525SV Reproduction (Human)
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Extract 1-10 As you are aware, humans are sexually reproducing and viviparous. The reproductive events in humans include formation of gametes (gametogenesis), i.e., sperms in males and ovum in females, transfer of sperms into the female genital tract (insemination) and fusion of male and female gametes (fertilisation) leading to formation of zygote. This is followed by formation and development of blastocyst and its attachment to the uterine wall (implantation), embryonic development (gestation) and delivery of the baby (parturition). You have learnt that these reproductive events occur after puberty. There are remarkable differences between the reproductive events in the male and in the female, for example, sperm formation continues even in old men, but formation of ovum ceases in women around the age of fifty years. Let us examine the male and female reproductive systems in human. 3.1 THE MALE REPRODUCTIVE SYSTEM The male reproductive system is located in the pelvis region (Figure 3.1a). It includes a pair of testes alongwith accessory ducts, glands and the external genitalia. The testes are situated outside the abdominal cavity within a pouch called scrotum. The scrotum helps in maintaining the low temperature of the testes (2–2.5o C lower than the normal internal body temperature) necessary for spermatogenesis. In adults, each testis is oval in shape, with a length of about 4 to 5 cm and a width of about 2 to 3 cm. The testis is covered by a dense covering. Each testis has about 250 compartments called testicular lobules (Figure 3.1b). Each lobule contains one to three highly coiled seminiferous tubules in which sperms are produced. Each seminiferous tubule is lined on its inside by two types of cells called male germ cells (spermatogonia) and Sertoli cells (Figure 3.2 ). The male germ cells undergo meiotic divisions finally leading to sperm formation, while Sertoli cells provide nutrition to the germ cells. The regions outside the seminiferous tubules called interstitial spaces, contain small blood vessels and interstitial cells or Leydig cells (Figure 3.2). Leydig cells synthesise and secrete testicular hormones called androgens. Other immunologically competent cells are also present. The male sex accessory ducts include rete testis, vasa efferentia, epididymis and vas deferens (Figure 3.1b). The seminiferous tubules of the testis open into the vasa efferentia through rete testis. The vasa efferentia leave the testis and open into epididymis located along the posterior surface of each testis. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct (Figure 3.1a). These ducts store and transport the sperms from the testis to the outside through urethra. The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus. The penis is the male external genitalia (Figure 3.1a, b). It is made up of special tissue that helps in erection of the penis to facilitate insemination. The enlarged end of penis called the glans penis is covered by a loose fold of skin called foreskin. The male accessory glands (Figure 3.1a, b) include paired seminal vesicles, a prostate and paired bulbourethral glands. Secretions of these glands constitute the seminal plasma which is rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands also helps in the lubrication of the penis. Extract10-20 3.2 THE FEMALE REPRODUCTIVE SYSTEM The female reproductive system consists of a pair of ovaries alongwith a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region (Figure 3.3a). These parts of the system alongwith a pair of the mammary glands are integrated structurally and functionally to support the processes of ovulation, fertilisation, pregnancy, birth and child care. Ovaries are the primary female sex organs that produce the female gamete (ovum) and several steroid hormones (ovarian hormones). The ovaries are located one on each side of the lower abdomen (Figure 3.3b). Each ovary is about 2 to 4 cm in length and is connected to the pelvic wall and uterus by ligaments. Each ovary is covered by a thin epithelium which encloses the ovarian stroma. The stroma is divided into two zones – a peripheral cortex and an inner medulla. The oviducts (fallopian tubes), uterus and vagina constitute the female accessory ducts. Each fallopian tube is about 10-12 cm long and extends from the periphery of each ovary to the uterus (Figure 3.3b), the part closer to the ovary is the funnel-shaped infundibulum. The edges of the infundibulum possess finger-like projections called fimbriae, which help in collection of the ovum after ovulation. The infundibulum leads to a wider part of the oviduct called ampulla. The last part of the oviduct, isthmus has a narrow lumen and it joins the uterus. The uterus is single and it is also called womb. The shape of the uterus is like an inverted pear. It is supported by ligaments attached to the pelvic wall. The uterus opens into vagina through a narrow cervix. The cavity of the cervix is called cervical canal (Figure 3.3b) which along with vagina forms the birth canal. The wall of the uterus has three layers of tissue. The external thin membranous perimetrium, middle thick layer of smooth muscle, myometrium and inner glandular layer called endometrium that lines the uterine cavity. The endometrium undergoes cyclical changes during menstrual cycle while the myometrium exhibits strong contraction during delivery of the baby. The female external genitalia include mons pubis, labia majora, labia minora, hymen and clitoris (Figure 3.3a). Mons pubis is a cushion of fatty tissue covered by skin and pubic hair. The labia majora are fleshy folds of tissue, which extend down from the mons pubis and surround the vaginal opening. The labia minora are paired folds of tissue under the labia majora. The opening of the vagina is often covered partially by a membrane called hymen. The clitoris is a tiny finger-like structure which lies at the upper junction of the two labia minora above the urethral opening. The hymen is often torn during the first coitus (intercourse). However, it can also be broken by a sudden fall or jolt, insertion of a vaginal tampon, active participation in some sports like horseback riding, cycling, etc. In some women the hymen persists even after coitus. In fact, the presence or absence of hymen is not a reliable indicator of virginity or sexual experience. A functional mammary gland is characteristic of all female mammals. The mammary glands are paired structures (breasts) that contain glandular tissue and variable amount of fat. The glandular tissue of each breast is divided into 15-20 mammary lobes containing clusters of cells called alveoli (Figure 3.4). The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli. The alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct. Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out. Extract21-30 3.3 GAMETOGENESIS The primary sex organs – the testis in the males and the ovaries in the females–produce gametes, i.e, sperms and ovum, respectively, by the process called gametogenesis. In testis, the immature male germ cells (spermatogonia) produce sperms by spermatogenesis that begins at puberty. The spermatogonia (sing. spermatogonium) present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. Each spermatogonium is diploid and contains 46 chromosomes. Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes, which have only 23 chromosomes each. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids (Figure 3.5). What would be the number of chromosome in the spermatids? The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis. After spermiogenesis, sperm heads become embedded in the Sertoli cells, and are finally released from the seminiferous tubules by the process called spermiation. Spermatogenesis starts at the age of puberty due to significant increase in the secretion of gonadotropin releasing hormone (GnRH). This, if you recall, is a hypothalamic hormone. The increased levels of GnRH then acts at the anterior pituitary gland and stimulates secretion of two gonadotropins – luteinising hormone (LH) and follicle stimulating hormone (FSH). LH acts at the Leydig cells and stimulates synthesis and secretion of androgens. Androgens, in turn, stimulate the process of spermatogenesis. FSH acts on the Sertoli cells and stimulates secretion of some factors which help in the process of spermiogenesis. Let us examine the structure of a sperm. It is a microscopic structure composed of a head, neck, a middle piece and a tail (Figure 3.6). A plasma membrane envelops the whole body of sperm. The sperm head contains an elongated haploid nucleus, the anterior portion of which is covered by a cap-like structure, acrosome. The acrosome is filled with enzymes that help fertilisation of the ovum. The middle piece possesses numerous mitochondria, which produce energy for the movement of tail that facilitate sperm motility essential for fertilisation. The human male ejaculates about 200 to 300 million sperms during a coitus of which, for normal fertility, at least 60 per cent sperms must have normal shape and size and at least 40 per cent of them must show vigorous motility. Sperms released from the seminiferous tubules, are transported by the accessory ducts. Secretions of epididymis, vas deferens, seminal vesicle and prostate are essential for maturation and motility of sperms. The seminal plasma along with the sperms constitute the semen. The functions of male sex accessory ducts and glands are maintained by the testicular hormones (androgens). Extract31-40 The process of formation of a mature female gamete is called oogenesis which is markedly different from spermatogenesis. Oogenesis is initiated during the embryonic development stage when a couple of million gamete mother cells (oogonia) are formed within each fetal ovary; no more oogonia are formed and added after birth. These cells start division and enter into prophase-I of the meiotic division and get temporarily arrested at that stage, called primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and is called the primary follicle (Figure 3.7). A large number of these follicles degenerate during the phase from birth to puberty. Therefore, at puberty only 60,000-80,000 primary follicles are left in each ovary. The primary follicles get surrounded by more layers of granulosa cells and a new theca and are called secondary follicles. The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum. The theca layer is organised into an inner theca interna and an outer theca externa. It is important to draw your attention that it is at this stage that the primary oocyte within the tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body (Figure 3.8b). The of each menstrual cycle. The major events of the menstrual cycle are shown in Figure 3.9. The cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3-5 days. The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertilised. Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other underlying causes like stress, poor health etc. The menstrual phase is followed by the follicular phase. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones (Figure 3.9). The secretion of

Notes

Male Reproductive System

It has two types of cells, spermatogenic (primary germ cells) and indifferent cells.

Indifferent cells give rise to Sertoli cells.

Spermatogenic cells form 4-8 layers.
The cells are destined to undergo spermatogenesis and form spermatozoa. Sertoli cells (named after Italian histologist, Enricho Sertoli, 1842-1910) are large pyramidal cells with bases resting on basal lamina and apical ends extending into lumen of the seminiferous tubule. They function as nurse cells for differentiating spermatozoa.

It is a loose pigmented pouch of skin arising from lower abdominal wall below the pubic symphysis and hanging between and infront of groin part of the thighs.
The pubic symphysis is a joint sandwiched between left pelvic bone and right pelvic bone. It is a secondary cartilaginous joint
Wall of scrotum has three layers -
outer wrinkled skin, connective tissue and smooth muscle.
An internal septum scroti divides the scrotum into two sacs (scrotal sacs or compartments), one for each testis.
The position of septum scroti is indicated externally by a raphe. Scrotum possesses a smooth involuntary dartos muscle.
A testis rests in its chamber over pad called gubernaculum, also called genito-inguinal ligament, .
In foetus the testes develop in the abdomen but descend into scrotum during 7th foetus month through passages called inguinal canals later plugged by connective tissue except for a spermatic cord.
If an inguinal canal remains open or is torn, a loop of intestine may descend in the scrotum to produce the disorder of inguinal hernia. A spermatic cord connects testis with abdominal cavity.
It consists of connective tissue that encloses an artery, a vein, a lymph vessel, a nerve, cremaster muscle and a vas deferens. Descent of testes in scrotum provides a low temperature (of 2 degree centigrade) for maintenance of spermatogenic tissue and formation of sperms.
Failure of testes to descend in scrotum is cryptorchidism, the disorder that causes sterility.
Movement of dartos and cremaster muscles help in changing position of testes to keep them at proper temperature.
In seasonally breeding mammals, the testes descend in scrotum only in breeding season.
They remain in the abdomen at other times, e.g., Otter, Bat. However, in some mammals the testes remain in the abdomen, e.g., elephant, seal, whale.
Scrotum is absent in such cases. Scrotum is in front of penis in Kangaroo.

4. Epididymes (singular-epididymis). They are two long (4-6 m), narrow (0.4 mm) tubules which lie compacted along the testes from their upper ends to lower back sides. Each epididymis is differentiated into upper caput epididymis, middle corpus epididymis and lower cauda epididymis (adjacent to gubernaculum). Wall of epididymis is thin muscular (with peristaltic and segmental movements) and glandular (secreting nutrient fluid for nourishing sperms and providing them mobility). In epididymis the sperms are stored for a few hours to a few days till sent out through ejaculation. Sperms not ejaculated are reabsorbed. Testis and epididymis are together called testicle.

5. Vasa Deferentia (singular-vas deferens). A pair of thick walled and muscular tubes, each of 30 - 35 cm, developing from cauda epididymis, coming out of inguinal canal as constituent of spermatic cord, passing over behind urinary bladder and gets dilated to form ampulla. At its distal end the ampulla receives a duct from seminal vesicle. Vasa deferentia (ducti deferentes) conduct sperms.

6. Ejaculatory Ducts. They are short (2 cm) straight muscular tubes each formed by union of a vas deferens and duct of seminal vesicle where ejaculate is formed by mixing of sperms with secretion of seminal vesicle. The two ejaculatory ducts join the urethra within prostrate gland.

7. Urethra. It arises from urinary bladder and is about 20 cm long, differentiated into four parts-urinary, prostatic, membranous and penile.
Urinary part carries only urine. It ends inside prostatc gland where urethra receives the two ejaculatory ducts to form urinogenital duct. Prostatic part of urethra lies inside the prostate gland. Urinogenital duct receives a number of ductules from the prostate gland. It comes out as membranous part. At the end, the membranous part of urethra receives ducts from Cowper's glands.
It then enters penis as penile part. The penile part is also called spongiose urethra because it lies inside corpus spongiosum.
Urethra opens at the tip of penis. Epididymes, vasa deferentia and urethra transport sperms towards penile meatus or passage. They are collectively called excretory genital ducts.

9. Seminal Vesicles. They are a pair of lobulated elongated musculoglandular sacs of 5 cm length, between urinary bladder and rectum.
Each has a tube that joins vas deferens to form ejaculatory duct.
Secrction of seminal vesicles forms the major part of semen (60- 70%).
It is thick, viscous, alkaline, having proteins including fibrinogen, fructose (nourishment for activity of sperms), citrate, inositol and prostaglandins (for stimulating movements in the female tract).

10. Prostate Gland.
It is a large pyramidal gland of 4 cm width and 3 cm height (a size of golf ball) that encloses a part of urethra including its junction with the ejaculatory ducts.
Prostate gland contains 30-40 glandular tubuloalveoli which open separately into urethra by fine ducts. Secretion of prostate gland is thick, milky and slightly acidic.
It contains calcium phosphate, citratic acid phosphatase and fibrolysin. Prostrate secretion constitutes upto 20 - 30% of semen.
It is essential for sperm motility (removal causes sterility).

11. Cowper's /Bulbourethral Glands.
A pair of small yellow pea seed sized tubuloalveolar glands, 4-5 cm below prostate and opening into membranous urethra by separate ducts.
The secretion has abundant mucus for lubrication of reproductive tract.
It neutralises the urethra from remains of urine. Secretion of Cowper's glands is produced before the ejaculation of semen.

Semen
It is a milky, viscous and alkaline (PH 7.3 - 7.5) fluid ejaculated by male reproductive system during orgasm. The quantity is 2.5 - 4.0 ml at one time having some 400 million sperms, roughly 80-100 million sperms per ml of semen. The fluid part is secreted by epididymes (plural of epididymis) and accessory glands (seminal vesicles, prostate gland and Cowper's glands). It is meant for providing medium for movement of sperms. Semen has chemicals for nourishing the sperms (e.g., fructose), neutralising the acidity of urethra and vagina (e.g., bicarbonate), stimulating movements in female tract (e.g., prostaglandins).

Male reproductive system becomes functional with the onset of puberty. External sex characters appear thereafter, under the influence of hormone testosterone poduced by the Leydig's cell of testes.
Male reproductive system remains operational throughout the life. However, sterility may appear due to prostatitis.
Sperm production is continuous phenomenon.
They are stored for some time inside epididymes. Unejaculated sperms are broken down and reabsorbed. The two major functions of male reproductive system are spermatogenesis and transfer of sperms to reproductive tract of female.

Female Reproductive System

It consists of a pair of ovaries (primary sex organs), a pair of fallopian tubes, uterus, vagina, external genitalia, pair of Bartholin's glands and mammary glands. The major parts are internal and occur in the lower part of abdomen around the area of urinary bladder.

1. Ovaries. They are a pair of greyish-pink ovoid or almond shaped primary sex organs 3 cm in length, 1. 5 - 2.0 cm in breadth, 1 cm in thickness) or female gonads which lie in the upper part of pelvis near kidneys and attached by ligaments to both uterus and abdominal wall. Each ovary is, surrounded by mesovarium or fold of peritoneum. It has hilus where nerves and blood vessels are connected.
Ovary is internally differentiated into four parts -
1. outer germinal epithelium of cubical cells,
2. a delicate sheath of connective tissue or tunica albuginea,
3. a cortex of dense connective tissue with reticular fibres, spindle-shaped cells, ovarian follicles and a few blood vessels
4. while the central part or medulla is made of less dense connective tissue with elastic fibres, smooth muscles, a number of blood vessels and a few nerves.

Cells of germinal epithelium give rise to groups of oogonia that project into cortex as cords called egg tubes of Pfluger, each with a round terminal mass of oogonia called egg nest.
Egg nests give rise to ovarian follicles. In neonate female baby the ovary contains about 2 million follicles but 50% of them are atretic or degenerate.

Atresia continues and by the time of puberty some 3,00,000 - 4,00,000 ovarian follicles are present in an ovary.
However, only 450 ovarian follicles mature, one by one alternately in the two ovaries at intervals of 28 days.
A mature ovarian follicle is called Graafian follicle. It has a diameter of 10 mm.
Outer fibrous theca externa and inner cellular theca interna are derived from spindle cells of cortex. Other constituents are follicular cells (nourishing cells formed from undifferentiated oogonia), an antrum or follicular cavity having liquor folliculi and an eccentrically placed oocyte.
Follicular cells form a cellular sheath (below theca interna) called membrana granulosa and cellular mass called cumulus ovaricus covering the oocyte.
Cumulus oophorus or cumulus ovaricus differentiates into outer discus proligerous and inner corona radiata. Alongwith oocyte it also secretes a mucoprotein membrane called zona pellucida.
Oocyte (secondary oocyte with metaphase of meiosis II) is 50 -100 µm.
It is microlecithal (alecithal according to some workers). Oocyte membrane is called vitelline membrane.
There are three coverings around the egg - inner zona pellucida, middle corona radiata and outer discus proligerous. A polar body is found between vitelline membrane and zona pellucida.
Graafian follicle develops under influence of FSH of anterior pituitary. Its follicular cells secrete estrogen.
Estrogen brings about proliferation of lining layer of uterus, vagina and fallopian tubes. Rising level of estrogen decreases production of FSH and stimulates secretion of LH. The two cause the mature Graafian follicle to rise to the ovarian surface and burst open releasing ovum (ovulation).
It occurs 10 -14 days of menstrual cycle. The empty ruptured / Graafian follicle is called corpus haemorrhagic. It usually contains a blood clot.
The ruptured follicle shows proliferation of cells of membrana granulosa, deposition of yellow pigment or lutein and formation of yellow body called corpus luteum.
It grows in size to about 2.5 cm. Corpus luteum secretes progesterone. Ultimately corpus luteum loses its yellow colour, becomes changed to corpus albicans and then degenerates.
Some thecal cells located around the follicle become active interstial cells which secrete small amount of androgen.

Uterus.
It is pyriform, hollow muscular thick-walled but distensible median structure located above and behind urinary bladder that is meant for nourishing and development of foetus. For this uterus is capable of tremendous enlargement. The empty uterus is 7.5 cm long and 5 cm broad and 2.5 em thick. Lining layer of uterus, called endometrium (mucous membrane), consists of an epithelium and lamina propria of connective tissue. Epithelium is a mixture of ciliated and secretory columnar cells. Lamina propria contains tubular glands, fibroblasts and blood vessels. Actual wall of uterus is myometrium. It has smooth muscles. On the outside is perimetrium which is either a serous coat or adventitia.
Uterus is differentiated into three parts.
(i) Fundus. Upper dome shaped part above the openings of fallopian tubes.
(ii) Body. Main part which is broad above and tapers towards lower side.
(iii) Cervix. Neck or inferior extremity of uterus which protrudes into vagina. It is connected to body by opening called internal os and vagina by external os. Upper part of uterus leans forwards. It is almost at right angles to vagina. Endometrium shows cyclic changes during the reproductive period of female. The phenomenon is called menstrual cycle.

4. Vagina. It is tubular female copulatory organ, passageway for menstrual flow as well as birth canal of about 7-9 cm length between external opening (vaginal orifice) in vestibule and cervix with depression or fornix around cervix, two longitudinal ridges and numerous transverse folds or vaginal rugae. Vaginal wall is made of an internal mucosa, muscular layer and an outer adventitia. Its mucous membrane is nonkeratinised stratified squamous epithelium. Glands are absent. However, cervical glands do pass on some mucus into it during ovulation. The epithelial cells contain glycogen (from puberty to menopause) which shows cyclic changes. Certain bacteria (species of Lactobacillus and Lactoneustoc, also called Doderlein's Bacillus) bring about fermentation and produce acid which inhibits growth of other microorganisms. In virgins the vaginal orifice is partially covered by an annular centrally perforate membrane called hymen.
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5. External Genitalia/ Vulva. The area having external genitalia is characterised by mons pubis (mons veneris) on the upper side, perineum on the lower side, depression or vestibule in the centre. Mons pubis or mons veneris is an eminence formed by fat over the pubic bones. Vestibule has urinary meatus with urethral opening on the upper side and vaginal orifice on the lower side. A small (pea-shaped) erectile clitoris occurs above urethral aperture. It contains two corpora cavernosa. The tip or glans clitoridis is a small tubercle of again erectile tissue. Its epithelium has high cutaneous sensitivity. A retractile foreskin covers the glans clitoridis. The foreskin or praeputium (prepuce) is actually a fold of labia minora. Clitoris is considered to be homologous to penis though it does not enclose urinogenital duct. Sides contain two pairs of fleshy folds, outer larger thicker and hairy labia majora (greater lips) with sebaceous glands and inner smaller thin and nonhairy labia minora (lesser lips) that are connected behind by fourchette. They also possess sebaceous glands.

6. Bartholin's Glands. They are a pair of small glands which open in the vestibule lateral to vaginal orifice. The secretion is thick, viscid and alkaline for lubrication and counteracting urinary acidity (similar to Cowper's glands in males).

7. Breasts. They are a pair of rounded thoracic prominences which develops during puberty but becomes active only after child birth. Each breast has a median nipple having multiporous rounded tip and pigmented sensitive muscular and sebaceous base called areola. Internally each breast has 15 – 25 lobulated tubuloalveolar milk A glands ; each having a number of lobules and each lobule having a number of alveoli. Each milk gland or lobe sends a laticiferous duct towards nipple where it opens by a separate pore of about 0.5 mm diameter. Its secretion is under the control of prolactin (of anterior pituitary) while milk ejection is under control of oxytocin (of posterior pituitary). First or premilk after parturition is called colostrum.

Functioning of Female Reproductive System

Female reproductive system becomes operational at puberty. It is characterised by menarche or on-set of menstrual.cycle. The cycle continues till the age of 45 - 55 years when it stops. Stoppage of menstrual cycle is called menopause. After menopause the females are unable to bear children. Both menarche and menopause are controlled by Gonadotropic14 Agonadotropin hormones. ; Menstrual cycle is also regulated by differing types of hormones.

Four hormones are involved - follicular stimulating hormone (FSH),
luteinising hormone (LH),
oestrogen and
progesterone.

FSH promotes maturation of ovarian or Graffian follicle. The latter secretes oestrogen.

Oestrogen promotes proliferation of endometrial lining,

LH alongwith FSH triggers ovulation. Oestrogen level falls.

LH stimulates empty Graffian follicle to get changed into corpus luteum. Corpus luteum secretes hormone progesterone. Progesterone causes further thickening of endometrial lining. However, LH level falls which causes regression of corpus luteum and hence stoppage of progesterone production. It results in tearing of endometrial lining and production of menstruation.

Human female reproductive system has multitude of functions - oogenesis, reception of sperms during copulation, capaciation of sperms, providing suitable environment for . fertilization, implantation, nourishment and protection of foetus and post-natal nourishment of baby, Therefore, responsibility of human female is considerably more than that of the male.

Menstrual Cycle (Ovarian Cycle)

It is a series of cyclic changes that occur in the reproductive tract of human females and other primates with a periodicity of 28 days, right from menarche to menopause except during period of pregnancy. It is characterised by menses or loss of blood for a few days. Menstrual cycle consists of the \ following phases:

1. Menstrual Phase. It is the phase of menstrual flow/menses which continues for 3 - 5 days and involves discharge of blood (a total of 45 -100 ml), blood clots, cell debris and mucosal fragments from cast off endometrial lining (uterus, fallopian tube and vagina) due to reduced titre of both estrogen and progesterone hormones. Menstrual phase is also called funeral of unfertilised egg or shedding tears of lost ovum. First day of menstrual phase is also considered the first day of menstrual cycle.
2. Post-Menstrual/Follicular Phase. It lasts for 10 -12 days. On stimulation by GnRH, anterior pituitary secretes FSH which stimulates some ovarian follicles to undergo enlargement. However, only one ovarian follicle continues growth. The others degenerate or undergo atresia. The nonfunctional follicles are also called atretic follicles. The growing ovarian follicle ultimately forms Graafian follicle. Follicle cells of Graafian follicle begin producing the hormone called oestrogen (= estrogen).
(a) Recovery Phase. It lasts for 2 days and brings about repair of ruptured blood vessels and mucous lining or endometrium of reproductive tract. (b) Proliferative Phase. The endomitrial lining begins to thicken, especially that of uterus. There is development of blood capillaries, elongation and coiling of uterine glands, greater activity of uterine muscles, thickening and development of more cilia in epithelial lining of fallopian tubes. Endometrium becomes about 3 mm thick.
3. Fertility Phase/Ovulation. Production of FSH decreases while that of LH increases. The mature Graffian follicle rises to the surface of ovary and ruptures to release ovum. The phenomenon is called ovulation. The ovum is drawn into fallopian tube. It is viable for two days when fertilization can occur. OvfIlation takes places between 10th to 14th day (fertility period 10th to 16th day of menstrual cycle), Two characteristics of the fertility phase help in fertilization: (i) Uterine movements help in spread of sperms in female reproductive tract. (ii) Ciliary movements in the epithelium of fallopian tubes for bringing in the ovum.
4. Pre-Menstruation/Luteal/Secretory Phase. It lasts for 12-14 days. The phase is characterised by proliferation of empty ovarian follicle from a yellow mass called corpus luteum. Corpus luteum begins to secrete hormone called progesterone. Under the influence of progesterone and LH, endometrial lining undergoes further thickening to about 5 mm. Its glands become secretory. Uterine movements are reduced. The stage is meant for receiving fertilized ovum and its implantation. The implanted embryo produces another hormone called human chorionic gonadotropin. It maintains luteum for a long time. In the absence of fertilization, corpus luteum degenerates. LH level falls. Progesterone level is reduced. Reduced level of both progesterone and estrogen causes menses.
Gravid Phase
It is the phase of pregnancy when the uterus contains the developing embryo or foetus. The period between fertilisation and delivery is called gestation period. It is 266 days in human beings. If counted from last menses, the period is 40 weeks or 280 days. Endometrial lining of uterus is ready to receive the embryo during secretory phase. As the embryo get implanted, it begins to secrete human chorionic gonadotropin or hCG. The hormone stimulates further growth of endometrium instead of its shedding. Pregnancy is indicated by missing of menstruation but the can be confirmed by the presence of hCG in newly gravid female. hCG maintains corpus luteum. Later on placenta takes over the secretion of progesterone for maintaining endometrium. With the growth of embryo the uterus also enlarges to accommodate it as well as amniotic fluid in which the embryo lies. Events of Human Reproduction
Human reproduction involves
formation of gametes,
cyclic changes in female body for receiving spermatozoa, fusion of gametes,
development of zygote,
implantation of embryo,
formation of foetus, its nourishment and
parturition. They are components of embryonic development.

Embryonic Development

It deals with the study of processes involved in formation of gametes, their fusion and formation of embryo or foetus upto the time of its delivery or parturition. There are two methods of formation of daughter individuals,
blastogenesis and embryogenesis.
Blastogenesis and Embryogenesis
Blastogenesis is the formation of daughter individuals through budding, gemmation and other means of asexual reproduction. Embryogenesis is the production of individuals through fertilization of ovum and development of embryo. Embryology deals with the study of changes that involve embryogenesis or formation of zygote and its development upto the birth of young one.
The various steps of embryonic development are gametogenesis, gametes, fertilisation, cleavage, blastulation, gastrulation, organogenesis, etc.

Gametogenesis

The process of formation of gametes in the gonads is called gametogenesis. Gametes are produced by way of meiotic division resulting in the reduction of number of chromosomes to one half. The production of sperms in the seminiferous tubules of testis is called spermatogenesis and production of ova in ovaries is called oogenesis.
Both ova and sperms develop from primordial germ cells or PGCs. They are extragonidial in origin being formed from extra embryonic mesoderm during early development. They then migrate to yolk sac endoderm and ultimately to gonads of developing embryo.
Oogenesis
It is the process of formation, development and maturation of haploid ova from diploid germinal cells of ovary. Early steps of oogenesis are completed when the female foetus is only 25 weeks old. All the oogonia have been formed by them. Some 45000 - 65000 oogonia form primary oocytes which begin early meiotic division upto diakinesis of meiosis I. Further growth is resumed at the time of puberty. Oogenesis occurs in three phases (i) Multiplication Phase. Diploid primary germ cells from germinal epithelium of ovary multiply mitotically and form oogonia. The latter produce ovigerous cords or egg tubes of Pfluger in mammals.
(ii) Growth Phase. It is prolonged and slow. Oogonia form rounded masses or egg nests at the tips of egg tubes of Pfluger (Pron. pfulgar). An egg nest forms ovarian follicle. One central oogonium grows and functions as primary oocyte. The others form the covering follicular cells. The latter provide nourishment to primary oocyte. Some nourishment also comes from outside. Yolk is, deposited in this stage. The phenomenon is called vitellogenesis. In cooperation with follicular cells, the enlarged primary oocyte secretes mucoprotein membrane or zona pellucida outside its own plasma membrane or vitelline membrane. There is increase in reserve food, size of nucleus, number of mitochondria, functioning of Golgi apparatus and complexity of endoplasmic reticulum.
(iii) Maturation Phase.
Meiosis occurs. Nucleus shifts towards animal pole and undergoes meiosis I. A daughter nucleus alongwith small quantity of cytoplasm is extruded as primary polar body or polocyte below zona pellucida. Simultaneously primary oocyte is changed into haploid secondary oocyte. It proceeds with meiosis II but stops at metaphase II. Ovum is generally shed in secondary oocyte stage. Meiosis II is completed in fallopian tube at the time of fertilization. Entry of sperm brings about degeneration of metaphase promoting factor or MPF and activates anaphase promoting factor APF; Completion of meiosis II then occurs in fertilised ovum. It produces a small secondary polar body. The primary polar body does not divide further in humans and most vertebrates. Spermatogenesis
Spermatogenesis is the process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules. Lining layer of seminiferous tubules possesses primary germ cells and indifferent cells that mature into nurse cells or Sertoli cells.
(i) Multiplication Phase. Diploid primary germ cells or spermatogonial cells are small cells about 12 um in diameter. At sexual maturity these cells undergo repeated mitosis to form a number of diploid spermatogonia. Spermatogonia get differentiated into two types, type A and type B. Type A spermatogonia function as stem cells and produce more spermatogonia. Type B spermatogonia undergo spermatocytogenesis.
(ii) Spermatocytogenesis. Each type B spermatogonium divides mitotically to form primary spermatocytes.
(iii) Growth Phase. Primary spermatocytes grow to become almost double in size.
(iv) Maturation Phase. Each diploid primary spermatocyte undergoes meiosis I to form two haploid secondary spermatocytes. All the secondary spermatocytes derived from a single spermatogonium I remain attached to one another. Secondary spermatocytes divide by meiosis II, each giving rise to two haploid spermatids. The spermatids become partially embedded in Sertoli cells for nourishment and support.
(v) Spermiogenesis. Spermiogenesis is differentiation of a spermatozoon from a spermatid. Golgi apparatus forms acrosome. Centriole divides into two. Distal centriole forms axial filament. Mitochondria collect around upper part of axial filament. Nucleus undergoes condensation. A spermatozoon now separates while the unused cytoplasm degenerates. Heads of spermatozoa remain embedded for some time in Sertoli cells but ultimately the spermatozoa are released into lumen of seminiferous tubule for onward passage. About 64 days are required for the development of spermatozoa from spermatogonia. An adult human male manufactures 10¹² - 10¹³ sperms each day.
Sperm
Human sperm is dart-like flagellate structure of 60 um length and maximum breadth of 3- 5 um.
It has four parts-head, neck, middle piece and tail.
(i) Head. Knob-like terminal part, 4-5 um long and 2. 5 – 3.5 um broad. It has two components, acrosome and nucleus. Acrosome is derived from Golgi apparatus of spermatid. It contains sperm lysins. Nucleus is compact mass of DNA having some prolamines. On the outside is present a double membrane head cap. It possesses a decapacitation factor which is removed by oestrogen hormone of female. Decapacitation is a phenomenon that takes place in the hind region where sperms are stored. The epididymis secretes some proteins that block the receptors on the plasma membrane of sperm head which renders sperm infertile inside the male tract. This is called decapacitation. Capacitation of sperms takes place when they enter the female reproductive tract.
(ii) Neck. It is short narrow part between head and middle piece which contains two centrioles, unconnected proximal centriole and distal centriole attached to axial filament (that passes into middle piece). A temporary band called manchette is present around the neck.
(iii) Middle Piece. It is cylindrical part, 5 - 7 um long and 1 um in breadth. It has axial filament surrounded by 10-14 spiral turns of mitochondria and bearing towards the end a ring centriole or annulus. Mitochondria provide energy for swimming but food is limited. All are embedded in a thin sheath of cytoplasm. Head, neck, middle piece and a part of tail are all covered on the outside by plasma membrane or sheath.
(iv) Sperm Tail. It is narrow vibratile long part about 50 um in length, with two regions, main and end piece. Main piece of tail is 0.5 um in diameter near the beginning but gradually narrows behind. It has an axial filament, small amount of cytoplasm and plasma membrane. In the end piece, cytoplasm and membrane are absent.
It shows lashing movements which provide forward push to the sperm. Hormonal Control of Spermatogenesis
Testosterone is essential for spermatogenesis. It is secreted by interstitial cells or Leydig's cells under the control of LH or ICSH of pituitary. Pituitary gland also produces another hormone called FSH (follicle stimulating hormone).
The hormone stimulates spermatogenesis. Secretion of both LH and FSH is under control of GnRH of hypothalamus. Rising level of testosterone inhibits release of GnRH while its lower level stimulates formation of GnRH. Sertoli cells concentrate testosterone by means of androgen binding protein (ABP) inside the seminiferous tubule under control of FSH and testosterone. In case of excess activity, sertoli cells secrete protein inhibin which supresses formation of FSH.
Cleavage

Types of Ova
On the basis of amount of yolk, the eggs are classified into following types.
Alecithal. Negligible yolk, e.g., human egg.
Microlecithal. Small amount of yolk, e.g., Sea urchin, Branchiostoma.
Mesolecithal. Moderate amount of yolk, e.g., Frog and other amphibian eggs.
Macrolecithal or Megalecithal or PolylecithaI. Eggs having a large amount of yolk e.g., Reptilian and Avian eggs.

. On the basis of distribution of yolk, the eggs are classified into:
a. Isolecithal (= Homolecithal). Having homogeneously distributed yolk all over the ooplasm e.g., protochordates and echinoderms.
b. Heterolecithal. Egg with unevenly distributed yolk.
Telolecithal. Having yolk concentrated in one half e.g., amphibian eggs.
Centrolecithal. Yolk is concentrated in the centre and the cytoplasm is peripheral e.g., Insect eggs.
Discoidal or Meiolecithal. Almost the-whole of the egg is occupied by the yolk except a small disc of active ooplasm e.g., eggs of birds and reptiles.

On the basis of presence or absence of shell, eggs are differentiated into cleidoic (surrounded with water proof shell e.g., birds, reptiles) and noncleidoic (shell absent).

Ovum
Human egg or ovum is noncleidoic (without shell) and alecithal (nearly microlecithal), rounded female gamete having a diameter of about 100 um. The ovum possesses three coverings - inner thickened plasma membrane or vitelline membrane, middle mucoprotein zona pellucida and outer cellular corona radiata with radially elongated scattered cells held in mucopolysaccharide (hyaluronic acid). In between plasma membrane ( = vitelline membrane) and zona pellucida is perivitelline space in which are present 1- 2 polar bodies towards animal pole. The opposite end is vegetal pole.
Cytoplasm of ovum is called ooplasm. It has a large nucleus or germinal vesicle. Typical nucleus or pronucleus is formed only at the time of fertilisation. Prior to it, the same is generally in the stage of metaphase II. Ectoplasm possesses mucopolysaccharide granules and microtubules. Endoplasm has mitochondria, Golgi apparatus, ribosomes, RNA, fat droplets, glycogen particles and proteins. Only a very small amount of yolk is present. It is distributed throughout the cytoplasm.

Egg Coverings
(i) Primary Membranes. Membranes secreted by ovum, e.g., vitelline membrane. A perivitelline space occurs between plasma membrane and vitelline membrane.
(ii) Secondary Membranes. Membranes formed over the ovum by cells of ovary, e.g., chorion of insect eggs.
(iii) Tertiary Membranes. Membranes deposited over the ovum by female reproductive tract, e.g., egg shell by oviducts.

Fertilization
Fusion of male and female gametes i.e., sperm and ovum forming a zygote is called fertilization. It can be external.(outside the body of female) or internal (inside the body of female). Depending upon the site of development of the zygote, the animals may be grouped into three categories i.e.,
(i) Oviparous. Egg laying animals. Fertilization may be internal or external. Development takes place out of the body of the female, e.g., frog, reptiles, birds, etc.
(ii) Ovo-Viviparous. Fertilization and development are internal, but there is no placenta formation. Nourishment is provided by the yolk, e.g., some fishes (dog fish).
(iii) Viviparous. Fertilization and development are internal. Nourishment is provided through true placenta by the uterine wall of the mother, e.g., mammals.

Fertilization in Humans

It is fusion of male and female gametes to form zygote. In human beings fertilisation is internal. Human beings are also viviparous. Here the embryo is retained and nourished inside the uterus of the female by means of an attachment called placenta. At one time only a single ovum is released in human females from one of the two ovaries towards the middle of ovarian/menstrual cycle. It passes into fallopian tube and rests inside ampulla for some time. The journey time is 12-24 hours. Human male produces 300-400 million sperms per ejaculation. They are deposited in vagina during coitus.
The process of deposition of sperms in the female genital tract is called insemination. A number of them are demobilised or eaten but a number of them remain functional and begin to pass into uterus and from there to oviducts. For this they become active swimmers. Viscous liquid secreted by female genital tract further enhances sperm motility. The phenomenon is called capacitation. It takes 5 - 6 hours. The term capacitation is also used for the ability of a sperm to penetrate an ovum after it reaches ampulla. A number of sperms reach the ampulla part of oviduct where the egg rests temporarily.

Fertilization involves the following steps.
(i) Approximation of Sperm and Ovum.
(ii) Acrosome Reaction.
(iii) Egg Reaction.
(iv) Penetration of Sperm. (v) Activation of Ovum. (vi) Fusion of Sperm and Egg Nuclei.

(i) Approximation of Sperm and Ovum. Sperms can remain motile for 24 - 48 hours. They swim at the rate of 1.5 - 3.0 mm/min. * They are able to reach the ampulla part of female genital tract partly by their own swimming and partly by contraction of uterus and fallopian tubes. One sperm comes to lie against the ovum and undergo fertilizin (from ovum) and antifertilizin (from sperm) compatibility reaction (Lillie, 1919) in the region of animal pole.

(ii) Acrosome Reaction. In contact with corona radiata, the acrosome covering lyses to release Iysins. The latter consist of hyaluronidase that dissolves ground substance of follicle cells, corona penetrating enzyme for dissolving corona radiata and zona lysine or acrosin for dissolving zona pellucida.

(iii) Egg Reaction. A small protuberance or fertilization cone (cone of reception) develops from the surface of ovum in the region of animal pole.

(iv) Penetration of Sperm. Sperm head establishes contact with lateral surface of fertilization cone. Plasma membranes of the two dissolve. Contents of head (nucleus), neck and middle piece of sperm enter ooplasm. Tail is left outside. Fertilization cone subsides. Depolarisation of egg membrane kills other sperms. Plasma membrane (cortical reaction) as well as zona pellucida (zona reaction) of the egg are now modified with the help of mucopolysaccharide cortical granules into fertilization membrane. A perivitelline space is created between it and zona pellucida.

(v) Activation of Ovum. Ovum (previously in secondary oocyte stage) undergoes meiosis II and extrudes a secondary polar body. It is now the actual ovum or female gamete.

(vi) Fusion of Sperm and Egg Nuclei. The envelopes of the sperm and egg pronuclei degenerate and their chromosomes intermingle to form 'synkaryon'. The act is called karyogamy or syngamy. The proximal centriole brought by sperm helps form the spindle for the division of synkaryon (cleavage nucleus). Fertilized egg is also called zygote. It immediately begins cleavage.

Cleavage

Process of early mitotic divisions of the zygote which do not involve growth of daughter cells or blastomeres causing increased nucleocytoplasmic ratio is called cleavage.

Cleavage differs from mitosis in the respects that

(i) There is no growth phase between the successive divisions.
(ii) The size of cells gradually decreases.
(iii) The metabolism becomes fast.
(iv) There is rapid DNA replication and (v) High consumption of oxygen.

Types of Cleavage.

There are two main types of cleavage.

(i) Holoblastic. When whole of the egg is divided. It is found in microlecithal and mesolecithal eggs. It may further be
(a) equal- when both the blastomeres are equal e.g., Amphioxus
(b) unequal-when the blastomeres are unequal in size, e.g., Frog.

(ii) Meroblastic division. When a part of the egg is divided. It is found in polylecithal eggs. It may be discoidal (e.g., birds) or superficial (e.g., insects).

Planes of cleavage include meridional, vertical, equatorial and transverse. Patterns can be radial, biradial, spiral or bilateral.

Morula. Early cleavage produces a solid ball of cells called morula.

Blastula

Multicellular ball like embryo produced at the end of cleavage and usually having a fluid filled blastocoel, is called blastula. It is of the following types:
Stereoblastula (Solid Blastula). It is blastula without blastocoel, e.g., Nereis.
Coeloblastula. A blastula with a prominent blastocoel, e.g., Frog.
Discoblastula. A blastula having a many layered disc of biastomeres above the yolk. It develops as a result of meroblastic divisions in polylecithal eggs e.g., Hen.
Superficial Blastula (Periblastula). A blastula having a single layer of blastomeres around the central yolk, e.g., Insects.

Gastrulation

Sum total of all the processes which convert a solid or hollow ball of cells or many layered disc of blastula into two (coelenterates) or three (platyhelminthes onwards) germinal layers of gastrula is called gastrulation. Gastrulation takes place by the migratory or formative or morphogenetic movements of blastomeres from the surface of blastula to the proper position in the gastrula. These movements are classified into

(a) Epiboly. Growth of one part over another like prospective ectoderm over the rest except blastopore.

(b) Emboly. Morphogenetic movements like migration of ectoderm, mesoderm and notochord cells from surface to interior.
The emboly may occur by way of
(i) involution (rolling of cells into interior)
(ii) invagination (infolding)
(iii) ingression (new cells migrating into blastocoel) and
(iv) delamination (formation of second layer by tangential division of surface cells). During gastrulation blastocoel is obliterated and a new cavity archenteron or gastrocoel (lined by endoderm) is formed which is the future alimentary canal of the animal. Blastopore is opening of archenteron which is absent in amniotes.

Organogenesis.

The development of tissues and organs from the three germ layers is called organogenesis.

Morphogenesis.

The assumption of shape, size and other morphological features by the embryo is called morphogenesis.
differentiation. It is the formation of different types of cells, which become different in size, form, chemical composition and perform different functions. .

Fate of three Germinal Layers
Ectoderm. Central nervous system, nerves, retina, lens, cornea of eyes, conjunctiva, ciliary and iridial muscles, lining of nasal chambers, membranous labyrinth, epidermis, cutaneous glands, hair, nails, claws, hypothalamus, pineal gland, neurohypophysis, adrenal medulla, salivary glands, stomodaeum and proctodaeum and enamel of teeth.

Mesoderm. Dermis of skin, connective tissue, muscles, notochord, skeleton, blood, heart, blood vessels, adrenal cortex, urino-genital system, lining of coelom, spleen and eyes (except lens, cornea and retina).

Endoderm. Mesodaeum, digestive glands (except salivary), liver, pancreas, middle ear, eustachian tubes, lining of urinary bladder, respiratory system, adenohypophysis, thymus, parathyroid and thyroid glands, lining of vagina and urethra, prostate.

Foetal Membranes

They are extra embryonic membranes that provide protection and nourishment to foetus. Foetal membranes are of four types –
chorion,
amnion,
allantois and yolk sac.
They are derived from trophoblast (A thin layer of cells that helps a developing embryo attach to the wall of the uterus, protects the embryo, and forms a part of the placenta. During pregnancy, normal trophoblast cells of the placenta secrete glycoprotein hormone: human chorionic gonadotropin (hCG). The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of the fetus. The appearance of a fluid-filled inner cavity marks the transition from morula to blastocyst and is accompanied by cellular differentiation: the surface cells become the trophoblast (The trophoblast provides nourishment to the growing embryo and forms chorion and amnion and placenta) and the inner cell mass gives rise to the embryo. Three layers of the inner cell mass are the endoderm, the ectoderm, and the mesoderm. Cells in each germ layer differentiate into tissues and embryonic organs. The ectoderm gives rise to the nervous system and the epidermis, among other tissues. The mesoderm gives rise to the muscle cells and connective tissue in the body.)
Chorion- Outer foetal membrane that also takes part in formation of placenta.
Amnion- Inner foetal membrane that invests the embryo and forms a space called amniotic cavity. It is filled with fluid called amniotic fluid (useful for studying chromosomal abnormalities of foetus as well as sex determination). Amnion protects the foetus from shocks.
Allantois. Sac-like, develops from gut of embryo, supplies blood vessels to placenta. In reptiles, birds and egg laying mammals, it helps in respiration, nutrition and excretion.
Yolk Sac. Membranous sac attached to embryo near allantois, having yolk in egg laying animals and forms corpuscles in mammals till liver takes over.
placenta
Placenta is a structure produced by fusion of uterine endometrium with extra embryonic foetal membranes like chorion, for physiological exchange between foetus and mother's blood. It has two parts, maternal and foetal.
Placenta is classified variously.
(a) Nature of Foetal Membranes. According to nature of foetal membranes involved, placenta is of three types
(i) Chorio-Vitelline Placenta (Yolk Sac Placenta). It is formed from yolk sac and chorion, e.g., marsupials like Kangaroo and Opossum.
(ii) Chorio-Allantoic Placenta. It is derived from both allantois and-chorion e.g., marsupial Bandicoot and all eutheria except humans.
(aa) Chorionic Placenta. It consists of chorion alone, e.g., humans.
(bb) Nature of Maternal and Foetal Tissues. Five types
(i) Epithelio-Chorial Placenta. All the six barriers between foetal and maternal bloods are present e.g., marsupials, Horse, Ass.
(ii) Syndesmo-Chorial Placenta. Only five barriers exist between foetal and maternal bloods due to breakdown of uterine epithelium, e.g., Cow, Buffalo, Sheep, Goat, Camel, Giraffe.
(iii) Endothelio. Chorial Placenta. Four barriers exist between foetal and maternal bloods due to erosion of uterine connective tissue and uterine epithelium, e.g., Tiger, Lion, Cat, Dog.
(iv) Haemo-Chorial Placenta. Three barriers exist between foetal and maternal bloods due to erosion of all three maternal barricrs (uterine epithelium, uterine connective tissue, endothelium of uterine capillaries), e.g., Apes, Lemurs, Humans.
(v) Haemo-Endothelial Placenta. There is only one barrier of endothelium of foetal capillaries between foetal blood and maternal blood due to breakdown of five out of the six barriers, e.g., Rat, Rabbit, Pig.
(c) Nature of Uterine Wall at Birth. Three types
(i) Nondeciduate Placenta. No part of uterine tissue breaks, e.g., Horse, Ass.
(ii) Deciduate Placenta. A part of uterine tissue called decidua is torn and passed out at parturition, e.g., many mammals.
(iii) Contra-Deciduate Placenta. Even the foetal part of placenta is retained in uterus and absorbed there, e.g., Talpa, Parameles.
(d) Nature of Arrangement of Villi. Six types
(i) Diffuse Placenta. The villi are distributed uniformly along the entire surface of chorio-allantois, e.g., Horse. Pig.
(ii) Cotyledonary Placenta. Villi occur in tufts and received in uterine wall in specially thickened areas (carunc1es), e.g., Cow, Buffalo, Sheep, Goat.
(iii) Intermediate Placenta. Villi occur both singly and in tufts, e.g., Camel, Giraffe.
(iv) Zonary Placenta. Villi occur in one or two transverse bands, e.g., Tiger, Lion, Cat, Dog, Elephant.
(v) Discoidal Placenta. Villi occur on a small disc-shaped area, e.g., Rat, Rabbit, Bat.
(vi) Metadiscoidal Placenta. Initially villi occur all over but later remain over a small disc-shaped area embedded in uterine wall, e.g., Apes,-Humans.
Derivatives of Ectoderm:
(1) Epidermis of skin, hair, arrector pili muscles, nails, sudoriferous (sweat) and sebaceous (oil) glands and chromatophores (pigment cells) of skin.
(2) Enamel of teeth, salivary glands, mucous membrane of lips, cheeks, gums, part of the floor of the mouth and part of palate, nasal cavities and paranasal sinuses. Lower part of anal canal.
(3) Nervous system including all neurons, neuroglia (except microglia), and Schwann cells. Piamater and arachnoid mater.
(4) Conjunctiva, cornea, lens of eye, muscles of iris, vitreous humour, retina, lacrimal gland.
(5) External ear, outer layer of tympanic membrane, membranous labyrinth (internal ear).
(6) Pituitary gland, pineal gland and medulla of adrenal glands.
(7) Mammary glands, outer surface of labia minora and whole of labia majora.
(8) Terminal part of male urethra.
Derivatives of Mesoderm:
(1) Muscles except iris muscles.
(2) Connective tissues including loose areolar tissue, ligaments, tendons and the dermis of skin.
(3) Specialised connective tissues like adipose tissue, reticular tissue, cartilage and bone.
(4) Dentine of teeth.
(5) Heart, all blood vessels, lymphatics, blood cells, spleen.
(6) Kidneys, ureters, trigone of urinary bladder.
(7) Coelomic epithelium (mesothelium of pleural, pericardial and peritoneal cavities).
(8) Duramater, microglia.
(9) Sclera, choroid, ciliary body and iris.
(10) Basis of tympanic membrane.
(11) Cortex of adrenal glands.
(12) Mesenteries
(13) Notochord.
(14) Reproductive system except prostate (derived from the endoderm).
Derivatives of Endoderm:
(1) Epithelium of mouth, part of palate, tongue, tonsils, pharynx, oesophagus, stomach, small and large intestines including upper part of anal canal (not lower part of anal canal).
(2) Epithelium of Eustachian tube, middle ear, inner layer of tympanic membrane.
(3) Epithelium of larynx, trachea, bronchi and lungs.
(4) Epithelium of gall bladder, liver, pancreas including islets of Langerhans, gastric and intestinal glands.
(5) Epithelium of urinary bladder except trigone.
(6) Epithelium of lower part of vagina, vestibule and inner surface of labia minora.
(7) Epithelium of prostate (except inner glandular zone), bulbourethral glands, greater vestibular and lesser vestibular glands.
(8) Epithelium of thyroid, parathyroid and thymus glands.) The duration of pregnancy in dogs, elephants and cats is 63(58-68), 624(18 - 22 months) and 63 (58-67) days respectively. Placenta plays an important role in pregnancy.
Placenta:
Placenta is the intimate connection between the foetus and uterine wall of the mother to exchange the materials. The outer surface of the chorion in humans develops a number of finger like projections, known as chorionic villi, which grow into the tissue of the uterus.
These villi, penetrate the tissues of the uterine wall in which they are embedded, make up the organ known as the placenta by means of which the developing embryo obtains nutri¬ents and oxygen and gets rid of carbon dioxide and metabolic wastes.
Because the chorion takes part in the formation of placenta, the human placenta is called the chorionic placenta. It consists of foetal part, the chorion and a ma¬ternal part the decidua basalis.
The foetal part of placenta grows to invade the uterine mucosa with its chorionic villi. The degree of inti¬macy is so strong that the blood vessels of the chorionic villi are bathed in the mother’s blood. This is due to erosion of the uterine mucosa, including its epithelium, connective tissue and the endothelial lining.
This type of placenta which is based on the intimacy between foetal and maternal parts of the placenta, is referred to as haemochorial placenta. The placenta is connected to embryo through an umbilical cord which helps in the transport of substances to and from the embryo. On the basis of the distribution of villi on chorion, human placenta is called metadiscoidal placenta.
The placenta performs the following functions:
(i) Nutrition. All the nutritive elements from the maternal blood pass into the foetus through the placenta,
(ii) Respiration. Oxygen passes from the maternal blood to the foetal blood through the placenta, and carbon dioxide passes in the reverse direction,
(iii) Excretion. The foetal excretory products diffuse into the maternal blood through placenta and are excreted by the mother,
(iv) Storage. Placenta stores glycogen, fat, etc.
(v) As a Barrier. Placenta serves as an efficient barrier and allows those materials to pass into the foetal blood that are necessary. Teratogens are certain agents (viruses or chemicals) or drugs that cause abnormal development in developing embryo/ foetus. The most well known synthetic teratogen drug is thalidomide. This drug causes multiple defects in the growing embryo,
(vi) Endocrine Function. Placenta secretes some hormones such as oestrogens, progesterone, human chorionic gonadotropin (hCG), human chorionic somatomammotropin— hCS (it was formerly known as human placental lactogen hPL), chorionic thyrotropin, chorionic corticotropin and relaxin.
The hCG stimulates and maintains the corpus luteum to secrete progesterone until the end of pregnancy. The hCS stimulates the growth of the mammary glands during pregnancy.
Relaxin facilitates parturition (act of birth) by softening the connective tissue of the pubic symphysis.
In addition, the levels of hormones like oestrogens, progestogens, cortisol, prolactin, thyroxine, etc. are increased in the maternal blood during pregnancy. Increased production of these hormones is necessary for supporting the foetal growth, metabolic changes in the mother and maintenance of pregnancy.
Important Developmental Changes in the Human Embryo: Time from Fertilization Organs Formed
Week 1 Fertilisation cleavage starts about 24 hours after fertilization. Cleavage to form a blastocyst 4—5 days after fertilisation. More than 100 cells. Implantation 6-9 days after fertilisation.
Week 2 The three primary germ layers (ectoderm, endoderm and mesoderm) develop.
Week 3 Woman will not have a period. This may be the first sign that she is pregnant. Beginnings of the backbone. Neural tube deve¬lops, the beginning of the brain and spinal cord (first oragans).
Week 4 Heart, blood vessels, blood and gut start forming. Umbilical cord developing.
Week 5 Brain developing, ‘Limb buds’, small swellings which are the beginnings of the arms and legs. Heart is a large tube and starts to beat, pumping blood. This can be seen on an ultrasound scan.
Week 6 Eyes and ears start to form.
Week 7 All major internal organs developing. Face forming. Eyes have some colour. Mouth and tongue develop. Beginnings of hands and feet.
By week 12 Foetus fully formed, with all organs, muscles, bones, toes and fingers. Sex organs well developed. Foetus is moving.
By Week 20 Hair beginning to grow, including eyebrows and eyelashes. Fingerprints developed. Fingernails and toenails growing. Firm hand grip. Between 16 and 20 weeks baby usually felt moving for first time.
Week 24 Eyelids open. Legal limit for abortion in most circumstances.
By week 26 Has a good chance of survival if born prematurely.
By week 28 Baby moving vigorously. Responds to touch and loud noises. Swallowing amniotic fluid and urinating.
By week 30 Usually lying head down ready for birth. 40 weeks (9 months ± 7 days) Birth
Parturition and Lactation:
Parturition (L. Parturio = to be in labour): Meaning: The duration of pregnancy in human beings is about 9 months ± 7 days which is called gestation period. Infact, the gestation period is the time from conception to birth. At the end of the pregnancy vigorous contraction of uterus causes delivery or expulsion of the foetus. This act of expelling the full term young one from the mother’s uterus at the end of gestation period is called parturition.
Process:
Process of parturition is induced by both nervous system and hormones se¬creted by the endocrine glands of the mother. The signals for child birth (parturition) originate from the fully matured foetus and placenta which induce mild uterine contractions called foetal ejection reflex. This causes quick release of oxytocin from the maternal posterior lobe of pituitary gland. The amount of oxytocin is increased just before and during “labour pains” (pains of child birth). Childbirth begins with a long series of involuntary contractions of the uterus experienced as labour pains. Oxytocin (birth hormone) promotes contraction of the uterine muscles. Relaxin increases the flexibility of the pubis symphysis and ligaments of the sacroiliac and sacrococcygeal joints and helps dilate the uterine cervix during labour pains. Both of these actions give relief to the body from the pain during delivery of the baby.
The hormone most recently found to be produced by the placenta is corticotro- pin-releasing hormone (CRH), which in nonpregnant women is secreted only by neuro¬secretory cells in the hypothalamus. CRH is now thought to be part of the “clock” that establishes the timing of birth. Secretion of CRH by placenta increases enormously toward the end of pregnancy. Women who have higher levels of CRH earlier in pregnancy are more likely to deliver prematurely, whereas those who have low levels are more likely to deliver after their due date.
Stages: Labour pains can be divided into three stages:
(i) Stage of dilation: The time from the onset of labour pain to the complete dilation of the cervix is called the stage of dilation. This stage lasts 6-12 hours. During this stage, regular contractions of the uterus, usually rupturing of the amniotic sac and complete dilation of the cervix occur. The first result of labour pains is the opening of the cervix. The amniotic fluid (the “waters”) starts flowing out through the vagina.
(ii) Stage of expulsion: The time from complete cervical dilation to delivery of the baby is the stage of expulsion. It lasts 10 minutes to several hours. The baby passes through the cervix and vagina and is ‘delivered’ or born.
(iii) Placental Stage: The time after the delivery until the placenta or “afterbirth” is expelled by powerful uterine contraction is the placental stage. These contractions also constrict blood vessels that were torn during delivery thereby reducing the possibility of haemorrhage.
In about 28-35 days, the uterus returns fully to its non-pregnant state by reduction in size and restoration of endometrium of the uterus.
Lactation:
Meaning:
Production of milk in the mammary glands is called lactation.
Period:
The female’s mammary glands undergo differentiation during pregnancy and start producing milk towards the end of pregnancy and after the birth of the young one.
Role of Hormones:
At puberty in females mammary glands begin to develop under the influence of oestrogen and progesterone. Secretion and storage of milk generally begins after birth of young one, usually within 24 hours under the influence of hormone prolactin (PRL) secreted by anterior lobe of the pituitary gland. However, the ejection of milk is stimulated by the hormone oxytocin (ОТ) released from the posterior lobe of the pituitary gland.
Colostrum:
The first milk which comes from the mammary glands of the mother just after child birth, for 2 or 3 days is called the colostrum. This is yellowish fluid that contains Cells from the alveoli and rich in protein (lactalbumin and lactoprotein) but low in fat. Colostrum contains antibodies (IgA is the major immunoglobin in it) that provide passive immunity to the new born infant.
Composition of Milk:
Human milk consists of water and organic and inorganic sub¬stances. Its main constituents are fat (fat droplets), casein (milk protein), lactose (milk sugar), mineral salts (sodium, calcium, potassium, phosphorous, etc.) and vitamins. Milk is poor in iron content. Vitamin С is present in very small quantity in milk. The process of milk secretion is regulated by the nervous system. It is also influenced by the psychic state of the mother. The process of milk production is also influenced by hormones of the pituitary gland (already mentioned), the ovaries and other endocrine glands. A nursing woman secretes of 1 to 2 litres of milk per day.
Importance of Breast feeding:
Breast feeding during initial stage of infant growth is recommended by doctors for the healthy baby. Milk contains an inhibitory peptide. If the mammary glands are not fully emptied, the peptide accumulates and inhibits milk production. Breast feeding is also a means of birth control, but it is not reliable.
Developmental Disorders:
1. Amnionitis (amnion + Gr suffix – ids – inflammation): Inflammation of amnion, usually resulting from premature rupture of the amnion and often associated with neonatal infection.
2. Abortion: It is giving birth to an embryo or foetus prior to the stage of viability at about 20 weeks of gestation (foetus weighs less than 500 gm). It may occur from natural causes or induced.
3. Teratogeny (Gr. terato = monster-abnormally misshaped animal or plant or person or thing, suffix gen = producing): Production of malformed infant is called teratogeny. It is due to use of drugs or other agents such as tobacco and alcohol by pregnant mother. These teratogens cause abnormal development.
Embryo Formation in Human Beings
1. Morula Formulation. Soon after fertilization, the zygote begins cleavage or segmentation. Cleavage consists of early mitotic divisions of fertilized egg without involving growth of daughter cells. There is rapid synthesis of new DNA and increased oxygen consumption. Surface-volume and nucleo-cytoplasmic ratios increase. The cells formed after cleavage are called blastomeres. Cleavage is simple and holoblastic (division of whole egg) in human beings as there is little yolk. The first cleavage is along animal-vegetal axis or primary axis. It is slow and is completed within 30 hours of fertilization. One of the two blastomeres is, however, slightly larger. Hence, the first cleavage is holoblastic and slightly unequal. Second cleavage is at right angles to the first one. It takes about 20 - 30 hours and is completed slightly earlier in the larger blastomere so that a transitional 3-celled stage appears. Subsequent divisions are rapid (third taking only 12 hours) and occur in different planes. They produce a solid ball of blastomeres called morula. Formation of morula is also called phase of compaction. Morula has almost the same size as that of fertilized egg due to presence of zona pellucida. Morula has 16 - 32 cells. The cells are compacted and of two types, outer slightly smaller peripheral cells with tight junctions than the inner mass of cells with gap junctions. During cleavage the young embryo descends in the fallopian tube and reaches uterus. It takes 4 - 6 days. Corona radiata dissolves away during this period.
2. Blastulation. In uterus, the outer cells of morula absorb nourishment being secreted by endometrium and begin growth while covered by zona pellucida. Consequently the outer cells enlarge, flatten and form trophoblast or tropho-ectoderm. Trophoblast pours a fluid towards interior producing a cavity called blastocoel or blastocyst cavity. The embryo is now called blastocyst. It is equivalent to blastula of other animals. The size of blastocyst is roughly three times the size of morula from 0.1 mm. to 0.3 mm.
Trophoblast then separates from inner cells except at one point called embryonic pole. The inner cells now occur at one side and called inner cell mass or embryonal knob as the latter is to form the body of embryo.
Trophoblast cells in contact with inner mass are called cells of Rauber. Embryonic pole is also called animal pole. The opposite end of blastocyst is called abembryonic pole.
Blastocyst stage is completed after about 5 days of fertilisation. Trophoblast later becomes two-layered, outer syncytiotrophoblast and inner cytotrophoblast.
It secretes hEG (human chorionic gonadotrophin), forms villi for implantation and later on produces chorion, amnion and foetal part of placenta. Implantation. It begins in blastocyst stage after about a week of fertilization. Zona pellucida dissolves. The exposed trophoblast or tropho-ectoderm comes in contact with endometrial lining in the region of embryonic pole. Trophoblast cells secrete lytic enzymes, cause breakdown of some endometrial cells, absorb nourishment and divide themselves to form villi.
Villi penetrate endometrium for fixation and absorption of nourishment. The attachment of young embryo or blastocyst to endometrium of uterus is called implantation. Implantation requires 3-4 days. It is completed by 9 -10th day after fertilisation. Implantation brings about changes in endometrium or mucosal lining which is now called decidua. Decidua differentiates into three - decidua parietalis, decidua basalis and decidua capsularis.
Blastocyst/Blastodermic Vesicle. It is name of blastula. Blastocyst has three parts - trophoblast, inner cell mass and blastocoel. Trophoblast is the outer cellular wall of blastocyst that forms hCG, villi, ehorion, amnion and foetal part of placenta. Inner cell mass forms embryo. Blastocoel or segmentation cavity is fluid filled space which helps in rapid expansion of blastocyst or blastodermic vesicle.
3. Gastrulation. The stage is characterised by cell movements or morphogenetic movements that establish the germinal layers and initiate morphogenesis.
(i) Formation of Endoderm. Cells of inner cell mass in contact with blastocoel flatten, divide and grow to form a complete layer around blastocoel. The layer is called endoderm. Endoderm is the first germinal layer that appears in human embryo. It forms an endodermal tube/archenteron/primitive gut. The cavity enclosed by it is now called gastrocoel. Endoderm in contact with embryonal knob is embryonic endoderm which forms gut tract of embryo. The remaining endodermal tube forms yolk sac that encloses a fluid.
(ii) Embryonic Disc. After formation of endoderm, the remaining cells of inner cells mass undergo regular arrangement and called embryonic disc. It is also called epiblast. It forms both mesoderm and ectoderm. The rest of blastocyst is extraembryonic. It docs not form any part of embryo but is required for its survival.
(iii) Formation of Ectoderm. The embryonic disc grows to produce a complete layer below the trophoblast. It is ectoderm.
(iv) Amniotic Cavity. A space develops between ectoderm and trophoblast. Trophoblast adds special cells to its roof. They are called amniogenic cells. The cavity is called the amniotic cavity. It is filled with fluid called amniotic fluid.
(v) Formation of Extra-embryonic Mesoderm. Cells from trophoblast proliferate between endoderm and trophoblast at the abembryonic pole and beween trophoblast and amnion at the embryonic or animal pole. They form extra-embryonic mesoderm.
(vi) Formation of Intraembryonic Mesoderm. Cells of embryonic ectoderm proliferate at one end (future posterior end of embryo) to form a ridge projecting into amniotic cavity. It is called primitive streak. It is initially oval or rounded but later becomes linear. Cells proliferating from primitive streak enter the space between ectoderm and endoderm and spread to all parts of embryonic disc. It is intra embryonic mesoderm.
Lateral plate mesoderm is a type of mesoderm that is found at the periphery of the embryo

4. Neurulation and Organogenesis. Ectoderm develops a fold and forms a neural plate. It is the primordium of nervous system. The process of formation of rudiments of nervous system is called neurulation. Soon rudiments of other organs begin to appear. It ushers in the phase of organogenesis. Various organs develop and become functional.

5. Foetal Membranes.
Four types of extra embryonic foetal membranes develop-amnion, chorion, allantois and yolk sac.
Chorion is the outer foetal membrane which give rise to villi, hence chorionic villi. In mammals the placenta is chorioallantoic being formed by both chorion and allantois. In human beings it is largly chorionic.
Amnion is the inner foetal membrane that invests the embryo. It produces an amniotic cavity filled with amniotic fluid. A fluid medium is provided to growing embryo as safety measure against desiccation and shocks.
Allantois is a sac like membrane that develops in the area of foetal gut. It produces blood vessels for placenta. Yolk sac is a remnant of an active structure present in non-mammals. It develops near the allantois. It is believed to form corpuscles till the liver of foetus becomes functional. Afterwards, it gradually shrinks and degenerates.

6. Placenta. Foetus is attached to uterine wall by placenta. It is disc-shaped having a number of finger-like chorionic villi. The villi have rich supply of blood capillaries. Uterine tissue around these villi have a number of blood sinuses so that the two bloods are very near to each other for exchange of materials. Such a placenta is called haemochorial placenta. Placenta is connected to foetus by a rope-like umbilical cord. It has two arteries (with deoxygenated blood) and one vein (with oxygenated blood). Placenta acts as a barrier as well as ultrafilter between foetus and mother. Inorganic and organic nutrients, hormones and antibodies against various toxins and pathogens and oxygen pass from mother to foetus. CO2, nitrogenous and other wastes of foetus pass back into mother for elimination. Metabolic activity of placenta is almost equal to that of foetus. It produces a number of hormones -- hCG (human chorionic gonadotropin), chorionic thyrotropin, chorionic somatomammotropin, oestrogen and progesterone. hCG keeps the corpus luteum active. The latter secretes progesterone and relaxin. Relaxin softens pubic symphysis and allows uterus to expand so as to accomodate developing foetus.
7. Foetus. First trimester or three months of pregnancy involves division, migration and differentiation of cells to form various basic structures of embryo. Organogenesis occurs in the third month when the term foetus is employed for the developing baby. At this time the barrier between foetus and mother is weak so that viral infection of mother (e.g., Rubella or German Measles), several antibiotics and toxins can enter foetus and bring about its malformation. The agents which cause malformation of foetus are called teratogens or monster forming.
After 3 months of pregnancy development of foetus mainly involves growth and minor modifications in various organs and systems. Teratogen effect is little. Hormones help in the development of various structures.
Progesterone is called pregnancy hormone as it helps maintain pregnancy. Hormones also bring about labour or parturition after the completion of gestation period. Head of foetus comes to apply against cervix.
Cervix begins to open. It is the first step of labour. Uterine contractions become more powerful. Amnion ruptures and amniotic fluid or 'waters' flows out through vagina. Cervix and vaginal orifice expand further. The baby comes out.
Umbilical cord is cut.
Infant shows a major switch over in its respiratory and circulatory system. It is mediated by gaseous hormone nitric oxide (NO). Lungs expand and begin breathing. Blood flow through foramen ovale, ductus arteriosus and umbilical cord stops. Instead, it begins to flow through heart, aorta and pulmonary arteries. After birth of the baby, placenta and remains of umbilical cord are expelled in "after birth".
The first milk after the birth of baby is called colostrum. It is rich in proteins, calories and antibodies. The antibodies provide passive immunity to the neonate. Milk synthesis is under control of prolactin (PRL) while its release is controlled by oxytocin of pituitary. Milk possesses self inhibitory peptide which will inhibit milk production if breasts are not emptied properly. It is an autocrine action where supply matches demand. NCERT > Knowledge Bank-

Glands - Types

They are specialised epithelial cells which synthesise intracellular macromolecules (protein in pancreas, lipids in adrenal glands, glycoprotein in salivary glands and all the three in mammary glands) and pour out the same in the form of a useful fluid secretion which is different from blood or any other extracellular fluid. Glands can be unicellular or multicellular.

Unicellular Glands. Single celled, e.g., goblet (mucous) cells producing protein-polys1:ccharide complex or mucus.

Multicellular Glands. Several cells compose the gland. A multicellular exocrine gland has a duct and a secretory portion. Duct is unbranched in simple glands. They are further differentiated into unbranched and branched depending upon the condition of secretory part. Both the duct and secretory part are branched in compound glands.

Further differentiation is made on the basis of shape of secretory part.
(a) Simple Tubular, e.g., intestinal glands.
(b) Simple Coiled Tubular, e.g., sweat gland, glands of seminal vesicle.
(c) Simple Saccular (acinar if flask shaped and alveolar if rounded) e.g., cutaneous gland of Frog.
(d) Simple Branched Tubular, e.g., some gastric glands
(e) Simple Branched Saccular e.g., sebaceous gland.
(f) Compound Tubular e.g., Brunner's glands.
(g) Compound Saccular, e.g., sublingual and submandibular salivary glands.
(h) Compound Tubulo-Saccular, e.g., pancreas, mammary glands, parotid salivary gland.

Exocrine Glands. They are glands which pour their secretion either directly over the substrate or through a duct, e.g., goblet cells, salivary glands, tear glands, gastric glands, intestinal glands.
Endocrine Glands. They are also called ductless glands. The glandular secretion is poured into blood or lymph for reaching the target region. The secretion is called hormone, e.g., pituitary gland, thyroid gland, parathyroid glands, adrenal glands.
Heterocrine Glands. Both exocrine and endocrine, e.g., pancreas.
Merocrine/Eccrine/Epicrine Glands. Secretion is discharged through diffusion, e.g., goblet cells, sweat glands.
Apocrine Glands. Glandular secretion accumulates in the terminal part of the cell which is pinched off, e.g., mammary glands.
Holocrine Glands. The cell filled with secretory product disintegrates during discharge of the product, e.g., sebaceous gland.
Gonadotropic14
The Gonadotropic Hormones : LH and FSH The Gonadotropic hormones or the Gonadotropins are a group of hormones that are released from the anterior pituitary gland in vertebrates. It includes two primary hormones namely, Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) that act on the gonads (the ovaries and testes) of the mammals and help in sexual development and the process of reproduction. Apart from these two anterior pituitary hormones, the gonadotropins also include human chorionic gonadotropin (HCG) that is released during the time of pregnancy from the placenta. The hormone family also includes equine chorionic gonadotropin (released in female horses during pregnancy) and two of fish gonadotropins as well. Gonadotropic cells The gonadotropic cells also known as gonadotrophs are the cells that secrete the gonadotropic hormones. The gonadotrophs are endocrine cells situated in the anterior pituitary gland and are stimulated by the gonadotropin releasing hormones (GnRH). The gonadotrophs have insulin receptors on their surface; high levels of insulin can disrupt the hormone releasing levels and lead to infertility. The cells are also inhibited by the hormone estradiol. Now that we know about the gonadotropic hormones and the cells that release them. Let us study their primary hormones in detail below. Luteinizing Hormone Luteinizing hormone (LH) or interstitial cell stimulating hormone (ICSH) is one the gonadotropic hormones that is released by gonadotropic cells of the anterior pituitary gland. It is a heterodimeric glycoprotein hormone that has one alpha and one beta subunit to make it a fully functional protein. Structure The alpha and the beta subunits are connected by a non covalent bond. The alpha subunit is made up of 92 amino acids in humans and upto 96 amino acids in other vertebrates. These hormones are not found in invertebrates. The beta subunit is made up of upto 120 amino acids and constitutes a specific sequence that stimulates the LH receptor. The LH works in conjunction with FSH to stimulate various sexual developments in both males and females. Let us look at those functions now. LH in Males The luteinizing hormones stimulate the release of androgens in both males and females. In males, they help in the production of testosterone from the Leydig cells under the regulation of gonadotropin releasing hormone. The luteinizing hormones bind to the LH receptor on the surface of leydig cells, increasing the levels of cAMP molecules. This increase leads to the translocation of cholesterol into mitochondria where it is first converted into dehydroepiandrosterone (DHEA) and then finally into testosterone. The secretion of the hormone testosterone is pulsatile in nature. The LH levels are low at the time of puberty but increase with great surge at the time of puberty. LH in females The luteinizing hormones bind to the receptors present on the ovarian follicles and promote their growth. During the menstrual cycle, there is a surge in the secretion of the LH hormones which is important for the rupture of the Graffian follicle. LH also stimulates the secretion of progesterone in females which helps in transforming the Graffian follicle into corpus luteum and thus completing the cycle of ovulation. LH is important in females as it helps in the secretion of endometrium walls in the uterus and also the implantation of fertilised eggs. Trends of the LH A surge in LH levels in the second week of menstrual cycle in females causes the release of egg from the ovary, indicating high chances of getting pregnant. Thus, in females with regular menstrual cycles the LH increases every month to complete the process of ovulation and also supports the secretion of progesterone. In older females who are menopausal, the levels of LH are high but the levels of progestrone and estrogen are low. The LH levels in males rises after puberty for sexual growth and development. It maintains a normal level throughout the male life after puberty. There are very low levels of LH in infants and children. It rises only after puberty. Follicle Stimulating Hormone Follicle stimulating hormone (FSH) is another gonadotropic hormone released by the anterior pituitary gland. It is required for the development and functioning of reproductive organs in both males and females. It is named so because of its ability to stimulate follicle development in females. Structure It is a heterodimeric glycoprotein that is composed of an alpha and a beta subunit. The FSH alpha subunit is similar to the LH alpha subunit. The beta subunit of FSH is composed of 111 amino acids with a specific sequence that interacts with the follicle stimulating hormone receptor (FSHR). FSH in Males In males, follicle stimulating hormone is responsible for the formation of gametes. It stimulates the division of primary spermatocytes to form secondary spermatocytes. It also induces the Sertoli cells (site of spermatogenesis) to release androgen binding proteins that help in the process of sperm production. FSH in females In females, the follicle stimulating hormone helps in the maturation of the ovarian follicles. The hormone binds to the receptors on ovarian follicles and acts as a major factor for their growth. The FSH surges in between the menstrual cycle and promotes ovulation. It also stimulates the production of oestrogen that helps in the maturation of the Graffian follicle. Similar to LH, the FSH surges in a cyclical manner in females to complete the process of ovulation. Trends of the FSH In men, the FSH levels stay constant after a sudden rise during puberty. This constant level is important for the development of spermatozoa. In women, the FSH level fluctuates according to their menstrual cycles. The level is the highest after the release of egg. In perimenopausal (transition from menstruation to menopause) women, the FSH is high one day and lower the next day. In other words, there is no fixed pattern for FSH levels in those women. Inhibin secreted by the Sertoli cells and Graffian follicle negatively inhibit the secretion of FSH. Human Chorionic Gonadotropin The human chorionic gonadotropin (hCG) is one of the earliest hormones produced by the trophoblast cells of the embryo. This hormone is used as a parameter to detect pregnancy in early stages. Structure Similar to LH and FSH, it is a heterodimeric glycoprotein that is composed of an alpha and a beta subunit. The alpha subunit is similar to the LH and FSH subunit and is made up of 92 amino acids. The beta subunit is composed of 145 amino acids. The two subunits together create a hydrophobic core but the outer amino acids are mostly hydrophilic. Function The hCG hormone binds with the LH receptor and helps in the maintenance of corpus luteum. It is the parameter for recognition of maternal pregnancy. The hCG hormone also acts as a link for the development of immune tolerance. Some evidence also suggests that the presence of hCG hormones leads to morning sickness in women during the time of pregnancy. Forms of hCG The hCG hormone exists in three forms – regular hCG, hyperglycosylated hCG and free beta subunit of hCG. The regular and beta hCG levels are used for the detection of pregnancy. The hyperglycosylated hCG is mainly secreted during the time of implantation. Stimulation of the Gonadotropic Hormones The gonadotropic hormones – follicle stimulating hormone and luteinizing hormone are stimulated by another hormone called Gonadotropin releasing hormones, abbreviated as GnRH. The GnRH is secreted in the hypothalamus by gonadotropin neurons. It belongs to the family of gonadotropin-releasing hormone family. Structurally, it is a decapeptide which can be identified by the following amino acid sequence – pyroGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 The GnRH is released in a pulsatile manner from the hypothalamus and reaches the pituitary gland via the portal bloodstream. Upon reaching the pituitary gland, the hormone activates its own receptor that is present on the surface of gonadotropic cells. This leads to the release of calcium and protein kinase C which promotes secretion of LH and FSH. The activity of GnRH is low during birth and in children. It increases once the child reaches puberty. At puberty, the pulsatile release of GnRH stimulates the production of LH and FSH. This mechanism is negatively inhibited by androgens and estrogens. Low pulses of GnRH are required for FSH secretion, whereas high pulses of GnRH are required for LH secretion. In males, the GnRH is released in pulses at a constant frequency. However in females the GnRH levels fluctuate according to the menstrual cycle. It is at the highest level just before ovulation. Related Diseases High Levels of FSH Higher levels of FSH is usually seen in women with menopause that accompanies hot flashes and vaginal itchiness and dryness. High levels of FSH during reproductive years can be indicative of – Absence of normal inhibition mechanism Premature menopause Klinefelter syndrome, Turner syndrome or Gonadal dysgenesis Castration Lupus Low Levels of FSH Diminished secretion of FSH can cause gonadal failure, known as hypogonadism. In males, there is no or decreased production of sperm whereas in females normal menstrual cycle is hindered. Low levels of FSH is also seen in other conditions such as – Polycystic Ovarian Syndrome Hypothalamic suppression Gonadotropin deficiency Kallmann syndrome High Levels of LH The levels of LH is normally high in menopausal women, but if it is abnormally high in the reproductive years, it can be indicative of – Congenital adrenal hyperplasia Klinefelter syndrome, Turner syndrome or Gonadal dysgenesis Testicular failure Polycystic Ovarian Syndrome Note: Children who undergo puberty at a rather early age show high levels of LH and FSH according to their age. Low Levels of LH Similar to FSH, low levels of LH can cause hypogonadism which causes failure in production of sperm in males and irregular menstruation (amenorrhea) in females. Other conditions that show low levels of FSH are – Eating disorder Pasqualini syndrome Kallmann syndrome Female athlete triad Others GnRH insensitivity: It is caused by a rare autosomal mutation that leads to inactivity of the gonadotropin releasing hormone receptor and thus sex hormones are not synthesised. LH Insensitivity: Also known as Leydig cell hypoplasia is a rare genetic autosomal disorder that occurs in males. It renders the body unresponsive to LH leading to infertility. FSH Insensitivity: Also known as ovarian follicle hypoplasia is a rare autosomal genetic disorder in females that makes the body insensitive to FSH and causes amenorrhea, reduced puberty or total infertility. 25 Apr 2023 (No of questions -3)

1. Explain the following terms

a. Fimbriae - The fimbriae tubae or the fimbriae of the uterine tube are small finger-like projections lying at the terminal of the fallopian tubes.
It is via these that the eggs move from ovaries to the uterus. The fimbriae are linked to the ovary. During the process of ovulation when an egg releases in the peritoneal cavity from the ovary, the fimbriae’s cilia sweep the ova inside the fallopian tube.

b. cervical canal - The cervical canal passes through the cervix. It allows blood from a menstrual period and a baby (fetus) to pass from the womb into the vagina. The sperm travel up the cervical canal, then through the uterine cavity into the fallopian tubes to fertilize the egg.

c. perimetrium - The perimetrium is the outer serous layer of the uterus. The serous layer secretes a lubricating fluid that helps to reduce friction. The perimetrium is also part of the peritoneum that covers some of the organs of the pelvis.

d. endometrium - The physiological functions of the uterine endometrium (uterine lining) are preparation for implantation, maintenance of pregnancy if implantation occurs, and menstruation in the absence of pregnancy.

e. interstitial cells or Leydig cells - Leydig cells are present in the interstitial spaces in the testicular lobules. They secrete androgens, primarily testosterone. Androgens stimulate the process of spermatogenesis, i.e. the formation of sperms, which is one of the main steps in reproduction.

f. testosterone - Testosterone is an androgen that are produced in the testes. It is responsible for the development of male accessory sex organs, muscular growth, and secondary sexual characters. It regulates spermatogenesis and libido in males. They also regulate bone mass, fat distribution, muscle mass, and strength.

g. Prolactin - Prolactin is a hormone made by the pituitary gland, a small gland at the base of the brain. Prolactin causes the breasts to grow and make milk during pregnancy and after birth. Prolactin levels are normally high for pregnant women and new mothers. Levels are normally low for nonpregnant women and for men.

What is oxytocin? Oxytocin is a natural hormone that manages key aspects of the female and male reproductive systems, including labor and delivery and lactation, as well as aspects of human behavior. Your hypothalamus makes oxytocin, but your posterior pituitary gland stores and releases it into your bloodstream.

2. Explain the following with suitable diagrams-

a. Spermatogenesis - Spermatogenesis is the process of the production of sperms from the immature germ cells in males. It takes place in seminiferous tubules present inside the testes. During spermatogenesis, a diploid spermatogonium (male germ cell) increases its size to form a diploid primary spermatocyte. This diploid primary spermatocyte undergoes first meiotic division (meiosis I), which is a reductional division to form two equal haploid secondary spermatocytes. Each secondary spermatocyte then undergoes second meiotic division (meiosis II) to form two equal haploid spermatids. Hence, a diploid spermatogonium produces four haploid spermatids. These spermatids are transformed into spermatozoa (sperm) by the process called spermiogenesis.

What are the 5 stages of spermatogenesis? The process of germ cell development during spermatogenesis can be divided into five succesive stages:
(1) spermatogonia,
(2) primary spermatocytes,
(3) secondary spermatocytes,
(4) spermatids, and
(5) spermatozoa.
b. Oogenesis - Oogenesis is the process of the formation of a mature ovum from the oogonia in females. It takes place in the ovaries. During oogenesis, a diploid oogonium or egg mother cell increases in size and gets transformed into a diploid primary oocyte. This diploid primary oocyte undergoes first meiotic division i.e., meiosis I or reductional division to form two unequal haploid cells. The smaller cell is known as the first polar body, while the larger cell is known as the secondary oocyte. This secondary oocyte undergoes second meiotic division i.e., meiosis II or equational division and gives rise to a second polar body and an ovum. Hence, in the process of oogenesis, a diploid oogonium produces a single haploid ovum while two or three polar bodies are produced.
There are three phases to oogenesis; namely, multiplication phase,
growth phase and
maturation phase.

3. Fill in the blanks -

a. The seminiferous tubules of the testis open into the ........... through rete testis. The vasa efferentia leave the testis and open into ........ located along the posterior surface of each testis.
b. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ...........
c. The urethra originates from the urinary bladder and extends through the penis to its external opening called ............
d. The male accessory glands include paired .............. , a .......... and paired ..........
e. The female reproductive system consists of a pair of ...... alongwith a pair of .........., .........., .........., ......... and the external genitalia located in pelvic region......

Answers

a. The seminiferous tubules of the testis open into the vasa efferentia through rete testis.
The vasa efferentia leave the testis and open into epididymis located along the posterior surface of each testis.
b. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder. It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct.
c. The urethra originates from the urinary bladder and extends through the penis to its external opening called urethral meatus.
d. The male accessory glands include paired seminal vesicles, a prostate and paired bulbourethral glands.
e. The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region.