tunica albuginea. Underlying this layer is stroma. Stroma is further divided into dense outer cortex and less dense inner medulla. Many Graafian follicles/ovarian follicles are present in the cortex and show different stages of development. Initial stage of development is primary oocyte. As the primary oocyte develops it changes to secondary oocyte. Secondary oocyte is released from the ovary by the rupturing of ovarian wall. This process is known as ovulation.

There are around four lakhs follicles in both the ovaries of an adult woman. Most of the follicles disappear by phagocytosis during reproductive years. This process is called follicular atresia. Due to this a female produces only around 450 ova in her entire reproductive life which ends between 40 and 50 years of age.

Secondary oocyte surrounded by layers of follicular cells (discus poligerous or cumulus oopharous) is called as Graafian follicle. It is suspended in the antrum which is filled with semi-viscous fluid called liquor folliculi, secreted by follicular cells and is held by a stalk of follicular cells called germ hill. This stalk arises from membrana granulose which is multicellular layer. The outer most layer is called theca externa and the inner layer is called theca interna. Follicular cells act as endocrine cells and secrete hormone estrogen in blood. Cortex of ovary also consists of conical, yellowish cells known as corpus luteum which on degeneration is called as corpus albicans. Corpus luteum also functions as endocrine cells and secretes progesterone, estradiol and relaxin.

b. Fallopian Tube or Oviduct

Fallopian tube is around 10 cm long, muscular, tubular and ciliated structure. It lies in pelvic region, just above the urinary bladder. It is composed of outer serosa, middle muscularis and inner mucosa. Mucosa is made of simple ciliated columnar cells and secretory cells. A viscous liquid is secreted by secretory cells, which provides protection and nourishment to the ovum. Ciliated cells help in the movement of ovum. Each fallopian tube is divided into infundibulum, ampulla, isthmus and uterine part.

i. Infundibulum

This is a broad, funnel shaped proximal part. Finger like projections arise from this proximal part and are called as fimbriae. Infundibulum opens into the body cavity by an aperture called as ostium. Ostium lies near the ovary and receives egg from the ovary with the help of fimbriae.

ii. Ampulla

It comprises of the major portion of fallopian tube. It is long, thin walled and wide.

iii. Isthmus

It is a short, thick walled, ciliated and narrow straight path.

iv. Uterine Part

It is narrow and inner part which opens in the upper part of uterus.

c. Uterus

It is hollow, muscular, vascular and large (8 cm x 5 cm x 2 cm) pear shaped Structure which is present in the pelvic region above the bladder. It can be divided into three parts – fundus, body and cervix.

Fundus is upper, dome shaped part above the opening of fallopian tube. The middle and major part of uterus is the body. It has three layers – outer peritoneal perimetrium, middle muscular myometrium and highly vascular endometrium. The lower narrow part which opens in the body of the uterus by internal os and in vagina by external os is called cervix.

Uterus is the site of foetal placentation, its growth and parturition.

d. Vagina

This is a tubular structure, 10-12 cm long and extends from cervix to the outside of body. It receives the sperms during copulation, is passage for menstrual flow and forms the birth canal during labour. Hymen is the membranous structure which covers the opening of vagina – the vaginal orifice. Vagina is lined by non-keratinised stratified squamous epithelium. Glands are absent in vaginal wall.

e. Vulva

This is the external genitalia of females. It consists of the vestibule or urino-genital sinus which is in the form of depression and is in the front of anus. It has two apertures, upper external urethral orifice and lower vaginal orifice.

The anterior part is fatty and covered with pubic hair. This portion is called as Mons pubis. Corresponding to the male penis, clitoris is present in the females which are made of erectile tissue. Two large, thick- walled fold of skin form the boundary of vulva. These are labia major and contain sebaceous glands. Between labia major two small folds are present and are called as labia minora. Labia minora fuse posteriorly to form fourchette.

On either side of vaginal orifice there is a pair of Bartholin’s gland. This gland secretes a clear, viscous fluid which works as lubricating agent during copulation. The area below fourchette and anus is perineum.

f. Mammary Gland or Breast

In human beings mammary glands are one pair and present on ventral thoracic wall. They are modified sweat glands. In males it is rudimentary whereas in females it is well developed (Fig. 5). Hormone estrogen and progesterone are responsible for their development. After child birth, anterior lobe of pituitary secretes oxytocin. The former is responsible for production of milk and later stimulates its release.

The breast is externally covered with skin and in the centre there is nipple made of erectile tissue. Nipple is surrounded by pigmented area called areola. Areola has numerous sebaceous glands called areolar gland.

Human milk is made of organic, inorganic compounds and water. Milk is poor in iron. It mainly consists of fat droplets, lactase, casein, vitamins and minerals like sodium, potassium, calcium, phosphate, etc. Glandular, fibrous and adipose tissues constitute the mammary glands.

i. Glandular Tissue

This tissue consists of around 20 lobes and each lobe has 15-20 lobules. Each lobule is made of group of glandular alveoli and unit to form lactiferous duct. These ducts expand to form lactiferous sinuses which store milk during lactation. Each sinus opens to the outside by narrow ducts which are 0.5 mm in diameter.

ii. Adipose Tissue

The surface of gland is covered by adipose tissue. It is also found between the lobes. The size of the breast is determined by adipose tissue.

iii. Fibrous Tissue

Glandular tissues and ducts get support from this tissue.

Hormonal Control

Follicular Stimulating Hormone (FSH) controls the transformation of young primary oocyte into Graafian follicle. It also controls maturation of ovum and secretion of estrogen by the follicular cells. Luteinizing hormone controls the ovulation process, formation of corpus luteum from Graafian follicle and secretion of progesterone from corpus luteum.

Puberty in Females

Puberty age in females is between 10 and 14 years and is characterised by menstrual cycle and ovulation. Estrogen secretion maintains growth and maturation of reproductive tract and development of accessory sexual characters.

There are physical and psychological changes that take place in females during this phase.

They are

a. Enlargement of breasts.

b. Growth of pubic and axillary hair.

c. Increase in subcutaneous fat in buttocks, thighs and face.

d. Beginning of menstrual cycle and ovulation.

e. Broadening of hip region due to widening of pelvis.

f. Stoppage of growth of long bones.

 

 

 

 

 

 

 

 

 

 

HISTOLOGICAL STRUCTURE OF FEMALE REPRODUCTIVE SYSTEM IN HUMAN

 

Pair of ovaries is situated in pelvic region, one on either side of uterus. They are attached to uterus by ovarian ligament and are suspended from dorsal abdominal wall by a fold of peritoneum called Mesovariam. Histologically ovary shows following different parts. Ovary is externally lined by a single layer of germinal epithelium. Inner to the germinal epithelium there is a fibrous covering of connective tissue called Tunica albuginea. Inside the tunica albuginea connectives tissue stroma is present, which is differentiated into outer cortex   and inner medulla. The medulla is the central region of the ovary and is made up of loose connective tissue containing blood vessels, nerve fibers etc. The germinal epithelium undergoes the process of oogenesis to form follicles in different stages of development in cortex as primordial follicle, primary follicle, and secondary follicle and finally, mature follicle called Graafian follicle.

A] PRIMORDIAL FOLLICLE

In this oocyte is surrounded by single layer of follicular cells resting on basement membrane.   The   single   layer    of            follicular       cells               divides           into  many  layers  called  stratum Granulosa.

B] PRIMARY FOLLICLE

The oocyte enlarges and  gets  separated  from  follicular  layer  by  a  zone  called  zona pellucida.

C] SECONDARY FOLLICLE

A cavity appears in the follicle called the antrum that gets filled with Liquor folliculi outside the follicle the stroma cells forms a layer called Theca folliculi.

D] GRAAFIAN FOLLICLE/MATURE FOLLICLE

It is discovered by Dutch scientist Reginior Degraaf. It is spherical in outline and measures about 2mm in diameter Externally it is covered by stroma cells layers, outer Theca Extema .& inner Theca interna. Below the Theca interna many follicular layers are present called stratum granulosa. It is divided into membrane granulosa and cumulus oophorus/discus proligerous due   to presence antrum, which is filled with a viscous fluid called liquor folliculi. In the cavity of follicle, the ovum is suspended eccentrically by a stalk of cumulus oophorus cells to membrane granulosa cells forms germ hill. The ovum is externally surrounded by three layers such As    

1)      Vitellin Membrane: It is the transparent, glasslike limiting membrane of ovum itself.

2)      Zona pellucida: A non cellular zone which   separates   the   ovum   from follicular  cells.   

3)      Corona Radiata: Cumulus oophorus    cells    immediately    surrounding    the   zona   pellucida are    called   Corona Radiata.

   In the ovary out of many developmental follicles only one attends the maturity to form Graafian follicle in every month. During mid day of menstrual cycle, the Graafian follicle rapture and ovum is released out from ovarian walls at spot stigma. It is called Ovulation. After ovulation empty Graafian follicle is covered into yellow glandular mass made up of leutin cells called as Corpus Luteum. It secretes another female sex hormone Progesterone. If the egg is not fertilized the corpus luteum forms corpus Albicans. The Graafian follicle secretes female sex hormone Estrogen. It regulates secondary sexual characteristics in female.

E] ATRETIC FOLLICLES

   The number of primordial follicles does not reach maturity, but they get degenerated. Such degenerating follicle is called as Atretic follicle. & the process is called Atresia. In the stroma of cortex such follicles are also observed under microscope.

 

 

HISTOLOGY OF UTERUS

 

 

Human uterus is hollow, pear shaped, highly distensible muscular sac situated in between vagina and fallopian tube. It is specialised for development of embryo. Uterus is differentiated in to   three parts.

a)         FUNDUS: It is upper dome shaped part. At its upper corner oviducts opens on both side  called as cornua.

b)         BODY/CORPUS: It is the middle part of the uterus. Its wall is made up of three layers- outer covering is primetrium, middle muscular myometrium and inner highly vascular and glandular endometrium.

c)        CERVIX: It is the lower part of the uterus opens in to vagina. The cervix communicates with the body of uterus by an aperture called internal Os. Two is called cervical canal.

Histologically the wall of uterus shows three layers

1)        Primetrium – outermost coat

2)        Myometrium – middle muscular coat

3)        Endometrium – Innermost coat

1)        PRIMETRIUM / SEROSA

Is the outermost protective serous coat of the peritoneum. It extends from the sides of the uterus forming the broad ligaments, through which blood vessels, lymphatics and nerves reach uterus   on each side.

2)        MYOMETRIUM / MUSCULARIS

It is the middle muscular coat of uterus. It is very thick layer of smooth muscle fibers with connective tissue. Muscle fibers are arranged in three district layers. The middle layer is circular, and outer & inner layers are longitudinal or oblique. The middle region contains many large blood vessels. The hormone oxytocin controls the movement of these muscles.

3)        ENDOMETRIUM OR MUCOSA

It is the innermost layer of uterus. This is the layer which undergoes cyclic changes in structure and secretory activity in response to the female sex cycle (menstrual) such a changing layer of mucosa is called endometrium functionalis. The region of endometrium that remains unchanged even during destructive period of sex cycle is called endometrium basalis.

A)         ENDOMETRIUM FUNCTIONALS

Histologically it is characterized to contain mucosal epithelium and underlying submucosa. Mucosal epithelium consists of a simple columnar epithelium which is ciliated at some regions. The uterine glands are developed by the imagination of this epithelium in to the stroma. It has large spiral arteries, veins and sub mucosal uterine glands.

B)        ENDOMETRIUM BASALIS

It mainly consists of sub mucosal uterine glands and loose connective tissue.

2.                  OVARY: FOLLICULOGENESIS, OVULATION, CORPUS LUTEUM FORMATION AND REGRESSION.

FOLLICULOGENESIS

 Folliculogenesis is the maturation of the ovarian follicle, a densely packed shell of somatic cells that contains an immature oocyte. Folliculogenesis describes the progression of a number of small primordial follicles into large preovulatory follicles that occurs in part during the menstrual cycle. Contrary to male spermatogenesis, which can last indefinitely, folliculogenesis ends when the remaining follicles in the ovaries are incapable of responding to the hormonal cues that previously recruited some follicles to mature. This depletion in follicle supply signals the beginning of menopause.

The primary role of the follicle is oocyte support. From birth, the ovaries of the human female contain a number of immature, primordial follicles. These follicles each contain a similarly immature primary oocyte. At puberty clutches of follicles begin folliculogenesis, entering a growth pattern that ends in death (apoptosis) or in ovulation (the process where the oocyte leaves the follicle).

During follicular development, primordial follicles undergo a series of critical changes in character, both histologically and hormonally. First they change into primary follicles and later into secondary follicles. The follicles then transition to tertiary, or antral, follicles. At this stage in development, they become dependent on hormones, particularly FSH which causes a substantial increase in their growth rate. The late tertiary or pre-ovulatory follicle ruptures and discharges the oocyte (that has become a secondary oocyte), ending folliculogenesis.

Folliculogenesis is continuous, meaning that at any time the ovary contains follicles in many stages of development. The majority of follicles dies and never completes development. A few develop fully to produce a secondary oocyte which is released by rupture of the follicle in a process called ovulation.

The growing follicle passes through the following distinct stages that are defined by certain structural characteristics:

In a larger perspective, the whole folliculogenesis, from primordial to preovulatory follicle, belongs to the stage of ootidogenesis of oogenesis.

In addition, follicles that have formed an antrum are called antral follicles or Graafian follicles. Definitions differ in where this shift occurs in the staging given above, with some stating that it occurs when entering the secondary stage,^[1] and others stating that it occurs when entering the tertiary stage.^[2]

Until the preovulatory stage, the follicle contains a primary oocyte that is arrested in prophase of meiosis I. During the late preovulatory stage, the oocyte continues meiosis and becomes a secondary oocyte, arrested in metaphase II.

Primordial

At 18–22 weeks post-conception, the cortex of the female ovary (foetal female ovary) contains its peak number of follicles (about 4 to 5 million in the average case, but individual peak populations range from 6 to 7 million).^[3] These primordial follicles contain immature oocytes surrounded by flat, squamous granulosa cells (support cells) that are segregated from the oocytes environment by the basal lamina. They are quiescent, showing little to no biological activity. Because primordial follicles can be dormant for up to 50 years in the human, the length of the ovarian cycle does not include this time.

The supply of follicles decreases slightly before birth, and to 180,000 by puberty for the average case (populations at puberty range from 25,000 to 1.5 million).^[3] By virtue of the "inefficient" nature of folliculogenesis (discussed later), only 400 of these follicles will ever reach the preovulatory stage. At menopause, only 1,000 follicles remain. It seems likely that early menopause occurs for women with low populations at birth, and late menopause occurs for women with high populations at birth, but there is as yet no clinical evidence for this.^[3]

The process by which primordial cells 'wake up' is known as initial recruitment. Research has shown that initial recruitment is mediated by the counterbalance of various stimulatory and inhibitory hormones and locally produced growth factors.^[4]

Primary

During ovarian follicle activation, the granulosa cells of the primordial follicles change from a flat to a cuboidal structure, marking the beginning of the primary follicle. The oocyte genome is activated and genes become transcribed. Rudimentary paracrine signalling pathways that are vital for communication between the follicle and oocyte are formed. Both the oocyte and the follicle grow dramatically, increasing to almost 0.1 mm in diameter.

Primary follicles develop receptors to follicle stimulating hormone (FSH) at this time, but they are gonadotropin-independent until the antral stage. Research has shown, however, that the presence of FSH accelerates follicle growth in vitro.

A glycoprotein polymer capsule called the zona pellucida forms around the oocyte, separating it from the surrounding granulosa cells. The zona pellucida, which remains with the oocyte after ovulation, contains enzymes that catalyze with sperm to allow penetration.

Secondary

Stroma-like theca cells are recruited by oocyte-secreted signals. They surround the follicle's outermost layer, the basal lamina, and undergo cytodifferentiation to become the theca externa and theca interna. An intricate network of capillary vessels forms between these two thecal layers and begins to circulate blood to and from the follicle.

The late-term secondary follicle is marked histologically and structurally by a fully grown oocyte surrounded by a zona pellucida, approximately nine layers of granulosa cells, a basal lamina, a theca interna, a capillary net, and a theca externa. The development of the antrum also starts taking place in secondary follicle stage

Antrum formation

Further information: Antral follicle

The formation of a fluid-filled cavity adjacent to the oocyte called the antrum designates the follicle as an antral follicle, in contrast to a so-called preantral follicle that still lacks an antrum. An antral follicle is also called a Graafian follicle.

Definitions differ as to which stage this shift occurs in, with some designating follicles in the secondary stage as antral,^[1] and others designating them as preantral.^[2]

Early tertiary

In the tertiary follicle, the basic structure of the mature follicle has formed and no novel cells are detectable. Granulosa and theca cells continue to undergo mitotis concomitant with an increase in antrum volume. Tertiary follicles can attain a tremendous size that is hampered only by the availability of FSH, which it is now dependent on.

Under action of an oocyte-secreted morphogenic gradient, the granulosa cells of the tertiary follicle undergo differentiation into four distinct subtypes: corona radiata, surrounding the zona pellucida; membrana, interior to the basal lamina; periantral, adjacent to the antrum and cumulus oophorous, which connects the membrana and corona radiata granulosa cells together. Each type of cell behaves differently in response to FSH.

Theca interna cells express receptors for luteinizing hormone (LH). LH induces the production of androgens by the theca cells, most notably androstendione, which are aromatized by granulosa cells to produce estrogens, primarily estradiol. Consequently, estrogen levels begin to rise.

Late tertiary and preovulatory (the follicular phase of the menstrual cycle)

At this point, the majority of the group of follicles that started growth have died. This process of follicle death is known as atresia, and it is characterized by radical apoptosis of all constituent cells and the oocyte. Although it is not known what causes atresia, the presence of high concentrations of FSH has been shown to prevent it.

A rise in pituitary FSH caused by the disintegration of the corpus luteum at the conclusion of a menstrual cycle precipitates the recruitment of five to seven class 5 follicles to participate in the next cycle. These follicles enter the end of the prior menstrual cycle and transition into the follicular phase of the next one. The selected follicles, called antral follicles, compete with each other for growth-inducing FSH.

The pattern of this emergence of a cohort of five to seven antral follicles is debated. There are theories of continuous recruitment of antral follicles, theories of a single recruitment episode at the end of the luteal phase, and more recently there has been evidence for a recruitment model marked by 2 - 3 waves of follicle recruitment and development during the menstrual cycle (only one of which is actually an ovulatory wave). ^[5]

In response to the rise of FSH, the antral follicles begin to secrete estrogen and inhibin, which have a negative feedback effect on FSH.^[6] Follicles that have fewer FSH-receptors will not be able to develop further; they will show retardation of their growth rate and become atretic. Eventually, only one follicle will be viable. This remaining follicle, called the dominant follicle, will grow quickly and dramatically—up to 20 mm in diameter—to become the preovulatory follicle.

The growth of the dominant follicle during the follicular phase is about 1.5 mm per day (±0.1 mm), both in natural cycles and for any dominant follicle developing while taking combined oral contraceptive pill.^[7] Performing controlled ovarian hyperstimulation leads to a greater recruitment of follicles, growing at about 1.6 mm per day.^[7]

Ovulation

Estrogen levels peak towards the end of the follicular phase. This causes a surge in levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). This lasts from 24 to 36 hours, and results in the rupture of the ovarian follicles, causing the oocyte to be released from the ovary.^[14]

Through a signal transduction cascade initiated by LH, proteolytic enzymes are secreted by the follicle that degrade the follicular tissue at the site of the blister, forming a hole called the stigma. The secondary oocyte leaves the ruptured follicle and moves out into the peritoneal cavity through the stigma, where it is caught by the fimbriae at the end of the fallopian tube. After entering the fallopian tube, the oocyte is pushed along by cilia, beginning its journey toward the uterus.^[8]

By this time, the oocyte has completed meiosis I, yielding two cells: the larger secondary oocyte that contains all of the cytoplasmic material and a smaller, inactive first polar body. Meiosis II follows at once but will be arrested in the metaphase and will so remain until fertilization. The spindle apparatus of the second meiotic division appears at the time of ovulation. If no fertilization occurs, the oocyte will degenerate between 12 and 24 hours after ovulation.^[15] Approximately 1-2% of ovulations release more than one oocyte. This tendency increases with maternal age. Fertilization of two different oocytes by two different spermatozoa results in fraternal twins.^[8]

The mucous membrane of the uterus, termed the functionalis, has reached its maximum size, and so have the endometrial glands, although they are still non-secretory.^{[citation needed]}

CORPUS LUTEUM FORMATION AND REGRESSION

The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries and is involved in the production of relatively high levels of progesterone and moderate levels of estradiol and inhibin A.^[1][2] It is the remains of the ovarian follicle that has released a mature ovum during a previous ovulation.^[3]

The corpus luteum is colored as a result of concentrating carotenoids (including lutein) from the diet and secretes a moderate amount of estrogen that inhibits further release of gonadotropin-releasing hormone (GnRH) and thus secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). A new corpus luteum develops with each menstrual cycle.

The corpus luteum develops from an ovarian follicle during the luteal phase of the menstrual cycle or oestrous cycle, following the release of a secondary oocyte from the follicle during ovulation. The follicle first forms a corpus hemorrhagicum before it becomes a corpus luteum, but the term refers to the visible collection of blood, left after rupture of the follicle, that secretes progesterone. While the oocyte (later the zygote if fertilization occurs) traverses the Fallopian tube into the uterus, the corpus luteum remains in the ovary.

The corpus luteum is typically very large relative to the size of the ovary; in humans, the size of the structure ranges from under 2 cm to 5 cm in diameter.^[4]

Its cells develop from the follicular cells surrounding the ovarian follicle.^[5] The follicular theca cells luteinize into small luteal cells (thecal-lutein cells) and follicular granulosa cells luteinize into large luteal cells (granulosal-lutein cells) forming the corpus luteum. Progesterone is synthesized from cholesterol by both the large and small luteal cells upon luteal maturation. Cholesterol-LDL complexes bind to receptors on the plasma membrane of luteal cells and are internalized. Cholesterol is released and stored within the cell as cholesterol ester. LDL is recycled for further cholesterol transport. Large luteal cells produce more progesterone due to uninhibited/basal levels of protein kinase A (PKA) activity within the cell. Small luteal cells have LH receptors that regulate PKA activity within the cell. PKA actively phosphorylates steroidogenic acute regulatory protein (StAR) and translocator protein to transport cholesterol from the outer mitochondrial membrane to the inner mitochondrial membrane.^[6]

The development of the corpus luteum is accompanied by an increase in the level of the steroidogenic enzyme P450scc that converts cholesterol to pregnenolone in the mitochondria.^[7] Pregnenolone is then converted to progesterone that is secreted out of the cell and into the blood stream. During the bovine estrous cycle, plasma levels of progesterone increase in parallel to the levels of P450scc and its electron donor adrenodoxin, indicating that progesterone secretion is a result of enhanced expression of P450scc in the corpus luteum.^[7]

The mitochondrial P450 system electron transport chain including adrenodoxin reductase and adrenodoxin has been shown to leak electrons leading to the formation of superoxide radical.^[8][9] Apparently to cope with the radicals produced by this system and by enhanced mitochondrial metabolism, the levels of antioxidant enzymes catalase and superoxide dismutase also increase in parallel with the enhanced steroidogenesis in the corpus luteum.^[7]

Follicular structure	Luteal structure	Secretion
Theca cells	Theca lutein cells	androgens,[10] progesterone[10]
Granulosa cells	Granulosa lutein cells	progesterone,[5] estrogen(majority),[5] and inhibin A[5][10]

Steroidogenesis, with progesterone in yellow field at upper center.^[11] The androgens are shown in blue field, and aromatase at lower center - the enzyme present in granulosa lutein cells that convert androgens into estrogens (shown in pink triangle).

Like the previous theca cells, the theca lutein cells lack the aromatase enzyme that is necessary to produce estrogen, so they can only perform steroidogenesis until formation of androgens. The granulosa lutein cells do have aromatase, and use it to produce estrogens, using the androgens previously synthesized by the theca lutein cells, as the granulosa lutein cells in themselves do not have the 17α-hydroxylase or 17,20 lyase to produce androgens.^[5] Once the corpus luteum regresses the remnant is known as corpus albicans.^[12]

Function

The corpus luteum is essential for establishing and maintaining pregnancy in females. The corpus luteum secretes progesterone, which is a steroid hormone responsible for the decidualization of the endometrium (its development) and maintenance, respectively. It also produces relaxin, a hormone responsible for softening of the pubic symphysis which helps in parturition.

When egg is not fertilized

If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 10 days in humans). It then degenerates into a corpus albicans, which is a mass of fibrous scar tissue.

The uterine lining (endometrium) is expelled through the vagina (in mammals that go through a menstrual cycle). In an estrous cycle, the lining degenerates back to normal size.

When egg is fertilized

If the egg is fertilized and implantation occurs, the syncytiotrophoblast (derived from trophoblast) cells of the blastocyst secrete the hormone human chorionic gonadotropin (hCG, or a similar hormone in other species) by day 9 post-fertilization.

Human chorionic gonadotropin signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining (endometrium) of the uterus and providing an area rich in blood vessels in which the zygote(s) can develop. From this point on, the corpus luteum is called the corpus luteum graviditatis.

The introduction of prostaglandins at this point causes the degeneration of the corpus luteum and the abortion of the fetus. However, in placental animals such as humans, the placenta eventually takes over progesterone production and the corpus luteum degrades into a corpus albicans without embryo/fetus loss.

Luteal support refers to the administration of medication (generally progestins) for the purpose of increasing the success of implantation and early embryogenesis, thereby complementing the function of the corpus luteum.

3.                  SECRETION OF OVARIAN HORMONES.

4.                  REPRODUCTIVE CYCLES IN HUMAN AND THEIR REGULATION, CHANGES IN THE FEMALE TRACT.

REPRODUCTIVE CYCLES IN HUMAN AND THEIR REGULATION

Menstruation, also known as a period or monthly,^[1] is the regular discharge of blood and mucosal tissue (known as menses) from the inner lining of the uterus through the vagina.^[2] The first period usually begins between twelve and fifteen years of age, a point in time known as menarche.^[1] However, periods may occasionally start as young as eight years old and still be considered normal.^[2] The average age of the first period is generally later in the developing world, and earlier in the developed world.^[3] The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).^[2][3] Bleeding usually lasts around 2 to 7 days.^[2] Menstruation stops occurring after menopause, which usually occurs between 45 and 55 years of age.^[4] Periods also stop during pregnancy and typically do not resume during the initial months of breastfeeding.^[2]

Up to 80% of women report having some symptoms prior to menstruation.^[5] Common signs and symptoms include acne, tender breasts, bloating, feeling tired, irritability, and mood changes.^[6] These may interfere with normal life, therefore qualifying as premenstrual syndrome, in 20 to 30% of women.^[5] In 3 to 8%, symptoms are severe.^[5]

A lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days.^[2] Other problems with the menstrual cycle include painful periods and abnormal bleeding such as bleeding between periods or heavy bleeding.^[2] Menstruation in other animals occur in primates (apes and monkeys).^[7][8]

The menstrual cycle occurs due to the rise and fall of hormones.^[2] This cycle results in the thickening of the lining of the uterus, and the growth of an egg, (which is required for pregnancy).^[2] The egg is released from an ovary around day fourteen in the cycle; the thickened lining of the uterus provides nutrients to an embryo after implantation.^[2] If pregnancy does not occur, the lining is released in what is known as menstruation.^[2]

Menstruation

Menstruation (also called menstrual bleeding, menses, catamenia or a period) is the first phase of the uterine cycle. The flow of menses normally serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as a number of factors can cause bleeding during pregnancy; some factors are specific to early pregnancy, and some can cause heavy flow.)^[94][95][96] Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal).^[90][97] The average blood loss during menstruation is 35 milliliters with 10–80 ml considered normal.^[98] Women who experience Menorrhagia are more susceptible to iron deficiency than the average person.^[99] An enzyme called plasmin inhibits clotting in the menstrual fluid.^[100]

Painful cramping in the abdomen, back, or upper thighs is common during the first few days of menstruation. Severe uterine pain during menstruation is known as dysmenorrhea, and it is most common among adolescents and younger women (affecting about 67.2% of adolescent females).^[101] When menstruation begins, symptoms of premenstrual syndrome (PMS) such as breast tenderness and irritability generally decrease.^[90] Sanitary products include pads and tampons, and are essential items for use during menstruation.

Proliferative phase

The proliferative phase is the second phase of the uterine cycle when estrogen causes the lining of the uterus to grow, or proliferate, during this time.^[82] As they mature, the ovarian follicles secrete increasing amounts of estradiol, and estrogen. The estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. The estrogen also stimulates crypts in the cervix to produce cervical mucus, which causes vaginal discharge regardless of arousal, and can be tracked by women practicing fertility awareness.^[102]

Secretory phase

The secretory phase is the final phase of the uterine cycle and it corresponds to the luteal phase of the ovarian cycle. During the secretory phase, the corpus luteum produces progesterone, which plays a vital role in making the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy, by increasing blood flow and uterine secretions and reducing the contractility of the smooth muscle in the uterus;^[103] it also has the side effect of raising the woman's basal body temperature.^[104]

5.                  OVUM TRANSPORT IN THE FALLOPIAN TUBES.

Egg transport refers to the movement of the oocyte from the moment of expulsion from the ovarian follicle to entry into the distal segment of the fallopian tube before fertilization takes place. Once fertilized in the ampullary segment of the fallopian tube, the embryo spends about 5 days traveling into the remaining anatomical oviductal districts and arrives into the uterine cavity at the blastocyst stage. For purposes of clarity and accuracy, the term “egg transport” covers post-ovulation and pre-fertilization stages (i.e. the haploid life span of the ovulated oocyte). A subsequent section provides details concerning transport of the fertilized diploid oocyte (i.e. zygote) and pre-implantation embryo.

The anatomy and physiology of the fallopian tube play an important role in egg transport and fertilization. The fallopian tube is a muscular tube with an average length of about 11–12 cm and is composed of four regions. The most distal portion is called the infundibulum, it is approximately 1 cm in length, and it includes the finger-like fimbria. The epithelial lining of the fimbria is densely ciliated and highly convoluted. This structure, along with the muscle-controlled movements of the fimbria, is thought to be important for capture of the cumulus-oocyte complex. The next portion of the oviduct is called the ampulla. This segment averages 5–8 cm in length. It is within this highly ciliated portion of the oviduct that fertilization and early embryo development occur. The ampulla is most often also the site for ectopic implantation (ectopic pregnancy). The next region, approximately 2–3 cm in length, is the isthmus. Like the ampulla, it too is ciliated yet less densely so. The isthmus is thought to regulate sperm and embryo transport. The last segment of the fallopian tube is called the intramural segment; it is the link between the isthmus of the oviduct and uterine cavity.¹ 

The ciliated and non-ciliated cells of the fallopian tube undergo cyclic changes with the menstrual cycle similar to those occurring in the endometrium. Further, each portion of the fallopian tube appears to be preferentially regulated by hormones that cause a distinct regionalization of activities depending on the day in the female reproductive cycle.² For example, in the early follicular phase (day 4), propulsive forces operate throughout the length of the fallopian tube in the direction of the uterus. At day 8 (mid-follicular phase), the ampulla has alternating propulsive forces towards and away from the uterus. At the time of ovulation (around cycle day 14), ipsilateral transport to the ovary increases with increasing follicular diameter.³ It has been observed that pregnancy rates after intercourse are higher in those women who demonstrate ipsilateral transport, as opposed to those who fail to show lateralization. The fallopian tube function is critical for the early stages of fertilization.

At the time of ovulation, the oocyte is surrounded by a mass of specialized granulosa cells called the cumulus oophorus. Together, the oocyte and granulosa cells are called the cumulus-oocyte complex (COC). The innermost cell layers of the cumulus immediately overlying the zona pellucida of the oocyte are called the coronal cells. After cumulus maturation, the same cells are called the corona radiata because of their “sunburst” appearance. These cells have processes that extend through the acellular glycoprotein matrix of the zona to contact the oocyte plasma membrane for a rich metabolic exchange of nutrients via the so-called transzonal projections. The cumulus of the mature COC is sticky and is thought to facilitate the adherence of the COC to the surface of the fimbriae once it is expelled from the follicle at ovulation.

The exact mechanism by which the COC is picked up and gains entry into the fallopian tube lumen is unknown. One possibility is that the fimbriated end of the ipsilateral fallopian tube sweeps over the ovary, picks up the COC, and draws it into the tubular lumen by muscular control. Paradoxically, women have become pregnant who were missing the fallopian tube on the side where ovulation occurred. Also, oocytes placed in the peritoneal cavity have been picked up by the fallopian tube and resulted in intrauterine pregnancies.⁴ This evidence suggests that other forces help to facilitate oocyte pickup. Another possibility is that the rhythmic and unidirectional beating of cilia on the fimbriae – where the cilia have adhesive sites – and in the ampullary and isthmic regions of the fallopian tube, draw the COC into the lumen of the fallopian tube. However, this cannot be the sole mechanism by which the COC is picked up and transported through the fallopian tube because women with immotile cilia syndrome, otherwise known as Kartagener's syndrome, are often fertile. Another possibility is that muscular contractions of the fallopian tube create negative pressure that helps to aspirate the COC from the surface of the ovary into the lumen. However, capping and suturing of the fimbriated end in women has failed to prevent pregnancy.⁵ More recently, researchers have reported that the uterus and fallopian tube appear to act as a peristaltic pump. The pumping frequency increases on the ipsilateral side, in the direction where ovulation will occur, and as the follicular diameter increases.³ A novel alternative to the aforementioned mechanisms for COC pickup is one involving mucus strand connections between fimbria and ovary that act as a tether between the two structures to facilitate fimbrial capture of the COC.⁵ The entire process of pickup and deposition of the COC into the lumen takes between 2 and 3 minutes after ovulation. Therefore, it would seem that at least several mechanisms are involved with COC pickup, the most important of which are ciliary beating, sweeping of the ovarian surface by the fimbria, and peristaltic pumping of the female tract.

6.                  SPERM TRANSPORT IN THE FEMALE TRACT, FERTILIZATION 

After ovulation, the fertilizable life span of the mature human oocyte is estimated to be about 24 hours. In contrast, the fertilizable life span of the human spermatozoon is around 72 hours. Sperm motility can persist for much longer and has been documented in vivo for up to 5 days, but fertilizing ability is lost before motility. Sperm deposited in the proximal vagina can be found in the fallopian tube within 5 minutes.⁶

A number of sperm-related events must occur for successful fertilization. The first factor is that a sufficient number of mature, viable spermatozoa must be present in the ejaculate. Second, the morphology of the sperm must be such that the cervical mucus will allow passage into the uterus. Third, it is essential that a good percentage of the sperm have forwardly progressive motion to propel them through the cervical mucus into the uterine cavity and the fallopian tube for ultimate encounter with the COC. Fourth, sperm must undergo the acrosome reaction and hyperactivation during sperm transport into the female reproductive tract (vagina, uterus and tubes) to be enabled for cumulus cell penetration and zona pellucida binding.^7, 8

The term capacitation derives from the observation that sperm must spend time in the female reproductive tract in order to acquire the capacity or ability to fertilize an oocyte. Sperm can also undergo capacitation in vitro when they are incubated in media containing bovine serum albumin as well as energy substrates and electrolytes. Capacitation begins as sperm swim through the cervical mucus. Proteins absorbed in the plasma membrane are removed and sperm surface molecules are modified. An efflux of cholesterol from the sperm plasma membrane may be the initiating event for capacitation. The sperm plasma membrane and outer acrosomal membrane have increased permeability and fluidity as a result of these changes. The more permeable sperm plasma membrane allows for influx of calcium and bicarbonate resulting in activation of second messengers and initiation of signaling events. These unique changes that prepare the spermatozoon for fertilization have collectively been termed capacitation.^9, 10

Some events that occur to induce capacitation are (1) an increase in membrane fluidity;⁸ (2) a decrease in net surface charge; (3) an increase in oxidative processes and cyclic adenosine monophosphate (cAMP) production;^8, 11, 12 (4) a decrease in the ratio of plasma membrane cholesterol to phospholipid;^8, 13, 14 (5) expression of mannose binding sites as a consequence of cholesterol removal;¹⁴ (6) an increase in tyrosine phosphorylation;^11, 12 (7) an increase in reactive oxygen species;¹² and (8) changes in sperm swimming patterns, termed hyperactivation.^8, 15 The hyperactive beat of the sperm flagellum is believed to help the sperm traverse the cumulus cell complex and bind to the zona pellucida. Successful capacitation of the sperm results in a hyperactivated spermatozoon, which is able to bind to the zona pellucida and is susceptible to acrosome reaction induction.

The acrosome reaction is an exocytotic process occurring in the sperm head that is essential for penetration of the zona pellucida and fertilization of the oocyte. The acrosome is a unique organelle, located in the anterior portion of the sperm head analogous to both a lysosome and a regulated secretory vesicle.^16, 17 One of the principal enzymes involved is a serine glycoproteinase called acrosin. It exists in a proenzyme form called proacrosin, which is converted to the active form acrosin by changes in acrosomal pH.¹⁶ 

When sperm bind to the zona pellucida, intracellular calcium is low. The binding causes an opening of calcium channels and an influx of calcium and second messengers that result in the acrosome reaction. Other substances may also induce the acrosome reaction. For example, the addition of periovulatory follicular fluid or progesterone to capacitated spermatozoa stimulates an influx of calcium ions that is coincident with the acrosome reaction.^{18, 19, 20, 21, 22} Periovulatory follicular fluid contains progesterone and it is thought that perhaps the progesterone stimulates calcium influx and the acrosome reaction. However, other acrosome reaction-stimulating factors (e.g. atrial natriuretic peptide) have also been detected in this complex fluid and may play a role in fertilization.²³

The zona pellucida is an acellular glycoprotein matrix that surrounds the mammalian oocyte. The zona pellucida plays an important role in species-specific sperm-egg recognition, sperm-egg binding, induction of the acrosome reaction, prevention of polyspermy, and protection of the embryo prior to implantation.^{24, 25, 26, 27} The zona pellucida is composed of four glycoproteins designated as ZP1, ZP2, ZP3, and ZP4.  ZP3 is the primary ligand for sperm-zona binding and acrosome reaction induction.^28, 29, 30 ZP4 also induces the acrosome reaction, but unlike ZP3 it uses a G-protein independent signaling pathway to induce the acrosome reaction.^31, 32, 33 More recent research has shown that epididymal CRISP1 helps to mediate sperm-zona binding by interacting specifically with ZP3.³⁴

The molecular details of sperm-oocyte recognition have remained elusive. A major breakthrough was made in 2005 when researchers identified a protein on the surface of the capacitated sperm named Izumo1 after a Japanese marriage shrine. Sperm that lacked this receptor were unable to fuse with normal eggs. Researchers recently discovered the egg binding partner for Izumo1 and named it “Juno” after the Roman goddess of marriage and fertility. They showed that Juno-deficient eggs were not able to fuse with normal capacitated sperm, which proved that the Juno-Izumo receptor interaction was essential for mammalian fertilization. Additionally, there is evidence that Juno is undetectable on the oolemma about 40 minutes after fertilization, which suggests that this may be the mechanism for membrane block to polyspermy in mammals.³⁵  

Although ZP3 has been fairly well characterized as a ligand for sperm, such is not the case for ZP3 receptors on the sperm plasma membrane. The majority of current data concerning sperm receptors for zona glycoproteins is restricted to nonhuman mammalian and nonmammalian species. In the human, one of the best described ZP3 receptor candidates is a lectin that binds mannose-containing ligands.¹⁴ Another ZP3 receptor candidate on human sperm is a 95-kd receptor tyrosine kinase (RTK).³⁶ This receptor is thought to initiate intracellular pH changes that culminate in the acrosome reaction. Interestingly, both intact zona pellucida and progesterone stimulate tyrosine phosphorylation.³⁷ Whether these two agonists act via the same RTK is unknown. The possibility exists that one or more signaling or second-messenger pathways interact to result in the acrosome reaction, and subsequent penetration of the oocyte vestments by the spermatozoon.^25, 38, 39 The spermatozoon may have sensitive control mechanisms for regulating cellular responses as it swims through the varied environment of the female reproductive tract. In fact, this arrangement could provide sperm with the ability to sense and respond to molecules present in the female reproductive tract that have been shown to initiate the acrosome reaction, such as follicular and oviductal fluids and the cumulus oophorus.

After a spermatozoon passes through the zona pellucida, it must contact, bind to, and fuse with the oocyte plasma membrane. As a result of the prior acrosome reaction, new sperm membrane proteins become exposed that are likely to prove integral for sperm-oocyte fusion. Data indicate that sperm-oocyte fusion is initiated by signal transduction processes that involve adhesion molecules on both sperm and oocyte plasma membranes that belong to the family of integrins.^40, 41, 42 Integrins are a class of heterodimeric adhesion receptor molecules that participate in cell-to-cell and cell-to-substratum interactions, and they are present on essentially all human cells. Integrins that recognize the Arg-Gly-Asp sequence (RGD) have been detected on the plasma membrane of oocytes. Fibronectin and vitronectin are glycoproteins that contain functional RGD sequences, and they are present on spermatozoa.^{40, 41, 42, 43} When oligopeptides specifically designed to block fibronectin or vitronectin receptors were tested on human spermatozoa in a zona-free hamster oocyte assay, it was found that the peptide for blocking cell attachment to fibronectin was without effect but the other peptide, which blocks both fibronectin and vitronectin receptors, inhibited sperm-oocyte binding. These data suggest that a possible mechanism for sperm-oocyte adhesion and fusion involves an integrin-vitronectin receptor-ligand interaction.⁴⁴

Another potential ligand for oolemmal integrin is human fertilin.^45, 46 Fertilin, formerly PH30, is a heterodimeric sperm surface protein with binding and fusion domains compatible for interaction with integrin receptors on the oocyte. Because of its domains, human fertilin β can be identified as a member of the ADAM family (membrane-anchored proteins having A Disintegrin And Metalloprotease domain).^45, 46 The possibility exists that fertilin and vitronectin act together or in a parallel fashion during gamete interaction.

At some point during or after the fusion process, the oocyte is activated by the spermatozoon.⁴⁷ Activation involves the resumption of meiosis through inactivation of metaphase promoting factor (MPF) which functions to arrest the oocyte in metaphase of the second meiotic division. Extrusion of the second polar body occurs and cortical granules are released into the perivitelline space. The cortical granules modify zona glycoproteins 2 and 3 on the inner aspect of the zona pellucida, resulting in a loss of their ability to stimulate the acrosome reaction and tight binding, so as to prevent polyspermy. This latter event occurs before or simultaneously with the resumption of meiosis. Failure of the oocyte to synthesize or release the cortical granules in a timely fashion results in polyspermic fertilization.

The first event after incorporation of the spermatozoon into the oocyte is the production of sperm-induced calcium (Ca2+) transients. Calcium is the main intracellular signal responsible for the initiation of oocyte activation. These calcium fluxes occur in series and over time (termed “calcium oscillations”); when only a single transient is induced, either by chemical or mechanical stimulation, the oocyte fails to activate. The mechanism by which sperm induce calcium transients is unknown, but there are data that support essentially two models for sperm-induced oocyte activation.^47, 48

One proposed mechanism for sperm-induced oocyte activation is the binding of the spermatozoon to a receptor on the oolemma, which results in G-protein activation, activation of an amplifying enzyme, and generation of an intracellular second messenger within the oocyte.  A second possible mechanism for sperm-induced oocyte activation can loosely be termed the “fusion hypothesis”.⁴⁷ In this model, at the time of sperm and oocyte membrane fusion a “latent” period ensues. During this latent period, a soluble sperm-derived factor diffuses from the sperm into the oocyte's cytoplasm and results in oocyte activation.^{49, 50, 51, 52, 53} To date, however, there are no published reports demonstrating that the extract from a single spermatozoon was able to activate an oocyte.  Abnormalities in transcription, translation, or any other significant molecular process responsible for producing the oocyte-activating ligand/effector molecule during spermatogenesis or spermiogenesis will inhibit fertilization.

Progesterone secreted by the cumulus cells that surround the oocyte stimulates calcium signals that can control hyperactivation and the acrosomal reaction, however, the signaling mechanism has remained unclear.  Recent research has shown that progesterone activates a sperm-specific calcium channel named CatSper, which is primarily associated with hyperactivation of sperm.  CatSper has been shown to be necessary for hyperactivation in mice and several men with infertility have been found to have deletions of the CatSper gene.^54, 55   

As the sperm nucleus is undergoing oocyte-mediated decondensation, the sperm centrosome is orchestrating pronuclear mobilization, syngamy, and, ultimately, early cleavage. The sperm centrosome, with the assistance of maternal γ-tubulin, nucleates sperm astral microtubules and forms the mitotic spindle. The sperm aster, the name for the radial array of these microtubules, unites paternal and maternal pronuclei. At the time of fertilization, the sperm introduces the centrosome, which is the organizing center for microtubules. In doing so, it establishes the polarity and three-dimensional structure of the embryo.^51, 56 In humans, defects in microtubule organization are one cause of fertilization failures seen with in vitro fertilization and may explain fertilization failures that occur after intracytoplasmic sperm injection (ICSI).⁵⁷

EMBRYO TRANSPORT

As mentioned previously, fertilization occurs in the ampullary segment of the fallopian tube. Transit time of the zygote from the ampulla to the ampulla-isthmic junction is approximately 30 hours, after which the zygote remains in the isthmus another 30 hours before resuming transit through the isthmus. It is not until the 5th or 6th day after fertilization that the pre-implantation embryo arrives into the uterine cavity. During the time frame from fertilization to deposition of the embryo in the uterus, the propulsive forces in the fallopian tube are towards the uterus.²

The fallopian tube and its microenvironment are ideal for early embryo development. Indeed, when human embryos are co-cultured on human fallopian tube epithelial cells, higher implantation and lower spontaneous abortion rates are achieved.⁵⁸ Therefore, it would appear that complex interactions take place between the oviductal epithelium and the embryo. Human oviductal cells are known to secrete growth factors, cytokines, and other embryotropic factors (ETFs) that enhance and support the development of the pre-implantation embryos.^59, 60 Oviductal cells may also affect gene expression of the pre-implantation embryo.^61, 62 Much more knowledge is necessary before we can understand the contributions of the tubal environment to embryo development. However, synchrony between uterine endometrium and embryo development must be in place for successful implantation to be achieved.

7.                  FERTILIZATION.

The fusion of a haploid male gamete (sperm) and a haploid female gamete (ovum) to form a diploid zygote is called fertilization. The idea of fertilization was known to Leeuwenhoek in 1683. Fertilization in the human beings is internal and takes place at the ampullary-isthmic junction of fallopian tube of the female.

Fertilization in Human: Process, Events and Significance

Definition:

The process of union of a haploid male gamete or sperm with a haploid female gamete or ovum to form a diploid cell, the zygote, is called fertilization.

Site of Fertilization

In man fertilization is internal (external in case of frog) as in other mammals. It takes place usually in the ampulla of the fallopian tube.

Process of Fertilization

Male discharges the semen into the vagina of the female during copulation (coitus). From the vagina, the sperms reach the ampulla partly by the movement of their tails and partly by the action of uterus. The sperms present in the semen travel a long way from vagina through the uterus into the fallopian tube.

Sperms may reach fallopian tube within five minutes. The sperm can survive in the female’s reproductive tract for 1 to 3 days and it can fertilize the ovum in 12 to 24 hours following ovulation. During sexual intercourse, nearly 300 million sperms are introduced into the vagina, but only few hundreds of them reach near the ovum.

Events of Fertilization

1. Activation of sperm and ovum

The sperms can fertilize an ovum only they are able to secrete the chemical hyaluronidase and possess a surface protein called antifertilizin (composed of acidic amino acid). The ovum secretes a chemical named fertilizin (composed of glycoprotein = mono saccharides + amino acids). It mixes with the water to form egg water which attracts the sperms of its own species.

2. Penetration of sperm

The fertilizin of an egg interacts with the anti fertilizin of sperm of the same species. This attraction between fertilizin and antifertilizin makes the sperms stick to the egg surface. The process of acquiring the capacity to fertilize the egg by the sperm is called capacitation. In this process, the membrane surrounding the acrosome of the sperm breaks and releases its contents, the sperm lysin. It is the chemical substance present in the sperm’s acrosome.

The ovum is surrounded by three membranes such as corona radiata, zona pellucida and the vitelline membranes. At first the sperm passes through corona radiata to reach zona pellucida. There it releases the enzyme hyaluronidase or sperm lysin from its acrosome (Fig. 3(B).8). This enzyme dissolves zona pellucida as a result of which the sperm reaches the plasma membrane of the egg. The above changes on the sperm head are called acrosome reaction.

At the point of contact with the sperm, the egg forms a projection, termed the cone of reception or fertilization cone which receives the sperm. Once one sperm has entered the egg (ovum) the vitelline membrane thickens and is converted into fertilization membrane. This membrane is rigid and never allows other sperms to pass through this membrane. Penetration of the sperm initiates a second maturation division of the ovum and a second polar body is given off.

3. Amphimixis

A sperm consists of three parts: head, middle piece and tail. Shortly before or after entering the egg, the sperm loses its tail (Fig. 3(B).9). After the sperm entering the egg, the membrane of head and middle piece dissolves, and is liberating nucleus, centrosome and mitochondria. Now the sperm nucleus enlarges to form the male pronuclear and the nucleus of the ovum becomes female pronuclear.

The male pronuclear moves inwards and then changes its direction to meet the egg nucleus. The initial path is known as penetration path and the second path is known as copulation path. The chromosomes (haploid set) of (he sperm and the chromosomes (haploid set) of the egg or ovum are set free by the breakdown of their nuclear envelops.

Mixing up of the chromosomes of a sperm and an ovum resulting in a diploid zygote nucleus is known as amphimixis or karyogamy. The mother is now said to be pregnant. The centrosome form asters and spindle fibers. The paternal and maternal chromosomes move to lie in the equator of the spindle and the zygote is ready for division by cleavage.

Fertilization has the following significance

1)      Fertilization restores the diploid number of chromosomes, i.e. 46 in human being.

2)      It provides stimulus for the ovum to complete its maturation.

3)      Fertilization combines the characters of two parents. This brings about recombination of genes and introduces variations.

4)      It determines the sex of the embryo in humans.

5)      Fertilization introduces the centrioles which are absent in ovum.

6)      Fertilization membrane formed after the entry of the sperm prevents the entry of additional sperms.

8.                  HORMONAL CONTROL OF IMPLANTATION.

Implantation in Mammals Body: Period, Mechanism and Hormonal Control!

1. Definition:

The process of attachment of the blastocyst (mammalian blastula) on the endometrium of the uterus is called implantation.

2. Period:

Though the implantation may occur at any period between 6th and 10th day after the fertilization but generally it occurs on seventh day after fertilization.

3. Mechanism:

9.      First of all, the blastocyst is held closely against the uterine endometrial epithelium.

10.  The uterine capillaries and uterine wall in the immediate vicinity of the embryo become more permeable and a local stromal oedema is developed.

11.  Soon the endometrium around the embryo shows the first sign of a decidual cell reaction (DCR) which involves:

12.               (a) The epithelium becomes disrupted and the loosely packed fibroblast-like cells of the stroma are transformed into large rounded glycogen-filled cells.

13.              (b) The area of contact becomes more vascular.

14.              (c) The decidual cells form an “implantation chamber” around the embryo and probably help in nutrition to embryo before the formation of a functional placenta.

15.              (d) The trophoblast is developed from the superficial layer of the morula stage. Later, the trophoblast is lined by mesoderm to form the chorion which contributes to the placenta formation.

Hormonal control of implantation:

(a) Role of estrogens:

Hormonal control of implantation:

(a) Role of estrogens:

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These are a group of steroid hormones mainly secreted by follicular epithelial cells of Graafian follicle though these are also produced by adrenal cortex and placenta. These include β-estradiol, esterone, estriol etc. Out of which most important estrogen is P-estradiol.

Secretion of estrogens is stimulated by FSH of anterior lobe of pituitary gland. These stimulate the uterine endometrial epithelium to enlarge, become more vascular and more glandular. The uterine glands become tortuous and cork-screw shaped. So the endometrium prepares itself for implantation. This stimulation by the estrogens on the uterus generally occurs on the 4th day of pregnancy.

G.P. Talwar (1969 A.D.) reported that estrogens regulate the synthesis of specific proteins through de novo synthesis of specific kinds of mRNA by transcription on specific segments of DNA followed by specific translation.

Some of these proteins act as enzymes which activate the blastocyst for implantation which, in turn, stimulates the uterine endometrium to undergo decidual cell reaction which is essential for implantation.

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(b) Progesterone:

It is also a steriod hormone secreted by yellow-coloured endocrine gland, called corpus luteum, formed from empty Graafian follicle during the pregnancy. Small amount of progesterone is also secreted by adrenal cortex and placenta, Secretion of progesterone is stimulated by LH of anterior lobe of pituitary gland.

Progesterone acts on only those uterine cells which have been earlier stimulated by estrogens. Progesterone further stimulates the proliferation of endometrium of uterus and prepares it for implantation. It also helps in implantation, placenta formation and normal development of the foetus in the uterus.

PREGNANCY DIAGNOSIS.

DIAGNOSTIC FEATURES OF PREGNANCY

1] A woman normally recognizes that she is pregnant by the cessation of menstruation.

2] Swelling of the abdomen.

 3] Vomiting it generally occurs between the 6th to 12th weeks of pregnancy.

4] One cans here the heart beats of foetus.

5] The mother can feel the kicking movements of the foetus by the 18th to the 20^th week.

6] It is possible to detect the pregnancy by X-ray method on the 16thweek.

7] Sperm shielding test: 5ml of urine is injected into each of 2 male frogs. Presence of sperms in frog’s urine in 4 hours indicates positive pregnancy.

Pregnancy can be studied for convenience into two phases

A] Luteal Phase                                                

 B] The Placental Phase

A] THE LUTEAL PHASE OF PREGNANCY

            Due to fertilization the secretory phase or luteal phase of menstrual cycle does not terminate as usual but continues directly as a luteal phase of pregnancy. During this phase following events takes place.

1] The fertilized egg undergoes cleavages to form blastocyst.

2] The blastocyst migrates to the uterus and establish and implantation.

3] A number of fingers like processes arise from the outer coverings of the embryo and are embedded in the tissue of uterus.

4] In the ovary, the corpus luteum grows and starts secreting more amount of progesterone {which increases as pregnancy advances.}

5] The plancentation is stimulated by progesterone hormone of corpus luteum.

B] THE PLACENTAL PHASE OF PREGNANCY

1] After implantation of blastocyst into uterine wall, pools of blood gather around the finger like process of blastocyst.

2] A firm vascular connection between foetus and maternal tissue is established called Plancentation.

3] The placenta also serves to eliminate the secretory products of foetal metabolism.

4] The foetus derives its nourishment and oxygen from the mother through placenta.

5] Human placenta functions as an endocrine organ secreting hormones like oestrogen, progesterone human chorionic gonadotrophin {HCGn}, Human placental lactogen {HPL} and Relaxin.

6] Though the foetal and maternal tissues are in close contact in placenta, there is no mixing of maternal and foetal circulation. It is separated by thin membranous partition. Which allow the exchange of various substances by the diffusion or by the active transport.

7] The main blood vessel from the placenta enters the foetus through a thick cord called Umbilical cord.

8] After birth this cord is cut and after which the baby will not receive any nourishment through the placenta.

9] The growing foetus is also attached by the amniotic fluid with the maternal tissue which can be freely exchanged with the maternal fluids.

10] The placental phase continues up to nine months during which all systems are properly developed.

OTHER PHYSIOLOGICAL CHANGES DURING PREGNANCY

1] Birth canal is enlarged and relaxation of pelvic ligaments takes place.

2] Breasts are properly developed to start lactation after parturition. In this period the pigmentation of Areola and Nipple occurs due to ACTH & MSH.

3] The volume of the blood increases by 1-2 litters of extra blood. Blood count and blood cholesterol are increased.

4] Endocrine glands like adrenal cortex, anterior pituitary and thyroid shows enlargement.

5] The tidal volume and pulmonary ventilation are FSH and LH secreted by pituitary stimulate ovary to produce Grafian follicle which then start secretion of oestrogen.

            Oestrogen stimulates pituitary to secrete LH & LTH which brings about ovulation & corpus luteum formation. Corpus luteum secretes progesterone which brings about plancentation on one hand & prevents further ovulation on the other hand, thus maintains pregnancy.

pregnancy test detects human pregnancy hormone (human chorionic gonadotropin (hCG)) to determine whether an individual is pregnant.

HCG testing can be performed on a blood sample (typically done in a medical office or hospital) or on urine (which can be performed in an office, hospital or at home.)

The most common tests use markers found in blood and urine, specifically one called human chorionic gonadotropin (hCG). Identified in the early 20th century, hCG rises quickly in the first few weeks of pregnancy, peaking at 10 weeks.^[1] It is produced by the syncytiotrophoblast cells of the fertilised ova (eggs) as the cells invade the uterus' lining and start forming what will become placenta.^[2]

Urine tests will typically show positive around four weeks after the last menstrual period (LMP) and are best done in the morning as hCG levels are then highest.^[3] Because of their cut-off hCG level, a positive result is less likely to be incorrect than a negative one, and how much water/fluids have been consumed can affect the results as well.^[4] Blood hCG tests for a more specific part of the hCG molecule and can detect pregnancy earlier than urine, even before a period has been missed. Obstetric ultrasonography may also be used to detect pregnancy. The order of detection from earliest to latest is that hCG can be detected earliest in the blood, then a urine test, then ultrasound.^[3]

Modern tests[edit]

Chemical tests for pregnancy look for the presence of the beta subunit of human chorionic gonadotropin (hCG) in the blood or urine. For a qualitative test (yes/no results only), the thresholds for a positive test are generally determined by an hCG cut-off where at least 95% of pregnant women would get a positive result on the day of their first missed period.^[5] hCG can be detected in urine or blood after implantation around six to twelve days after fertilization, although some evidence suggests that hCG may be released preimplantation as well.^[6][7] Quantitative blood (serum beta) tests can detect hCG levels as low as 1 mIU/mL, and typically clinicians will call a positive pregnancy test at 5mIU/mL.^[5] Urine test strips have published detection thresholds of 10 mIU/mL to 100 mIU/mL, depending on the brand.^[8] Most home pregnancy tests are based on lateral-flow technology.

With obstetric ultrasonography the gestational sac sometimes can be visualized as early as four and a half weeks of gestation (approximately two and a half weeks after ovulation) and the yolk sac at about five weeks' gestation. The embryo can be observed and measured by about five and a half weeks. The heartbeat may be seen as early as six weeks, and is usually visible by seven weeks' gestation.^[9][10]

Although all current pregnancy tests detect the presence of beta hCG, research has identified at least one other possible marker that may appear earlier and exclusively during pregnancy. For example, early pregnancy factor (EPF) can be detected in blood within 48 hours of fertilization, rather than after implantation.^[11] That said, its reliable use as a pregnancy test remains unclear as studies have shown its presence in physiological situations besides pregnancy, and its application to humans remains limited.^[12]

Accuracy[edit]

The control line of this pregnancy test is blank, making the test invalid

The control line on the left of this pregnancy test is visible, suggesting that the test result is valid. A pale purple line has also appeared on the right hand side (the test line) which clearly signifies that the woman is pregnant

A systematic review published in 1998 showed that home pregnancy test kits, when used by experienced technicians, are almost as accurate as professional laboratory testing (97.4%). When used by consumers, however, the accuracy fell to 75%: the review authors noted that many users misunderstood or failed to follow the instructions included in the kits. Improper usage may cause both false negatives and false positives.^[13]

Timing of test[edit]

False negative readings can occur when testing is done too early. Quantitative blood tests and the most sensitive urine tests usually begin to detect hCG shortly after implantation, which can occur anywhere from 6 to 12 days after ovulation. hCG levels continue to rise through the first 20 weeks of pregnancy, so the chances of false negative test results diminish with time (gestation age).^[7] Less sensitive urine tests and qualitative blood^[14] tests may not detect pregnancy until three or four days after implantation. Menstruation occurs on average 14 days after ovulation, so the likelihood of a false negative is low once a menstrual period is late.

Ovulation may not occur at a predictable time in the menstrual cycle, however. A number of factors may cause an unexpectedly early or late ovulation, even for women with a history of regular menstrual cycles. Using ovulation predictor kits (OPKs), or charting the fertility signs of cervical mucus or basal body temperature give a more accurate idea of when to test than day-counting alone.^{[15][unreliable medical source?]}

The accuracy of a pregnancy test is most closely related to the day of ovulation, not of the act of intercourse or insemination that caused the pregnancy. It is normal for sperm to live up to five days in the fallopian tubes, waiting for ovulation to occur.^{[15][unreliable medical source?][16]} It could take up to 12 further days for implantation to occur,^[7] meaning even the most sensitive pregnancy tests may give false negatives up to 17 days after the act that caused the pregnancy. Because some home pregnancy tests have high hCG detection thresholds (up to 100 mIU/mL),^[8] it may take an additional three or four days for hCG to rise to levels detectable by these tests — meaning false negatives may occur up to three weeks after the act of intercourse or insemination that causes pregnancy.^{[citation needed]}

False positives[edit]

False positive test results may occur for several reasons, including errors of test application, use of drugs containing the hCG molecule, and non-pregnant production of the hCG molecule. Urine tests can be falsely positive in those that are taking the medications: chlorpromazine, phenothiazines and methadone among others.^[17]

Spurious evaporation lines may appear on many home pregnancy tests if read after the suggested 3–5 minute window or reaction time, independent of an actual pregnancy. False positives may also appear on tests used past their expiration date.

A woman who has been given an hCG injection as part of infertility treatment will test positive on pregnancy tests that assay hCG, regardless of her actual pregnancy status. However, some infertility drugs (e.g., clomid) do not contain the hCG hormone.^{[medical citation needed]}

Some diseases of the liver, cancers, and other medical conditions may produce elevated hCG and thus cause a false positive pregnancy test.^[18] These include choriocarcinoma and other germ cell tumors, IgA deficiencies, heterophile antibodies, enterocystoplasties, gestational trophoblastic diseases (GTD), and gestational trophoblastic neoplasms.^[18]

Viability[edit]

Pregnancy tests may be used to determine the viability of a pregnancy. Serial quantitative blood tests may be done, usually 3–4 days apart. Below an hCG level of 1,200 mIU/ml the hCG usually doubles every 48–72 hours, though a rise of 50–60% is still considered normal. Between 1,200 and 6,000 mIU/ml serum the hCG usually takes 72–90 hours to double, and above 6,000 mIU/ml, the hCG often takes more than four days to double. Failure to increase normally may indicate an increased risk of miscarriage or a possible ectopic pregnancy.

Ultrasound is also a common tool for determining viability. A lower than expected heart rate or missed development milestones may indicate a problem with the pregnancy.^[10] Diagnosis should not be made from a single ultrasound, however. Inaccurate estimations of fetal age and inaccuracies inherent in ultrasonic examination may cause a scan to be interpreted negatively. If results from the first ultrasound scan indicate a problem, repeating the scan 7–10 days later is reasonable practice.^[9]

HORMONAL REGULATION OF GESTATION.

MECHANISM OF PARTURITION AND ITS HORMONAL REGULATION.

Parturition in Human Beings: Definition, Mechanism and Control!

1. Definition:

It is the expelling of the fully formed young from the mother’s uterus after the gestation period (about 280 days or 40 weeks in human female) i.e., of about 9.5 months.

2. Mechanism:

Parturition is induced by a complex neuroendocrine mechanism which is triggered by fully formed foetus and the placenta called foetal ejection complex.A developing foetus secretes hormones from its adrenal glands. These hormones diffuse into the maternal blood and accumulate to stimulate the release of oxytocin (birth hormone) from the mother’s posterior pituitary.

Oxytocin causes the forceful contraction of smooth muscles of myomefrium, called labour pains, which pushes the young gradually out through the dilated cervix (caused by relaxin) and vagina, with the head foremost. Uterine contraction, in turn, stimulates further secretion of oxytocin.

The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions. It is aided by a reflex (whose centre lies in the lumbar region of spinal cord) and voluntary contraction of abdominal muscles. In the beginning, the labour pains occur once every half or quarter of an hour, called mild ejection reflex, but soon become more frequent.

The foetal membranes burst and amniotic fluid is released but foetal membranes remain behind. This expulsion stage lasts about 20 minutes to one hour. It is followed by placental stage of 10-45 minutes during which the umbilical cord, placenta and foetal membranes are expelled as decidua or after birth.

It is because after the child birth, the uterus reduces in size causing detachment of placenta. Umblical cord is tied and then cut which finally shrinks into a depressed scar called umblicus or navel. Sometimes, the foetus fails to come out then the baby is delivered by a surgical procedure. Such a baby is called cesarean.

Parturition is controlled by hormones:

a)      Oxytocin:

Causes powerful contractions of myometrium during parturition.

(b) Relaxin:

Causes widening of pelvis by relaxing the pubic symphysis of the pelvic girdles.

 

LACTATION AND ITS REGULATION.

Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process can occur with all post-pregnancy female mammals, although it predates mammals.^[1] In humans the process of feeding milk is also called breastfeeding or nursing. Newborn infants often produce some milk from their own breast tissue, known colloquially as witch's milk.

In most species, milk comes out of the mother's nipples; however, the monotremes, egg-laying mammals, lack nipples and release milk through ducts in the abdomen. In only one species of mammal, the Dayak fruit bat from Southeast Asia, is milk production a normal male function.

Galactopoiesis is the maintenance of milk production. This stage requires prolactin. Oxytocin is critical for the milk let-down reflex in response to suckling. Galactorrhea is milk production unrelated to nursing. It can occur in males and females of many mammal species as result of hormonal imbalances such as hyperprolactinaemia.

The chief function of a lactation is to provide nutrition and immune protection to the young after birth. Due to lactation, the mother-young pair can survive even if food is scarce or too hard for the young to attain, expanding the environmental conditions the species can withstand. The costly investment of energy and resources into milk is outweighed by the benefit to offspring survival. ^[2] In almost all mammals, lactation induces a period of infertility (in humans, lactational amenorrhea), which serves to provide the optimal birth spacing for survival of the offspring.^[3]

Hormonal influences[edit]

From the eighteenth week of pregnancy (the second and third trimesters), a woman's body produces hormones that stimulate the growth of the milk duct system in the breasts:

●        Progesterone influences the growth in size of alveoli and lobes; high levels of progesterone inhibit lactation before birth. Progesterone levels drop after birth; this triggers the onset of copious milk production.^[4]

●        Estrogen stimulates the milk duct system to grow and differentiate. Like progesterone, high levels of estrogen also inhibit lactation. Estrogen levels also drop at delivery and remain low for the first several months of breastfeeding.^[4] Breastfeeding mothers should avoid estrogen-based birth control methods, as a spike in estrogen levels may reduce a mother's milk supply.

●        Prolactin contributes to the increased growth and differentiation of the alveoli, and also influences differentiation of ductal structures. High levels of prolactin during pregnancy and breastfeeding also increase insulin resistance, increase growth factor levels (IGF-1) and modify lipid metabolism in preparation for breastfeeding. During lactation, prolactin is the main factor maintaining tight junctions of the ductal epithelium and regulating milk production through osmotic balance.

●        Human placental lactogen (HPL) – from the second month of pregnancy, the placenta releases large amounts of HPL. This hormone is closely associated with prolactin and appears to be instrumental in breast, nipple, and areola growth before birth.

●        Follicle stimulating hormone (FSH), luteinizing hormone (LH), and human chorionic gonadotropin (hCG), through control of estrogen and progesterone production, and also, by extension, prolactin and growth hormone production, are essential.

●        Growth hormone (GH) is structurally very similar to prolactin and independently contributes to its galactopoiesis.

●        Adrenocorticotropic hormone (ACTH) and glucocorticoids such as cortisol have an important lactation inducing function in several animal species, including humans. Glucocorticoids play a complex regulating role in the maintenance of tight junctions.

●        Thyroid-stimulating hormone (TSH) and thyrotropin-releasing hormone (TRH) are very important galactopoietic hormones whose levels are naturally increased during pregnancy.

●        Oxytocin contracts the smooth muscle of the uterus during and after birth, and during orgasm(s). After birth, oxytocin contracts the smooth muscle layer of band-like cells surrounding the alveoli to squeeze the newly produced milk into the duct system. Oxytocin is necessary for the milk ejection reflex, or let-down, in response to suckling, to occur.

It is also possible to induce lactation without pregnancy. Protocols for inducing lactation are called the Goldfarb protocols. Using birth control pills to mimic the hormone levels of pregnancy, then discontinuing the birth control, followed by use of a double electric breast pump for 15 minute sessions at regular 2-3 hour intervals (100+ minutes total per day)_ helps induce milk production.

Secretory differentiation[edit]

During the latter part of pregnancy, the woman's breasts enter into the Secretory Differentiation stage. This is when the breasts make colostrum (see below), a thick, sometimes yellowish fluid. At this stage, high levels of progesterone inhibit most milk production. It is not a medical concern if a pregnant woman leaks any colostrum before her baby's birth, nor is it an indication of future milk production.

Secretory activation[edit]

At birth, prolactin levels remain high, while the delivery of the placenta results in a sudden drop in progesterone, estrogen, and HPL levels. This abrupt withdrawal of progesterone in the presence of high prolactin levels stimulates the copious milk production of Secretory Activation.

When the breast is stimulated, prolactin levels in the blood rise, peak in about 45 minutes, and return to the pre-breastfeeding state about three hours later. The release of prolactin triggers the cells in the alveoli to make milk. Prolactin also transfers to the breast milk. Some research indicates that prolactin in milk is greater at times of higher milk production, and lower when breasts are fuller, and that the highest levels tend to occur between 2 a.m. and 6 a.m.^[5]

Other hormones—notably insulin, thyroxine, and cortisol—are also involved, but their roles are not yet well understood. Although biochemical markers indicate that Secretory Activation begins about 30–40 hours after birth, mothers do not typically begin feeling increased breast fullness (the sensation of milk "coming in the breast") until 50–73 hours (2–3 days) after birth.

Colostrum is the first milk a breastfed baby receives. It contains higher amounts of white blood cells and antibodies than mature milk, and is especially high in immunoglobulin A (IgA), which coats the lining of the baby's immature intestines, and helps to prevent pathogens from invading the baby's system. Secretory IgA also helps prevent food allergies.^[6] Over the first two weeks after the birth, colostrum production slowly gives way to mature breast milk.^[4]

Autocrine control - Galactapoiesis[edit]

The hormonal endocrine control system drives milk production during pregnancy and the first few days after the birth. When the milk supply is more firmly established, autocrine (or local) control system begins.

During this stage, the more that milk is removed from the breasts, the more the breast will produce milk.^[7][8] Research also suggests that draining the breasts more fully also increases the rate of milk production.^[9] Thus the milk supply is strongly influenced by how often the baby feeds and how well it is able to transfer milk from the breast. Low supply can often be traced to:

●        not feeding or pumping often enough

●        inability of the infant to transfer milk effectively caused by, among other things:

o    jaw or mouth structure deficits

o    poor latching technique

●        rare maternal endocrine disorders

●        hypoplastic breast tissue

●        inadequate calorie intake or malnutrition of the mother

Milk ejection reflex[edit]

This is the mechanism by which milk is transported from the breast alveoli to the nipple. Suckling by the baby stimulates the paraventricular nuclei and supraoptic nucleus in the hypothalamus, which signals to the posterior pituitary gland to produce oxytocin. Oxytocin stimulates contraction of the myoepithelial cells surrounding the alveoli, which already hold milk. The increased pressure causes milk to flow through the duct system and be released through the nipple. This response can be conditioned e.g. to the cry of the baby.

Milk ejection is initiated in the mother's breast by the act of suckling by the baby. The milk ejection reflex (also called let-down reflex) is not always consistent, especially at first. Once a woman is conditioned to nursing, let-down can be triggered by a variety of stimuli, including the sound of any baby. Even thinking about breastfeeding can stimulate this reflex, causing unwanted leakage, or both breasts may give out milk when an infant is feeding from one breast. However, this and other problems often settle after two weeks of feeding. Stress or anxiety can cause difficulties with breastfeeding. The release of the hormone oxytocin leads to the milk ejection or let-down reflex. Oxytocin stimulates the muscles surrounding the breast to squeeze out the milk. Breastfeeding mothers describe the sensation differently. Some feel a slight tingling, others feel immense amounts of pressure or slight pain/discomfort, and still others do not feel anything different.

A poor milk ejection reflex can be due to sore or cracked nipples, separation from the infant, a history of breast surgery, or tissue damage from prior breast trauma. If a mother has trouble breastfeeding, different methods of assisting the milk ejection reflex may help. These include feeding in a familiar and comfortable location, massage of the breast or back, or warming the breast with a cloth or shower.

Milk ejection reflex mechanism[edit]

This is the mechanism by which milk is transported from the breast alveoli to the nipple. Suckling by the baby innervates slowly-adapting^[10] and rapidly-adapting^[11] mechanoreceptors that are densely packed around the areolar region. The electrical impulse follows the spinothalamic tract, which begins by innervation of fourth intercostal nerves. The electrical impulse then ascends the posterolateral tract for one or two vertebral levels and synapses with second-order neurons, called tract cells, in the posterior dorsal horn. The tract cells then decussate via the anterior white commissure to the anterolateral corner and ascend to the supraoptic nucleus and paraventricular nucleus in the hypothalamus, where they synapse with oxytocinergic third-order neurons. The somas of these neurons are located in the hypothalamus, but their axon and axon terminals are located in the infundibulum and pars nervosa of the posterior pituitary, respectively. The oxytocin is produced in the neuron's soma in the supraoptic and paraventricular nuclei, and is then transported down the infundibulum via the hypothalamo-neurohypophyseal tract with the help of the carrier protein, neurophysin I, to the pars nervosa of the posterior pituitary, and then stored in Herring bodies, where they are stored until the synapse between second- and third-order neurons.

Following the electrical impulse, oxytocin is released into the bloodstream. Through the bloodstream, oxytocin makes its way to myoepithelial cells, which lie between the extracellular matrix and luminal epithelial cells that also make up the alveoli in breast tissue. When oxytocin binds to the myoepithelial cells, the cells contract. The increased intra-aveolar pressure forces milk into the lactiferous sinuses, into the lactiferous ducts (a study found that lactiferous sinuses may not exist.^[12] If this is true then milk simply enters the lactiferous ducts), and then out the nipple.

Afterpains[edit]

A surge of oxytocin also causes the uterus to contract. During breastfeeding, mothers may feel these contractions as afterpains. These may range from period-like cramps to strong labour-like contractions and can be more severe with second and subsequent babies. ^[13][14]

Without pregnancy, induced lactation, relactation[edit]

In humans, induced lactation and relactation have been observed frequently in some cultures, and demonstrated with varying success in adoptive mothers. It appears plausible that the possibility of lactation in women (or females of other species) who are not biological mothers does confer an evolutionary advantage, especially in groups with high maternal mortality and tight social bonds.^[15][16] The phenomenon has been also observed in most primates, in some lemurs, and in dwarf mongooses.^[17][18]

Lactation can be induced in humans by a combination of physical and psychological stimulation, by drugs, or by a combination of those methods.^[19] Some couples may stimulate lactation outside of pregnancy for sexual purposes.

Rare accounts of male lactation (as distinct from galactorrhea) exist in historical medical and anthropological literature, although the phenomenon has not been confirmed by more recent literature.^[20]

B) FUNCTIONAL ANATOMY OF MALE REPRODUCTION

1)      OUTLINE AND HISTOLOGY OF MALE REPRODUCTIVE SYSTEM IN HUMAN.

Male Reproductive System of Humans

a. Testes

There is a pair of testis whose size is 4.5 cm x 2.5 cm x 3 cm. It is oval in shape and pink in colour. It is the primary sex organ in males. Testes are lodged in a thin walled skin pouch called scrotum or scrotal sac. Testes are extra abdominal. The reason behind this is that testicular temperature should be 2°C lower than the body temperature for normal spermatogenesis to occur. Rise in temperature kills sperms. In case testes do not descend in the scrotum it causes infertility as the formation of sperms does not occur because of rise in temperature. When it is cold, testes shrink to bring it close to the body to keep it warm and in summers it is relaxed and thin. Scrotal sac is tilled with a tissue fluid called as hydrocoel. Testes are held in the scrotal sac by thick fibrous tissue called spermatic cord or gubernaculum.

There is a cavity between abdominal cavity and scrotal sac called as inguinal canal. When the testes descend in the sac they pull their nerves, blood vessels and conducting tubes after them. The connecting tissue along with the cremaster muscles form spermatic cord.

Any damage to inguinal tissue may cause budging out of intestine into scrotum. Such a condition is called inguinal hernia. Septum scorti divides the scrotum internally into two parts. Externally this division is marked by a scar, raphe.

Outer most cover of testes is called as tunica vaginalis which is the visceral layer of peritonium. Under this there is a dense fibrous cover called as tunica albuginea. Under this coat there is a loose connective tissue and blood vessels which together form the tunica vasculosa. Internally tunica albuginea divides each testes into 200-300 lobules

spermatogenetic tissues.

Each of these lobes consists of 1—3 convoluted seminiferous tubules. Seminiferous tubules is a tubular structure which has both the ends terminating into short tubules called tubular recti. Tubular recti connects seminiferous tubules to rete testis. Rete testis is convoluted labyrinth of cuboidal epithelium.

Each testes consists of 1000 seminiferous tubules (Fig. 2). There are two kinds of cells found in seminiferous tubules. They are spermatogenic cells (germ cells) and Sertoli or supporting cells (nurse cells). Sertoli cells were discovered by Enrichno Sertoli, an Italian histologist. As the name signifies, germ cells from the spermatozoa by spermatogenesis and nurse cells provide nourishment to the developing sperm.

Between the seminiferous tubules, Leydig cells are found. These are polygonal in shape and secrete a male steroid hormone called testosterone. Testosterone controls the development of secondary sexual characters in males. Leydig cells were discovered by Franz von Leydig, a German anatomist.

b. Vasa Efferentia

Rete testes gives rise to 10-20 ductules called as vasa efferentia or ductuli efferentes. Vasa efferentia enters the head of epididymis. They are lined by pseudo stratified epithelium which helps in sperm movement.

c. Epididymis

It is a 6 meter long coiled tube found in the poster lateral side of each testes.

It is divided into three parts

i. Upper head or caput epididymis or Globus major — This part is wide and receives vasa efferentia.

ii. Corpus epididymis or Globus minor or body – It stores sperms for a short duration, which undergoes maturation. It lies in the lateral side of testes.

iii. Cauda epididymis or Globus minor or tail –  Before entering the vas deferens the spermatozoa is stored here. This part lies on the caudal side of the testes and is wide.

d. Vas Deferentia

This is also called as seminal duct. It is around 30 cm long, narrow, muscular and tubular structure which starts from the tail of epididymis, passes through the inguinal canal, then over the urinary bladder and then joins the duct of seminal vesicle to form a 2 cm long ejaculatory duct. After passing through the prostate gland it joins the urethra. Before the sperms are transferred to urethra they are stored in spindle like ampulla of vasa deferens.

e. Penis

It is the male genetalia. It is erectable, copulatory, cylindrical organ. It is made up of three erectable tissues. Two of the three are posterior and made of yellow fibrous ligament and is called as Corpora cavernosa. One is anterior, spongy and highly vascular Corpus spongiosum.

It surrounds the urethra. The tip of penis is highly sensitive and is called glans penis. There is a retractile fold of skin on glans penis and is known as fore skin or prepuce. The erection of penis is due to rush of arterial blood into sinuses of corpus spongiosum.

Accessory Sex Glands of Males

These are a pair of seminal vesicles, prostate gland and a pair of Cowper or bulbourethral glands.

a. Seminal Vesicle

They are convoluted, glandular sacs of 4 cm length. They are lined by pseudo stratified epithelium and lie near the ampulae of the vasa deferentia. It provides seminal fluid, which is alkaline and viscous. It contains fructose and prostaglandins. Fructose provides energy to the sperms for swimming and prostaglandins stimulate vaginal contraction which helps in the fusion of gametes.

b. Pair of Cowper’s Gland

These are pea seed sized, white in colour and located at the base of penis. Its secretion helps in lubrication of vagina for smooth movement of penis during copulation.

c. Prostate Gland

It surrounds the proximal part of urethra. It is large and lobulated. It pours alkaline secretion through 20-30 openings. This secretion contains lipids, bicarbonate ions, enzyme and small amount of citric acid.

The secretion of accessory sex glands, i.e. prostate gland and mucus from seminal vesicles combine with sperm to form seminal fluid or semen. The pH is alkaline, i.e. 7.3 – 7.5.

Semen performs the following functions

i. It provides nourishment to the sperms which keeps them viable and motile.

ii. Since it is alkaline it neutralises the acidity of urine in urethra of male and vagina of female to save the sperm.

iii. It helps in transfer of sperm into the vagina of female.

Hormonal Control

The hormones responsible for the normal growth and functioning of seminiferous tubule and Leydig cells are, Follicle Stimulating Hormone (FSH) and Lutenizing Hormone (LH). These are secreted from anterior lobe of pituitary

2)      TESTIS: CELLULAR FUNCTIONS, GERM CELL; SPERMATOGENESIS. HORMONAL REGULATION.

Spermatogenesis: Notes on Spermatogenesis!

Mammalian spermatogenesis is a highly synchronized, regular, long and extremely complex process of cellular differentiation by which a spermatogonial “stem-cell” is gradually transformed into a highly differentiated haploid cell ‘Spermatozoon.”

This differentiation involves three distinct classes of germinal cells—the spermatogonia, the spermatocytes, and the spermatids, which usIn the adult mammals spermatogenesis is a continuous process, which can be divided into two distinct phases and each characterized by specific morphological and biochemical changes of nuclear and cytoplasmic components.

The two phases include:

(i) formation of spermatids (mitosis and meiosis) and

ually are arranged in concentric layers in the seminiferous tubules.

(ii) spermiogenesis.

Formation of spermatids:

This phase of spermatogenesis is further subdivided into three phases.

1. Multiplication phase:

This phase is also known as proliferation and renewal of spermatogonia. During this phase the diploid spermatogonia which are situated at the periphery of the seminiferous tubule, multiply mitotically to form spermatocytes and also to give rise to new spermatogonia! stem cells and enter the phase of growth.

2. Growth phase:

During this phase, a limited growth of spermatogonia takes place; their volume becomes double and they are now called primary spermatocytes which are still diploid in number. Now these primary spermatocytes enter into the next phase namely, maturation phase.

3. Maturation phase:

The primary spermatocyte enter into the prophase of meiotic or maturation division. Meiotic prophase is a very complex process characterised by an ordered series of chromosal rearrangements which are accompanied by molecular changes. During meiosis, first nuclear DNA duplicates, each homologous chromosome starts pairing (synapsis) and longitudinally spilts up into two chromatids, both of which remain joined by a common centromere.

By chiasma formation mutual exchange of some chromosome material between two non-sister chromatids of each homologous pair (tetrad) occurs (crossing over) to provide an almost indefinite variety of combinations of paternal and maternal genes in any gamete.

Lastly, two chromosomes of each homologous pair (tetrad) migrate towards opposite poles of the primary spermatocyte. Now each pole of primary spermatocyte has haploid set of chromosomes. Each set of chromosome is surrounded by the nuclear membrane developed from the endoplasmic reticulum. The first meiotic division, as a rule, is followed by the division of cytoplasm (cytokinesis) which divides each primary spermatocyte into two haploid, secondary spermatocyte.

Each secondary spermatocyte undergoes second meiotic or maturation division which is a simple mitosis and produces four haploid spermatids. These are non-functional male gametes. To become functional spermatozoa, they have to undergo a complex process of cytological and chemical transformations; a process usually referred to as spermiogenesis.

Spermiogenesis:

The changes in the spermatids leading to the formation of spertmatozoa constitute the process of spermiogenesis. Because a spermatozoon is a very active and mobile cell, in order to provide real mobility to it, all the superfluous materials of the developing spermatozoa are to be discarded and a high degree of specialization takes place in the sperm cell through a number of steps.

3)      EPIDIDYMAL FUNCTION AND SPERM MATURATION.

the epididymis (/ɛpɪˈdɪdɪmɪs/; plural: epididymides /ɛpɪdɪˈdɪmədiːz/ or /ɛpɪˈdɪdəmɪdiːz/) is a tube that connects a testicle to a vas deferens in the male reproductive system. It is present in all male reptiles, birds, and mammals. It is a single, narrow, tightly-coiled tube in adult humans, 6 to 7 meters (20 to 23 ft) in length ^[1] connecting the efferent ducts from the rear of each testicle to its vas deferens.

Structure

The epididymis can be divided into three main regions:

●        The head (Latin: Caput). The head of the epididymis receives spermatozoa via the efferent ducts of the mediastinium of the testis. It is characterized histologically by a thick epithelium with long stereocilia (described below) and a little smooth muscle^[2]. It is involved in absorbing fluid to make the sperm more concentrated. The concentration of the sperm here is dilute.

●        The body (Latin: Corpus). This has an intermediate epithelium and smooth muscle thickness^[2].

●        The tail (Latin: Cauda). This has the thinnest epithelium of the three regions and the greatest quantity of smooth muscle^[2].

In reptiles, there is an additional canal between the testis and the head of the epididymis and which receives the various efferent ducts. This is, however, absent in all birds and mammals.^[3]

Histology[edit]

The epididymis is covered by a two layered pseudostratified epithelium. The epithelium is separated by a basement membrane from the connective tissue wall which has smooth muscle cells. The major cell types in the epithelium are:

●        Principal cells: columnar cells that, with the basal cells, form the majority of the epithelium. In the caput (head) region these cells have long stereocilia that are tuft like extensions that project into the lumen.^[4] The sterocilia are much shorter in the cauda (tail) segment.^[4] They also secrete carnitine, sialic acid, glycoproteins, and glycerylphosphorylcholine into the lumen.

●        Basal cells: shorter, pyramid-shaped cells which contact the basal lamina but taper off before their apical surfaces reach the lumen. These are thought to be undifferentiated precursors of principal cells.

●        Apical cells: predominantly found in the head region

●        Clear cells: predominant in the tail region

●        Intraepithelial lymphocytes: distributed throughout the tissue.

●        Intraepithelial macrophages^[5][6]

Stereocilia[edit]

The stereocilia of the epididymis are long cytoplasmic projections that have an actin filament backbone.^[4] These filaments have been visualized at high resolution using fluorescent phalloidin that binds to actin filaments. ^[4] The stereocilia in the epididymis are non-motile. These membrane extensions increase the surface area of the cell, allowing for greater absorption and secretion. It has been shown that epithelial sodium channel ENaC that allows the flow of Na⁺ ions into the cell is localized on stereocilia.^[4]

Because sperm are initially non-motile as they leave the seminiferous tubules, large volumes of fluid are secreted to propel them to the epididymis. The core function of the stereocilia is to resorb 90% of this fluid as the spermatozoa start to become motile. This absorption creates a fluid current that moves the immobile sperm from the seminiferous tubules to the epididymis. Spermatozoa do not reach full motility until they reach the vagina, where the alkaline pH is neutralized by acidic vaginal fluids.

Development[edit]

In the embryo, the epididymis develops from tissue that once formed the mesonephros, a primitive kidney found in many aquatic vertebrates. Persistence of the cranial end of the mesonephric duct will leave behind a remnant called the appendix of the epididymis. In addition, some mesonephric tubules can persist as the paradidymis, a small body caudal to the efferent ductules.

A Gartner's duct is a homologous remnant in the female.

Function[edit]

Role in storage of sperm and ejaculant[edit]

Spermatozoa formed in the testis enter the caput epididymis, progress to the corpus, and finally reach the cauda region, where they are stored. Sperm entering the caput epididymis are incomplete—they lack the ability to swim forward (motility) and to fertilize an egg. Epididymal transit takes about 2.5 months in humans (longer in other species), but the sperm can be stored in the cauda for 2–3 days. During their transit in the epididymis, sperm undergo maturation processes necessary for them to acquire motility and fertility.^[7] Final maturation (capacitation) is completed in the female reproductive tract.

The epididymis secretes an intraluminal environment that suppresses sperm motility until ejaculation.

During ejaculation, sperm flow from the cauda epididymis (which functions as a storage reservoir) into the vas deferens where they are propelled by the peristaltic action of muscle layers in the wall of the vas deferens, and are mixed with the diluting fluids of the prostate, seminal vesicles, and other accessory glands prior to ejaculation (forming semen).

   

4)      ACCESSORY GLANDS FUNCTIONS.

Accessory Sex Glands of Males

These are a pair of seminal vesicles, prostate gland and a pair of Cowper or bulbourethral glands.

a. Seminal Vesicle

They are convoluted, glandular sacs of 4 cm length. They are lined by pseudo stratified epithelium and lie near the ampulae of the vasa deferentia. It provides seminal fluid, which is alkaline and viscous. It contains fructose and prostaglandins. Fructose provides energy to the sperms for swimming and prostaglandins stimulate vaginal contraction which helps in the fusion of gametes.

b. Pair of Cowper ’s gland

These are pea seed sized, white in colour and located at the base of penis. Its secretion helps in lubrication of vagina for smooth movement of penis during copulation.

c. Prostate Gland

It surrounds the proximal part of urethra. It is large and lobulated. It pours alkaline secretion through 20-30 openings. This secretion contains lipids, bicarbonate ions, enzyme and small amount of citric acid.

The secretion of accessory sex glands, i.e. prostate gland and mucus from seminal vesicles combine with sperm to form seminal fluid or semen. The pH is alkaline, i.e. 7.3 – 7.5.

Semen performs the following functions

i. It provides nourishment to the sperms which keeps them viable and motile.

ii. Since it is alkaline it neutralises the acidity of urine in urethra of male and vagina of female to save the sperm.

iii. It helps in transfer of sperm into the vagina of female.

SPERM TRANSPORT IN MALE TRACT

Sperm Transport:-

Sperm transport occurs in both the male reproductive tract and the female reproductive tract. In the male reproductive tract, transport of spermatozoa is closely connected with their structural and functional maturation, whereas in the female reproductive tract, it is important for spermatozoa to pass to the upper uterine tube, where they can meet the ovulated egg. After spermiogenesis in the seminiferous tubules, the spermatozoa are morphologically mature but are nonmotile and incapable of fertilizing an egg . Spermatozoa are passively transported via testicular fluid from the seminiferous tubules to the caput (head) of the epididymis through the rete testis and the efferent ductules. They are propelled by fluid pressure generated in the seminiferous tubules and are assisted by smooth muscle contractions and ciliary currents in the efferent ductules. Spermatozoa spend about 12 days in the highly convoluted duct of the epididymis, which measures 6 m in the human, during which time they undergo biochemical maturation. This period of maturation is associated with changes in the glycoproteins in the plasma membrane of the sperm head. By the time the spermatozoa have reached the cauda (tail) of the epididymis, they are capable of fertilizing an egg.

On ejaculation, the spermatozoa rapidly pass through the vas deferens and become mixed with fluid secretions from the seminal vesicles and prostate gland. Prostatic fluid is rich in citric acid, acid phosphatase, zinc, and magnesium ions, whereas fluid of the seminal vesicle is rich in fructose which is the principal energy source of spermatozoa and prostaglandins. The 2 to 6 ml of ejaculate semen typically consists of 40 to 250 million spermatozoa mixed with alkaline fluid from the seminal vesicles,and prostate.The pH of normal semen ranges from 7.2 to 7.8.

In the female reproductive tract, sperm transport begins in the upper vagina and ends in the ampulla of the uterine tube, where the spermatozoa make contact with the ovulated egg. During copulation, the seminal fluid is normally deposited in the upper vagina where its composition and buffering capacity immediately protect the spermatozoa from the acid fluid found in the upper vaginal area. The buffering effect lasts only a few minutes in humans, but it provides enough time for the spermatozoa to approach the cervix in an environment (pH 6.0 to 6.5) optimal for sperm motility.The next barriers that the sperm cells must overcome are the cervical canal and the cervical mucus that blocks it. Changes in intravaginal pressure may suck spermatozoa into the cervical wall, but swimming movements also seem to be important for most spermatozoa in penetrating the cervical mucus.

There are two main modes of sperm transport through the cervix. One is a phase of initial rapid transport, by which some spermatozoa can reach the uterine tubes within 5 to 20 minutes of ejaculation. Such rapid transport relies more on muscular movements of the female reproductive tract than on the motility of the spermatozoa themselves. These early-arriving sperm, however, appear not to be as capable of fertilizing an egg as do those that have spent more time in the female reproductive tract. The second, slow phase of sperm transport involves the swimming of spermatozoa through the cervical mucus i.e.traveling at a rate of 2 to 3 mm/hour.

Relatively little is known about the passage of spermatozoa through the uterine cavity, but the contraction of uterine smooth muscle, rather than sperm motility, seems to be the main intrauterine transport mechanism. At this point, the spermatozoa enter one of the uterine tubes. According to some more recent estimates, only several hundred spermatozoa enter the uterine tubes, and most enter the tube containing the ovulated egg.

Once inside the uterine tube, the spermatozoa collect in the isthmus and bind to the epithelium for about 24 hours. During this time, they are influenced by secretions of the tube to undergo the capacitation reaction. One phase of capacitation is removal of cholesterol from the surface of the sperm. Cholesterol is a component of semen and acts to inhibit premature capacitation. The next phase of capacitation consists of removal of many of the glycoproteins that were deposited on the surface of the spermatozoa during their tenure in the epididymis. Capacitation is required for spermatozoa to be able to fertilize an egg (specifically, to undergo the acrosome reaction. After the capacitation reaction, the spermatozoa undergo a period of hyperactivity and detach from the tubal epithelium. Hyperactivation helps the spermatozoa to break free of the bonds that held them to the tubal epithelium. It also assists the sperm in penetrating isthmic mucus, as well as the corona radiata and the zona pellucida, which surround the ovum. Only small numbers of sperm are released at a given time. This may reduce the chances of polyspermy.

On their release from the isthmus, the spermatozoa make their way up the tube through a combination of muscular movements of the tube and some swimming movements. The simultaneous transport of an egg down and spermatozoa up the tube is currently explained on the basis of peristaltic contractions of the uterine tube muscles. Fertilization of the egg normally occurs in the ampullary portion (upper third) of the fallopian tube.

It seems that only capacitated spermatozoa have the capability of responding to chemical or thermal stimuli. Because many of the sperm cells that enter the uterine tube fail to become capacitated, these spermatozoa are less likely to find their way to the egg.

C) REPRODUCTIVE HEALTH

1)                  INFERTILITY IN MALE AND FEMALE: CAUSES, DIAGNOSIS AND MANAGEMENT.

2)                  ASSISTED REPRODUCTIVE TECHNOLOGY: SEX SELECTION, SPERM BANKS, AND FROZEN EMBRYOS. IN VITRO FERTILIZATION, ET, EFT, IUT, ZIFT, GIFT, ICSI, PROST.

3)                  MODERN CONTRACEPTIVE TECHNOLOGIES.

1)      INFERTILITY IN MALE AND FEMALE: CAUSES, DIAGNOSIS AND MANAGEMENT

Causes for Male and Female Infertility

Infertility is the inability to conceive. But a precise definition is impossible since varying degrees of infertility is seen. Infertility is seen in both males and females, though it is traditional to blame a woman as the cause of infertility if a couple fail to have children. For many couples especially in India, having a child of their own is a very important event in their lives. To be unable to do so causes a lot of anguish and emotional pain to individuals.

1. FEMALE INFERTILITY

A woman may be infertile due to several causes.

Some important reasons are as follows:

a. Failure to Ovulate

Failure to ovulate is one common cause of infertility in females. This is because the pituitary or hypothalamus fails to produce the FSH which is required for follicle development or LH required for release of the egg from the ovary. It may also be because the ovaries fail to produce oestrogen or progesterone. Hormonal imbalances may be corrected by administering synthetic hormones to the affected individual.

The most commonly used drug is Clomiphene, a synthetic oestrogen like drug which stimulates ovulation. Tamoxifen is another drug used. These pills are taken orally for five days soon after the menstrual cycle starts. Injection of HCG, which is chemically similar to LH is given at the middle of the cycle to stimulate ovulation. ‘Fertility drugs’ which contains FSH and LH or only FSH is also used.

But these have the danger of multiple egg release and consequently multiple pregnancies. Advance techniques include small implants in the upper arm which releases small amounts of GnRH mimicking the activity of the hypothalamus.

b. Damage to Oviducts

The fallopian tubes may be blocked or narrowed in some women. This interferes with the movement of the eggs and fertilisation. This can be treated by laser surgery.

c. Damage to Uterus

In about 5-10% cases, infertility problems are due to a damaged uterus. The uterus is unable to maintain pregnancy, i.e., the fertilised zygote does not get implanted. Sometimes large non-malignant tumours called fibroids or smaller growths known as polyps which grow in the walls of the uterus can cause infertility.

These can be surgically removed. IUCD or PID also causes inflammation in the uterus and cause problems. This can be treated by using antibiotics. Adhesion in the uterus, i.e. sticking of parts of the uterus which occurs as a result of an abortion is another reason for infertility.

d. Damage to the Cervix

The cervix is the neck of the uterus. The cervix may become damaged because of the abortion or difficult birth. A narrow cervix may interfere with sperm movement.

e. Antibodies to Sperm

In some rare cases, women may produce antibodies against sperms. These are found in the cervix, uterus and oviducts. These may be treated using immunosuppressant drugs, but IVF is a better method of treatment.

2. MALE INFERTILITY

Infertility in males may be due to the following causes:

a. Azoospermia

Absence of sperms in the semen is known as azoospermia. This may occur because of lack of sperm production or because of blocked tubes which does not permit the sperms to appear in the semen. Blockage can occur due to an infection or injury.

Failure of the ejaculation mechanism is another possible reason of azoospermia. Failure to produce sperms may result because of injury to the testes or as a result of infection such as mumps virus or due to hormonal reasons (Fig. 3).

b. Oligospermia

Low sperm count is known as oligospermia. More than 90% males suffer from infertility due to low sperm count. The reasons of oligospermia is summarised in Fig. 3.

c. Abnormal Sperms

Abnormal sperms may possess two heads, or no tail or may have abnormal shapes (Fig. 4). The reasons are not known and may be because of hormonal malfunctions.

d. Autoimmunity

In some males, the immune system may attack the sperms and reduce the sperm numbers. Treatment is not usually possible.

e. Impotence and Premature Ejaculation

The inability to achieve an erection of the penis is known as impotence. Psychological counselling may help in some cases. Premature ejaculation is a condition where the man releases the semen even before penetration into the vagina. This condition is treatable with psychological treatment.

Treatment for Infertility

Infertility can be treated in a number of ways as explained above. Advanced techniques include in vitro fertilisation, donor insemination and surrogacy.

1. In Vitro Fertilisation (IVF)

In vitro fertilisation or IVF is a technique in which egg cells are fertilised by sperms outside the woman’s body. After fertilisation, the zygote is transferred to the patient. The transfer of the zygote is called ‘Embryo transfer’. The foetus is then allowed to grow in the uterus. The term ‘test tube babies’ are given to zygotes formed thus and grown. In vitro fertilisation is usually performed in shallow containers called petri dishes made of plastic resin or glass.

This technique is used as a treatment for infertility when the normal methods of conception have failed. It is included under a category known as assisted reproductive technology or ART. Embryos can also be formed by in vivo fertilisation i.e., fusion of sperm within the female. These embryos can also be transferred into the body of females who cannot conceive.

The technique of IVF was specifically developed for humans in the United Kingdom by Dr. Patrick Steptoe and Robert Edwards in United Kingdom. The first test tube baby, Louise Brown was born in England on July 25, 1978. Subash Mukhopadhyay was the first physician in India to perform IVF and the test tube baby, Durga was born on July 25 1978. But it was only after 1981 the technique became popular all over the world.

For IVF to be successful, healthy ova, sperm that can fertilise, and a uterus that can maintain pregnancy are required. The woman, from whom the ova are collected, is administered fertility medicines to stimulate multiple follicle development. Endogenous ovulation or ovulation inside the body is blocked by the use of GnRH antagonists. After follicular maturation is achieved, human chorionic gonadotropins or (β-hCG) is injected, which acts as an analogue of the lutenising hormone.

This hormone stimulates ovulation 36 hours after the injection. But the egg is retrieved using a transvaginal technique involving an ultrasound guided needle that pierces the vaginal wall to reach the uterus. From the follicular fluid collected, the ova are identified. The procedure is done in a woman under general anesthesia.

In the laboratory, the identified eggs are stripped of surrounding cells and prepared for fertilisation. In the meantime semen is collected from a donor. The sperm and the egg are incubated together in a culture media for about 18 hours. In a more refined technique a single sperm is injected directly into the egg using the intra cytoplasmic sperm injection or ICSI. This is used for sperm that have difficulty penetrating the egg and when sperm numbers are very low.

The fertilised egg is passed into special growth medium and maintained until the fertilised egg has achieved 6-8 celled stage. Embryos are graded by an embryologist based on the number of cells, evenness of growth and degree of fragmentation. The embryos judged to be the ‘best’ are transferred to the patient’s uterus through a thin, plastic catheter that enters the vagina. Often, several embryos are passed into the uterus to improve chances of implantation and pregnancy.

The patient confirms pregnancy after two weeks. She is administered progesterone, to keep the uterus lining thickened and suitable for implantation. But the major complication of IVF is the risk of multiple births. This is because of the practice of transferring multiple embryos.

The multiple embryos generated, may be frozen. These embryos are placed in liquid nitrogen and can be preserved for a long time. The advantage is that patients who fail to conceive may become pregnant using such embryos without having to go through a full IVF cycle. Alternately, they could use them for another pregnancy.

Ethical, Religious and Legal Issues Raised by IVF

Several ethical issues have been raised from the time IVF was introduced.

These concerns are as follows

a. Interference and bypassing of natural method of conception.

b. The idea of creating human life outside in the laboratory is not acceptable to many.

c. Multiple embryos and their uncertain fate are also appalling. The use of embryos for research, manipulation of the embryos are considered unethical.

d. Very expensive and unaffordable to majority of people.

There are several religions which oppose the technique of IVF. In several countries the IVF programmes are subject to regulations that regulate many aspects of IVF practice.

Some are as follows

a. The number of oocytes that can be fertilised is specified.

b. The number of embryos that can be transferred is also restricted to avoid the complications of multiple births.

c. The use of cryopreservation is not permitted in some countries. In Italy, it is a crime to freeze human embryos or to perform pre-implantation diagnosis.

d. The use of third party reproduction leads to related complications such as the surrogate mother developing a psychological bonding with the developing child and refusal to separate from the child after delivery.

2. Donor Insemination or Artificial Insemination:

Sperms may be obtained from sperm donors. Potential donors are screened for health fertility and genetic diseases. They are also screened for HIV antibodies, hepatitis B which may be transmitted through the semen. They donate their semen, which are then frozen and kept ready. When the woman is ready to ovulate, she visits a clinic, and the frozen sample is gently released into the cervix with the help of a small tube.

The success rate of this technique is very high. However, there are many ethical issues related to this technique. Artificial insemination can be used as a technique to treat impotence premature ejaculation, oligospermia and azoospermia.

3. Surrogacy:

Surrogacy is the phenomenon where the embryos are implanted into the uterus of a woman who may or may not be the donor of the eggs. She ‘rents’ her womb for the development of the embryos. An agreement is reached with the surrogate mother and the couple who desire to have the baby. In case the infertile woman is unable to produce eggs or have a damaged uterus, she may opt for surrogacy.

IVF or GIFT technique may be used to transfer the embryo or gamete into the surrogate mother. Alternatively, the egg of the surrogate mother may be used in which case the sperm may be donated by the male partner. There have been many instances of transfer of wrong embryos and misidentified gametes in some laboratories.

Infertility: Causes & Treatment

Infertility is the inability to produce children for a couple in spite of unprotected sexual co-habitation within one year or more. A large number of couples all over the world including India are infertile.

The causes of infertility may be physical, congenital, disease, drug, immunological or even psychological.

In India, when a couple is childless, the female is usually blamed. But more often, the males are detected to be responsible. However, now, specialized health care units known as infertility clinics are available. They could identify the cause of infertility and take up treatment to remove the disorder.

Causes:

The possible causes of infertility in males, females or both are discussed below:

Male infertility:

(i) Cryptorchidism:

It is a condition in which the testes are unable to descend into scrotal sacs, so that; sperms are not produced (azospermia).

(ii) Oligospermia:

It is a defect with testes due to which very less number of sperms is produced. Due to infections like mumps, infection of seminal vesicle and prostate there is less concentration of spermatozoa in semen, the ovum is not fertilized.

(iii) Alcoholism:

Regular intake of alcohol reduces spermatogenesis.

(iv) Impotency:

In this condition the male is unable to erect and penitrate the penis into vagina of female.

(v) Hormone deficiency:

Deficiency of gonadotropins (LH, FSH) thyroid disfunction may be the cause of male infertility.

(vi) Infertility may be due to prolonged use of antihypertensive and antipsychotic drugs.

(vii) Immotile cilia:

Absence of tail in sperm makes it immotile. Hence, sperms cannot move from vagina to upper portions of genital tract of female.

(viii) Absence of Y-chromosome:

Sometimes, deletion of Y-chromosomes in primordial germ cells leads to sperm production without Y-chromosome. Such sperms cannot form viable zygole.

FUNCTIONAL ANATOMY OF MALE REPRODUCTION

Outline and histology of male reproductive system in human; Testis: Cellular functions, germ cell; Spermatogenesis: hormonal regulation; Epididymal function and sperm maturation; Accessory glands functions; Sperm transportation in male tract.

Sperm transport in the female tract,

INTRODUCTION

It is well understood that a myriad of complex steps occur between the time of ejaculation and the union of the haploid number of chromosomes from each partner that results in the final process of fertilization. Consequently, it is easy to imagine the numerous potential problems that can occur at each step, and thus may prevent a successful pregnancy. Often overlooked are the complexities of sperm transport and the steps that must occur in the sperm, a process known as capacitation, before fertilization can occur. These processes of sperm delivery and potentiation are addressed in detail in this chapter.

TRANSPORT

Vaginal Insemination

The complex process of sperm transport through the female reproductive tract begins at the time of ejaculation. During coitus, 1.5- to 5.0-ml of semen containing between 200 and 500 million sperm is deposited at the posterior vaginal fornix, leaving the external cervical os partially submerged in this pool of fluid.¹ At this time, some sperm may be passively taken up by the cervix in a process described as “rapid transport;” otherwise, sperm undergo “delayed transport.” Both of these are discussed at length in this chapter.

The optimal pH for sperm viability is between 7.0 and 8.5,^2,3,4,5 and a reduction in sperm motility is seen at a pH less than 6.0.^6,7,8 Normal vaginal pH is only 3.5 to 4.0,⁹ and the acidic environment of the vagina is thus toxic to sperm. However, both seminal fluid and cervical mucus present within the posterior vagina are alkaline and act as buffers. Fox and coworkers have shown that vaginal pH rises to 7.0 within just seconds after ejaculation,¹⁰ and this decrease in acidity can be maintained for up to two hours after ejaculation.

Within about 1 minute after coitus, the ejaculate undergoes coagulation. This coagulum temporarily restricts movement of sperm out of the seminal clot, thus preventing their passage into the cervical mucus and ascension up the female reproductive tract. Over the next 20 to 30 minutes, however, a seminal-fluid proteolytic enzyme produced by the prostate gland gradually liquefies the clot. At this time, motile sperm may then enter the cervical mucus, leaving behind the seminal plasma. Although there are reports of motile sperm persisting within the vagina for up to 12 hours after ejaculation,¹¹ motility of most vaginal sperm is diminished within about 30 minutes, and after 2 hours almost all sperm motility in the vagina has been lost.

Rapid Sperm Transport

Sperm may begin to undergo the process of rapid sperm transport within seconds after ejaculation. This type of sperm movement is thought to be predominantly passive, resulting from coordinated vaginal, cervical, and uterine contractions. Although these contractions are of short duration, they are believed to be the primary force responsible for the rapid progression of sperm to the upper female reproductive tract—the oviduct. Settlage and coworkers in 1973 reported results of a study in which fertile ovulatory females were intravaginally inseminated with donor sperm at the time of bilateral salpingectomy for sterilization. Within 5 minutes after insemination, sperm were present within the Fallopian tubes, and the number of sperm found there was proportional to the number inseminated.¹² Similar results demonstrating this rapid transport process have also been documented in numerous animal studies.^13,14

The Cervix

Several important functions have been attributed to the cervix, and these include¹⁵

  Providing a receptive environment for sperm entry near the time of ovulation
  Preventing access of sperm, microorganisms, and particulate matter to the upper reproductive tract and thus, the peritoneal cavity
  Filtering spermatozoa and removal of seminal plasma
  Preventing sperm phagocytosis by white blood cells within the female reproductive tract
  Providing a biochemical environment sufficient for sperm storage, capacitation, and migration

The structure of the human cervix facilitates performance of the these stated functions. The endocervical canal has an average length of 3.0 cm, and it is lined by two types of columnar epithelial cells, ciliated and nonciliated.¹⁶ The cervix does not contain true glandular units; rather, the mucosa is arranged with a series of infoldings that form crypts off the central canal. The nonciliated columnar epithelial cells secrete mucin granules, and the ciliated cells propel the cervical mucus from the crypt of origination toward the external cervicalos.^17,18,19 Production of mucus is perhaps the most important function of the cervix, and this is discussed at length later in the chapter. Finally, cervical pH is alkaline, with a peak pH during the periovulatory period. This environment is much more hospitable to spermatozoa than the acidic pH of the vagina.

Cervical Mucus

Cervical mucus is continuously secreted through exocytosis by the nonciliated epithelial cells that line the cervical canal. This biomaterial serves many important functions, including exclusion of seminal plasma, exclusion of morphologically abnormal sperm, and support of viable sperm for subsequent migration to the uterus and oviduct. It is a heterogeneous fluid with both high- and low-viscosity components. The amount of mucus produced and its composition and characteristics fluctuate with circulating progesterone and estrogen levels. As estrogen levels peak at midcycle, cervical mucus is abundant in volume and thin in consistency because of increased water content.¹⁸ Under the influence of progesterone, water content decreases, and the mucus has a much higher viscosity.

Ultrastructurally, cervical mucus can be seen as a complex biphasic fluid with high viscosity and low viscosity components. The high viscosity gel phase is composed of a network of filamentous glycoproteins called mucin. Collectively, mucin macromolecules form a complex of interconnected micelles, which comprise a lattice whose interstices are capable of supporting the low viscosity phase, which is predominantly water.²⁰ Sperm movement through the cervical mucus is primarily through the interstitial spaces between the mucin micelles, and the sperm's progression depends on the size of these spaces.²¹ The size of the interstices is usually smaller than the size of the sperm heads; thus, sperm must push their way through the mucus as they proceed through the lower female genital tract.^22,23,24,25

Besides hormonal factors, physical processes, such as shearing, stretching, and compression can alter the spaces between molecules and, consequently, orientation of the mucin filaments. These mechanical forces can be imparted by thrusting and pelvic contraction during coitus, and also by cervical contractions in the pericoital period. Additionally, rheologic forces associated with the mucus outflow from the cervical crypts tend to align the mucin filaments in a longitudinal fashion within the cervical canal, thus creating aqueous channels between the filaments.²⁰ Given this longitudinal orientation, with mucus outflow originating in the crypts of the cervical epithelium, it has been postulated that sperm are constrained to swim in the direction of least resistance, that is, along the tracts of mucus outflow in the direction of the cervical crypts.^26,27 Using mucus stretched in vitro, several investigators have indeed demonstrated the parallel swimming patterns of sperm.^28,29 This theory complements the notion that spermatozoa entering the cervix are directed toward the cervical crypts, the site of mucus secretion that serves as a possible storage reservoir. Spermatozoa may retain their fertilizing capacity in human cervical mucus for up to 48 hours and their motility for as long as 120 hours.^30,31,32 From their temporary storage location within the cervical crypts, sperm can be released gradually over time, thus enhancing the probability of fertilization.

Another potentially important feature of human cervical mucus is the belief that it is able to restrict migration of human spermatozoa with abnormal morphology. The percentage of spermatozoa with normal morphology in the cervical mucus and in the uterine fluid is significantly higher than usually seen in semen.^{33,34,35,36,37,38,39} Quantitatively, these findings have been demonstrated following artificial insemination in which the percentage of sperm with normal morphology from the inseminated specimen was known ahead of time, thus allowing a more accurate comparison of the postinseminate semen within the cervical mucus.^33,40 These results suggest that spermatozoa with abnormal morphology may be constrained by a process of restricted entry into cervical mucus. Comparison of morphologically normal versus abnormal human sperm in semen has shown that abnormal sperm are less likely to be motile, and those that are motile tend to swim with a lower velocity than normal cells.^41,42 Katz and colleagues studied human sperm motility and morphology in vitro and they found that sperm with normal morphology swim faster than sperm with abnormal morphology, despite similar flagellar frequencies and amplitudes.⁴³ These results suggest that morphologically abnormal spermatozoa may experience decreased movement resulting from increased resistance of mucus.

Sperm Transport Through the Uterus

Little is known about sperm transport within the endometrial cavity. Sperm motility does not appear to be the only force directing the sperm toward the oviducts, because inert particles deposited within the uterus are transported to the Fallopian tubes.⁴⁴ Uterine muscular contractions likely play a role in this process. Unfortunately, much difficulty has been met in attempts to recover and quantify uterine sperm.⁴⁵ Moyer and colleagues examined sperm recovered at the time of ovulation from the uterus of women undergoing hysterectomies 25 to 41 hours after intercourse.^45A Sperm was recovered in only 6 of 26 women, and for these women the total number of sperm ranged from 1 to 4. None of the sperm were motile.

A study by Kunz and coworkers used vaginal sonography to demonstrate that uterine peristalsis during the follicular phase of the menstrual cycle exhibits an increasing frequency and intensity of subendometrial and myometrial peristaltic waves as the follicular phase progresses.⁴⁶ During this portion of the cycle, the number of contractions propagating in the fundocervical direction decreased, and number of contractions progressing in the cervicofundal direction increased.⁴⁶ In another part of this same study, the investigators placed technetium-labeled albumin macrospheres, about the size of spermatozoa, into the posterior vaginal fornix. The ascension of these particles was monitored by serial scintigrams. As soon as 1 minute after placement, the macrospheres reached the intramural and isthmic portion of the oviduct. Quantitatively, the number of macrospheres progressed dramatically as the follicular phase progressed, with only a few particles entering the uterine cavity during the early follicular phase of the menstrual cycle. By the midfollicular phase, the proportion of macrospheres entering the uterine cavity increased dramatically, and by the late follicular phase, the highest level of macrosphere transported to the oviducts was noted. Perhaps the most striking finding of this particular study was the preferential transport of these inert particles to the oviduct ipsilateral to the side of the dominant follicle. Other investigators have shown that near the time of ovulation, the number of spermatozoa is higher in the oviduct ipsilateral to the dominant follicle than in the contralateral oviduct on the side of the nondominant follicle.⁴⁷ Several responsible forces have been proposed, including chemotaxis of the sperm toward the dominant follicle. The results of the above study, however, seem to suggest that lateralizing muscular contractile forces may play a significant role in this preferential movement, in that inert particles are obviously unable to engage in chemotactic migration.

Fallopian Tube

The adult human Fallopian tube, about 9 to 11 cm long, consists of five distinct segments: the fimbria, infundibulum, ampulla, isthmus, and intramural segment.⁴⁸ The epithelial lining of the tube is composed of four cell types: ciliated, secretory, intercalary (peg), and undifferentiated cells. Epithelial cells undergo histologic changes in response to cyclic estrogen and progesterone variations, with the height of the epithelial cells being greatest at the time of the estrogen peak near midcycle.⁴⁹ Tubal musculature is organized in a spiral fashion, and at the tubouterine junction these muscles become continuous with the myometrium.⁴⁹

Sperm movement through the Fallopian tube relies on a combination of forces: intrinsic sperm motility, tubular muscular contraction, and fluid flow. Tubal fluid production is maximal at the time of ovulation, and this fluid sustains the sperm before fertilization.⁵⁰ Tubal fluid may also facilitate both sperm capacitation and acrosomal reaction.

Although the uterotubal junction does not act as a barrier to inert particles, it may serve as an additional functional barrier to sperm with abnormal morphology or motility.¹ The number of sperm that reach the oviduct is many orders of magnitude lower than the total number of sperm in the ejaculate. Although tens of millions to hundreds of millions of sperm are deposited in the vagina at the time of ejaculation, anatomic studies have shown that typically only hundreds of sperm are present in the oviduct at various postcoital timepoints.⁵⁰ Williams and colleagues studied the number and distribution of spermatozoa within the human oviduct near the time of ovulation. Parous women undergoing total abdominal hysterectomies for menorrhagia were inseminated with partner or donor semen, and 18 hours later, during surgery, both oviducts were ligated into ampullary, isthmic, and intramural regions. Using flushing techniques, scanning electron microscopy, and homogenization procedures, patients' oviducts were carefully evaluated for the presence of sperm. A median of only 251 total sperm was recovered from the oviducts of these women, and the ampulla near the ovulating ovary contained a significantly higher percentage of spermatozoa than did the nonovulatory side.⁵¹

The precise role played by tubal fluid in gamete transport and sperm activation is still not entirely understood. Zhu and colleagues used an in vitro technique to demonstrate that human oviductal fluid maintains sperm motility induced by exposure to follicular fluid longer than does exposure to a simple salt solution.⁵² Furthermore, these investigators reported that the sperm acrosome reaction, which is induced by follicular fluid, is modulated by exposure of spermatozoa to tubal fluid. These findings may suggest that tubal fluid potentiates the motility and viability of spermatozoa, thus enhancing the chances of fertilization. Yao and colleagues used in vitro oviductal cell cultures incubated with spermatozoa to determine that oviductal cells promote capacitation and stabilize the acrosome.⁵³ There is still much to learn about the dynamics of spermatozoa and the tubal environment. Although done in an in vitro setting, new studies such as the ones already discussed will likely provide clarity to the complex interplay between male gametes and the female reproductive tract.

SPERM CAPACITATION AND THE ACROSOME REACTION

Sperm Capacitation

In 1951 Chang, while studying rabbits, and Austin, while working on rats, each independently reported that mammalian sperm must reside in the female reproductive tract for a finite period of time before they gain the ability to fertilize ova.⁵⁴ One year later, Austin introduced the term “capacitation” when he stated that “the sperm must undergo some form of physiologic change or capacitation before it is capable of penetrating the egg”.^54A Capacitation is now commonly regarded as the reversible, prefertilization activation process of sperm which results in the spermatozoa gaining the ability to:

1.                  Develop hyperactivated motility, with vigorous nonlinear flagellar motion

2.                  Bind to the zona pellucida

3.                  Undergo the acrosome reaction

4.                  Proceed eventually to fusion with the oolemma and egg fertilization

Initial investigative work in the area of sperm capacitation was performed using animal models such as rabbits, rats, and hamsters. In fact, in 1963, Yanagimachi and Chang broke major scientific ground with their finding that hamster epididymal spermatozoa could be capacitated in vitro.⁵⁵ Work soon followed with the demonstration of in vitro sperm capacitation in a large number of other animal species.⁵⁶ A significant finding from these collective studies is that capacitation-related changes at the molecular level in the spermatozoa seem to vary from species to species. Temporally as well, there are also differences in capacitation between species with some species capable of much more rapid capacitation in vitro than others.

Studies of capacitation have sometimes met with controversy, largely because of lack of morphologic criteria by which to assess its occurrence.⁵⁷ Although sperm capacitation has been induced in vitro,⁵⁵ it is not clear whether changes caused by in vitro manipulation are the same as those that occur in vivo. Despite this, both in vivo and in vitro capacitation enable the spermatozoa to undergo fusion of the plasma and outer acrosomal membrane during the acrosome reaction and thus proceed to subsequent fertilization. These two steps, sperm capacitation and the acrosome reaction, are both essential precursors of normal fertilization. Evidence of this is seen in sperm that have not been incubated in the female reproductive tract or otherwise capacitated cannot effectively fertilize an egg.

Many substances within the female reproductive tract have been examined as potential capacitating factors, but at this time none has been uniquely identified. Nonetheless, we do know that at the molecular level, several key changes are noted to occur in the spermatozoa as a result of capacitation. These changes include:⁵⁸

1)                  Alteration or removal of sperm coating materials. These coating materials become adsorbed to or integrated within the sperm plasma membrane during epididymal transport and also during exposure to seminal plasma^59,60

2)                  A decrease in the net negative surface charge⁶¹

3)                  Changes in the content and location of surface antigens⁶²

4)                  Conformational changes to intrinsic membrane proteins⁶³

5)                  Changes in the permeability of the membrane to various ions, especially calcium⁶⁴

Capacitation in Human Spermatozoa

Very little is known about human sperm capacitation in the female reproductive tract. We do know that human sperm that are recovered from the cervical mucus and placed into a noncapacitating medium are able to penetrate the zona pellucida of the human oocyte and also fuse with zona-free hamster oocytes.⁶⁵ Thus, it appears that human sperm capacitation can occur in the cervical mucus. Because of the inherent difficulty in manipulating and subsequently evaluating the in vivo environment of the female reproductive tract, much of what we now know about human sperm capacitation is the result of in vitro studies.

Capacitation in Vitro

Capacitation is associated with significant alteration of the surface of the sperm, with various molecules being removed or rearranged.⁶⁶ Substances in the first group are called “decapacitation factors,” because when added to suspensions that have been previously capacitated, they quickly inhibit fertilizing ability. This inhibition, like capacitation, is reversible.⁶⁷ Rosselli and coworkers investigated human spermatozoa using transmission electron microscopy and found that aliquots of spermatozoa incubated with either cervical mucus or a capacitating medium enriched with 3% bovine serum albumin each showed ultrastructural “stripping” of the sperm coat.⁶⁸ Yudin and colleagues proposed in 1989 that human sperm may experience physical stresses while moving through cervical mucus which result in a removal of sperm coat molecules from the gamete's surface.⁶⁹ Balerna and associates postulated that these sperm coat alterations may result from hydrogen bonding and electrostatic forces by the glycan moiety of the mucin molecules, thus causing the removal of certain sperm surface molecules.^66,70

Capacitation is also characterized by a loss or reduction of cholesterol from the plasma membrane of spermatozoa. Benoff and colleagues have shown that a loss of membrane cholesterol is a necessary feature of capacitation in human spermatozoa.⁷¹ Electron microscopy studies have shown a reduction in cholesterol concentration overlying the acrosome cap during in vitro capacitation.⁷² Further studies have shown that human spermatozoa can be kept in a noncapacitated state if placed in a suspension saturated with cholesterol.⁷¹ Capacitation in this setting will only occur after the sperm are transferred to an environment containing albumin or a similar molecule that can act as a cholesterol acceptor.^71,73

Membranes are a very dynamic collection of proteins and lipids that are capable of responding to various environmental signals that modify cellular activities. Part of this ongoing dynamic process involves alterations of membrane topography, with certain cell surface molecules moving to various locations or domains in response to environmental conditions. Cholesterol has been shown to limit the insertion of proteins into lipid bilayers, to prohibit the movement of receptors in cell membranes and to change membrane protein conformation and thus alter their activity.^74,75 The ratio of cholesterol to phospholipid, so important in the sperm membrane, controls fluidity and ion permeability in most biologic membranes, and the proportion of these two components change during capacitation.^58,59,76,77 Various studies have shown in vitro that plasma membrane cholesterol content is reduced by 20% to 50%, depending on the makeup of the capacitating medium.^58,78 Collectively, these changes in sperm membrane composition are believed to be interrelated to subsequent changes in membrane ion transport and possibly membrane fusion.

Hyperactivation of Motility

This is described as one of the hallmark characteristic changes seen as a result of capacitation. Sperm motility becomes more vigorous with a decreased rate of forward progression. Specifically, the sperm develops:

1.                  Wider amplitude of lateral head displacement

2.                  Marked increase in flagellar beating

3.                  A curved and tortuous trajectory⁷⁹

Although the functional significance of these changes remains unclear, they may facilitate sperm transit through the oviduct and provide the necessary force needed to penetrate the granulosa cell layer and zona pellucida surrounding the ovum.^69A,70A Some studies have shown hyperactivation in about 20% of spermatozoa after a sufficient incubation period with in vitro media, and sperm that display hyperactivated patterns tended to be those with normal morphology.⁸⁰ Factors determining which sperm incubated in capacitating solutions will ultimately demonstrate hyperactive motility are not well understood.

Sperm Membrane Changes

The sperm plasma membrane is composed of a lipid bilayer interspersed with a number of proteins. Lipid types present include cholesterol, glycolipids, and phospholipids. The proteins found here can traverse the entire membrane from cytosolic compartment to extracellular space. These proteins have important functions, including activation of receptors and transport of ions.

The Acrosome Reaction

The mature human ovum possesses a number of surrounding layers that must be penetrated by the spermatozoa for normal fertilization to occur. To assist with this task, the spermatozoa has a caplike region called the acrosome covering the anterior 80% of its head. This structure contains a number of digestive enzymes, such as hyaluronidase, corona-penetrating enzyme, and acrosin to facilitate membrane fusion and sperm entry into the ovum.

Ultrastructurally, the acrosome reaction involves regional fusion of areas of the outer acrosomal membrane and the overlying sperm plasma membrane. These fused areas then lyse, serving as portals through which soluble contents of the acrosome can be dispersed to act on the vestments of the ovum.

The acrosome reaction is initiated as the spermatozoa arrives at the ovum. The outermost covering of the ovum, the cumulus oophorus, is degraded by hyaluronidase located on the plasma membrane of the spermatozoa.⁸¹ Subsequently, corona-penetrating enzyme is released to facilitate spermatozoal transit through the corona radiata.⁸² After transit through the corona radiata is completed, the sperm binds to the zona pellucida. Next, proacrosin, a zymogen within the acrosomal region, is converted to acrosin, and this facilitates breakdown of the zona pellucida glycoproteins. For this biologic process to occur, the spermatozoa plasma membrane and the outer acrosomal membrane must be removed. This, in essence, is the hallmark of the acrosomal reaction. After the spermatozoa has proceeded through the zona pellucida, the sperm head crosses the perivitelline space and attaches to the cell membrane of the ovum. Subsequently, the sperm and ovum plasma membranes fuse, the sperm enters the ovum, and fertilization follows.

The acrosome reaction is a key component of the fertilization process, and its proper timing is essential. Inappropriately early release of the acrosomal enzymes within the female reproductive tract would result in spermatozoa being unable to fertilize. Initiation of the acrosome reaction seems to hinge specifically on spermatozoal binding to the zona pellucida. Although the human model is not entirely understood, the murine model has been extensively studied. With the murine model, spermatozoal exposure and binding to the structural zona glycoproteins, ZP3 (zona pellucida protein #3) have been identified as the molecule that sets the events of the acrosome reaction into motion.⁸³ After this binding has occurred, several changes follow:

  Influx of calcium into the spermatozoa
  Activation of the adenylate cyclase, adenosine 3',5'cyclic phosphate (cAMP), protein kinase pathway
  Activation of the guanylate cycle, cyclic guanosine monophosphate (cGMP), protein kinase pathway
  Activation of the phospholipase C, diacylglycerate, protein kinase C pathway

Together, these pathways likely share a complex regulation of the events collectively called the acrosomal reaction.

CLINICAL APPLICATION

Extensive clinical application has been made of the large body of information accumulated to date regarding sperm transport and capacitation. The most notable utilization has come with the widespread use of in vitro fertilization techniques since the early 1980s for couples with otherwise untreatable infertility. In particular, spermatozoa capacitation techniques in vitro are now performed readily in the laboratory as a routine part of the in vitro fertilization (IVF) treatment for both male and female infertility.

Because of the large number of sperm required for standard IVF as well as the modest initial fertilization and pregnancy rates associated with IVF, several gamete micromanipulation techniques were developed over the next decade in an attempt to improve successful outcomes. The first advance involved creation of a nick in the zona pellucida, followed by standard IVF. This was called partial zona dissection (PZD). Another advance, called subzonal insertion of sperm (SUZI), involved placing the sperm directly into the perivitelline space, the region between the zonal pellucida and the ovum. Both of these techniques have been used successfully in humans but did not give acceptable success rates.

Since the first report of success with intracytoplasmic sperm injection (ICSI) by Palermo and researchers in 1992, this form of treatment has drastically changed the options available to the infertile couple.⁸⁴ This approach obviates many processes, such as the acrosomal reaction, that are essential components of sperm-ovum interaction, during normal fertilization and IVF.

Despite these advances in gamete micromanipulation techniques, it is clear that further investigative work regarding sperm transport, capacitation, and sperm-ovum interaction will help to further advance efforts to efficiently treat infertile couples. Although ICSI certainly is a viable and effective treatment option, less invasive approaches for both male and female factor infertility may be developed as a result of further research in techniques for in vivo enhancement of sperm function. Expanded use of cell culture techniques and use of in vivo experimental models will likely be of great benefit in attempts to better understand the processes of sperm transport, capacitation, and ultimately, fertilization.

Process of Lactation

In this article we will discuss about the process of lactation, explained with the help of suitable diagrams.  

The hormones which influence the development of breasts are:

a. At puberty, it will be estrogen and progesterone. In addition to these, some of the other hormones which are also required are: thyroxine, growth hormone, Cortisol and insulin.

b. During pregnancy, it will be estrogen and progesterone which are secreted in large quantity either from corpus luteum or placenta. Apart from these, the human chorionic somatomammotropin (HCS) secreted by placenta also is responsible for growth of breasts. There is no lactation during pregnancy because, progesterone level is high and it inhibits the release of prolactin.

c. After delivery, since the concentration of progesterone falls earlier than the estrogen, prolactin secretion starts and lactation commences in about 1-3 days. During this phase, suckling is the most effective stimulus that brings about secretion of prolactin.

“The breasts were more skillful at compounding a feeding mixture than the hemispheres of the most learned professors brain”—Oliver Wendell Holmes

Hormones Influencing Lactation:

1. Prolactin:Suckling of breasts not only brings about release of oxytocin (Fig. 7.21), it will also stimulate the secretion of prolactin. For both the hormonal secretions, it is the neuroendocrine mechanism that is involved.

2. Some of the other hormones influencing lactation are thyroxine, and growth hormone. ACTH and glucocorticoids are necessary for maintenance of milk secretion which is known as galactopoiesis.

Emotional conditions, like cry of the baby and condition reflexes, also play an important role in lactogenesis.

Advantages of Breastfeeding:

1. Infant gets a well-balanced diet.

2. Stimulation of nipple releases oxytocin. Oxytocin brings about involution of uterus and size of uterus is reduced following parturition.

3. During the time of breastfeeding, ovulation is inhibited. This is because prolactin inhibits the release of luteinizing hormone.

4. The infant gets some amount of passive immunity since milk contains some of the antibodies.

5. Since it is directly coming from mammary gland, contamination is less and chances of child suffering from infantile diarrhea is minimized.

6. It builds psychological bond between mother and child.