The Reproductive System

- 233 - THE REPRODUCTIVE SYSTEM PRIMARY SEX ORGANS Gonads, which form gametes are called primary sex organs-testis (plural testes) in males and ovary (plural ovaries) in females. Testis produces sperms and secrets testosterone (formation and maintenance of secondary sex organs, accessory male glands and external sex characters). Ovary produces ova. Maturing Graffian follicles secrets follicles secrete estrogens for development and maintenance of secondary sex organs, accessory or external sex characters and part of menstrual cycle. Progesterone produced by ruptured Graffian follicles or corpus luteum controls a part of menstrual cycles, implantation and development of placenta. SECONDARY SEX ORGANS Sex organs, glands and ducts which do not produce gametes but are otherwise essential for sexual reproduction are known as secondary sex organs. In human male reproductive system, the secondary sex organ are vasa efferentia, epididymes, vasa deferentia, ejaculatory ducts, seminal vesicles, urethra, prostate gland, Cowper’s glands and penis. Secondary sex organs of a human female include fallopian tubes, uterus, vagina, external genitalia, Bartholin’s glands and mammary glands. ACCESSORY/EXTERNAL /SECONDARY SEX ORGANS They are traits, which do not have any direct role in reproduction but provide specific features and structures to the two sexes. The important external/accessory sex characters of human male are beard, moustaches, body hair on shoulder and chest, pubic hair on both lateral and vertical directions, comparatively more height with more muscular body, larynx apparent externally, voice low pitched with breathing ,more by means of diaphragm. The important accessory sex characters of human females are high pitched voice, breast, broader pelvis, lateral pubic hair, rounded body contours with more subcutaneous fat in thighs buttocks and face and sternal breathing. PUBERTY Beginning of sexual maturity or ability to reproduce is known as puberty. Primary sex organs begin functioning. Secondary sex organ develop fully under the influence of sex hormones produced by primary sex organs. Growth is rapid. It is accompanied by slow development of accessory/external sexual characters. Puberty occurs at the age of 10-14 years in girls and 13-15 years in boys. CHARACTERISTICS OF HUMAN REPRODUCTIONS (i) Human beings are non seasonal breeders. (ii) There is no oestrus/heat. (iii) In human females the ability to produce young ones begins at menarche (beginning of menses) and ends at menopause (stoppage of menses). (iv) In human females the reproductive phase has 28 day repeated menstrual cycle. (v) Fertilization is internal. (vi) There is vivipary, i.e, giving birth to young ones. (vii) Foetus develops inside uterus and is nourished by joint special structure called placenta. (viii) Infants can be fed on mother’s milk. (ix) Parental care is very well developed. MALE REPRODUCTIVE SYSTEM It is made of a pair of testes (primary sex organs), scrotum, vasa efferentia, a pair of epididymes, a pair of vasa deferentia, a pair of seminal vesicles, a pair of ejaculatory ducts, urethra, prostate gland, a pair of Cowper’s glands and penis. 1. Testes (singular testis). They are a pair of oval, pinkish primary sex organs of male reproductive system, each with a size of 5 cm (length), 3 cm (thickness), and 2.5 cm (width), weighing about 12 gm and lying obliquely in scrotum. It is suspended in its position by means of a spermatic cord. Each testis has three coverings – tunica vaginalis, tunica albuginea and tunica vasculosa. On one side each testis is covered by hollowed tunica vaginalis, a bilayer of peritoniam with a narrow coelomic cavity having coelomic fluid for sliding. The actual covering of testis is a dense, bluish white fibrous connective tissue sheath called tunica albuginea. Tunica albugenia also project inside testis to form a thick incomplete vertical column called mediastinum. And a number of transverse septa. The septa produce 200-300 conical testicular lobules each with a covering of vascular sheath of loose connective tissue called tunica vasculosa. A lobule is filled with connective tissue and 1-4 convoluted seminiferous tubules. A total of 900-1000 yellowish seminiferous tubules occur in each testis. Each tubule is about 70-80 cm long. In the connective tissue lying in between seminiferous tubules are present interstitial cells or Leydig cells (named after their discoverer a German anatomist Leydig 1821-1908) that contain yellow pigment granules. Leydig cells (Leydig 1850) are large polyhedral cells which have eccentric nucleus and small lipid containing vacuoles. They secrete testosterone and other androgens. Seminiferous tubules are closed at one end. They become straight or tubuli recti towards mediastinum. The tubuli recti enter a network of irregular but fine channels called rete testis. Besides a tunic of fibrous connective tissue, each seminiferous tubules has a well defind basal lamina or basement membrane and a complex germinal or seminiferous epithelium. It has two types of cells, spermetogenic (primary germ cells) and indifferent supporting or sustentacular cells. Indifferent cells give rise to Sertoli cells. Spermatogenic cells form 4-8 layers. The cells are distined to undergo spermatogenesis and for spermatozoa. Sertoli cells (named after Italian histologist, Enricho Sertoli, 1842- 1910) are large pyramidal or columnar polygonal cells with bases resting on basal lamina and apical ends extending into lumen of the seminiferous tubule. The lateral and terminal ends develop a number of processes for holding growing spermatids and spermatocytes. Nucleus is irregular. The cells have FSH receptors. FSH stimulates sertoli cells to secrete spermatogenic substance. Sertoli cells, therefore, function as nurse cells for differentiating spermatozoa. 2. 3. Scrotum. 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. Scrotal skin bears sebaceous lands that produce a characteristic odour, sweat glands and nerve endings. 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. Left testis lies a bit lower than the right one. 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 gubernaculums. 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 a 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°C) 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. 4. Vasa Efferentia. Rete testis is connected to caput epididymis by 12–20 fine tubules called vasa efferentia or ductuli efferents. Their lining epithelium is pseudostratified. It has large columnar ciliated cells and small nonciliated cells with endocytic activity. Ciliated cells help in conducting sperms. Tubuli recti, rete testis and ductuli efferentes constitute an intratesticular genital duct system. 5. Epididymes (singular – epididymis). They are two long (4–6m), highly coiled, 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 gubernaculums). Wall of epididymis is thin muscular (with peristaltic and segmental movements) with a lining of pseudostratified epithelium. Principal cells of pseudostratified epithelium possess stereocilia and secrete glycoprotein for nourishing and maturation of spermatozoa (Flickinger 1983). Spermatozoa gain motility. 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. 6. Vasa Deferentia (singular – vas deferens, sperm duct). A pair of thick walled and muscular tubes, each of 30 – 35 cm, developing from cauda epididymis, coming out of inguinal canal as consitituent 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) store, nourish and conduct sperms. 7. 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. 8. Urethra. It arises from urinary baldder and is about 20 cm long, differentiated into four parts – urinary, prostatic, membranous and penile. Urinary part carries only urine. It ends inside prostate gland where urethra receives the two ejaculatory ducts to form urinogenital duct. Prostatic part of urethral 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 penis. Epididymes, vasa deferentia and urethra transport sperms towards penile meatus or passage. They are collectively called excretory genital ducts. 9. Penis. It is male erectile copulatory organ, which also passes out urine. Penis has long shaft and expanded tip called glans penis. An oblique groove-like neck occurs below the glans. The shaft has three columns of spongy tissue – two dorsal corpora cavernosa and one ventral peri-urethral corpus spongiosum. The latter forms the broad sensitive tip or glans of penis. Glans can be covered by a loose retractile foreskin or prepuce (praeputium) which is often removed by circumcision. Sinuses present in spongy columns can get filled with blood to make the penis stiff for copulation. Dense connective tissue of the penis is tunica albuginea. 10. Seminal Vesicles. They are a pair of lobulated contorted musculoglandular sacs of 5 cm length (uncoiled 10 – 15cm), between urinary bladder and rectum. Each has a tube that joins vas deferens to from ejaculatory duct. Secretion of seminal vesicles forms the major part of semen (60 – 70%). It is thick, viscous, mucoid, alkaline, having proteins including fibrinogen, fructose (nourishment for activity of sperms), citrate, inositol and prostaglandins (for liquefying cervical mucus and stimulating movements in the female tract. 11. Prostate Gland. It is a large grayish to reddish 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 thin, milky and slightly alkaline. It contains calcium, phosphate, citrate, a clotting enzyme and profibrolysin. Prostrate secretion constitutes upto 20 – 30% of semen. It is essential for sperm motility (removal causes sterility). 12. Cowper’s/Bulbourethral Glands. A pair of small (1 cm diameter) yellow pea seed sized lobulated 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 neutralizes 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 and Cowper’s glands). It is meant for providing medium for movement of sperms. Semen has chemicals for nourishing the sperms (e.g., fructose), neutralizing the acidity of urethra and vagina (e.g., calcium bicarbonate), stimulating movements in female tract (e.g., prostaglandins). Action of clotting enzyme (of prostate) over fibrinogen (of seminal vesicles) changes the semen into a coagulum in vagina. However, after 15–30 minutes coagulum is dissolved as profibrinolysin (of prostate) is changed into firbinolysin. The sperms now become highly motile. FUNCTIONING OF MALE REPRODUCTIVE SYSTEM Male reproductive system becomes functional with the onset of puberty. External sex characters appear thereafter, under the influence of hormone testosterone produced by the Leydig cells of testes. Male reproductive system remain operational throughout the life. However, there is decrease in testosterone secretion in old age and male sexual function decreases. It is called male climacteric. The average age of male climacteric is 68 years (60 – 80 years). Sterility may also 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 to female. FEMALE REPRODUCTIVE SYSTEM It consists of a pair of ovaries (primary sex organs), a pair of oviducts or fallopian tubes, uterus, vagina, external genitalia, pair of accessory genital glands called 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 thickness) or female gonads, which lie in the upper lateral 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 for suspending it to dorsal body wall. Ovary has hilus where nerves and blood vessels pass into it. Ovary is solid structure which is internally differentiated into four parts – outer germinal epithelium of flattened (squamous) and cuboidal cells, a delicate sheath of connective tissue or tunica albuginea, a cortex of dense connective tissue with reticular fibres, spindle-shaped cells, ovarian follicles and a few blood vessels 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 follicles is called Graafian follicles . It has a diameter of `10mm. 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 place oocyte. Follicular cells form a cellular sheath (below theca interna) called membrane granulosa and cellular mass called cumulus ovaricus covering the oocyte. Cumulus oophorus or cumulus ovaricus differentiates into outer discus proligerous and inner corona radiata. A glycoprotein rich noncellular layer called zona pellucida develops around the oocyte by the joint activity of oocyte and follicular cells. Gap junctions occur between follicular cells and plasma membrane of oocyte for transfer of nutrients and other occur between follicular cells and plasma membrane of oocyte for transfer of nutrients and other biochemical’s. A perevitelline space appears at places betweenzona pellucida and oocyte membrane (or oolemma), Zona pellucida takes part in compatibility reaction with sperm head and brings about acrosome reaction. Oocyte (secondary oocyte) is 50 – 100 μm is diameter. It is alecithal. There are three coverings around it – inner zona pellucida, middle corona radiata and outer discus proligerons. Corona radiata has radially elongated cells. A polar body is found between oocyte 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 stimulates secretion of LH. The two cause the mature Graafian follicle to rise to the ovarian surface, form a protuberance or stigma 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 show proliferation of cells of membrane granulosa, deposition of yellow carotenoid 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 atretic follicles become active interstitial cells which secrete small amount of androgen. 2. Fallopian Tubes/Uterine Tubes/Oviducts. They are a pair of muscular and internally ciliated tubes of 10 – 12 cm length which lie horizontally over peritoneal cavity arising near the ovary and ending at uterus. A fallopian tube is differentiated into four parts: (a) Infundibulum. It is funnel shaped end of fallopian tube that lies in near contact with ovary. It has a pore called ostium. The funnel is called oviducal funnel. Its free end bears a number of finger like processes called fimbriae, One fumbria is longer than others and is attached to ovary. Fimbriae have longitudinal folds lined with cilia. Muscular movements of oviducts and cilia develop a current that directs the released ovum to pass into infundibulum. (b) Ampulla. A slightly swollen and curved part behind infundibulum where fertilization of ovum takes place. It has a large internal surface area due to presence of variously folded plicae or ridges. (c) Isthmus. It is narrow and straight part of fallopian tube. Plicae or ridges occur but are simpler. (d) Uterine Part. It is about 1 cm long part that passes into the uterine wall. Oviducal wall is composed of three layers – outer serosa of visceral peritoneum, middle muscularis and inner mucosa. Mucosa has an epithelium of ciliated and secretory columnar cells. Secretory cells produce a viscous secretion for nourishing and protection of ovum. Passage of ovum is facilitated by movements of cilia and muscular contractions of the wall. 3. 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 cm 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 2.5 cm long narrow inferior extremity of uterus which protrudes into vagina. It is connected to body by opening called internal os and vagina by external os. External os is circular but after normal child birth is becomes irregular bilipped. 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. 5. External Genitalia/Vulva (Pudendum). The area having external genitalia is characterized 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 symphysis bones. Vestibule has urinary meatus with urethral opening on the upper side and vaginal orifice on the lower side. A small (peashaped) crectile clitoris occurs above urethral aperture. It contains two corpora cavernosa. The tip or glans clitoridis is a small tubercle of again crectile 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 through 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) which form clitoris in front and are connected behind by fourchette. They also possess sebaceous glands. 6. Vestibular or Bartholin’s Glands. They are a pair of small tubuloaciner 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). A number of small lesser vestibular glands (glands of Skene) or paraurethral glands also occur in the area. 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 multipurpose rounded tip and pigmented sensitive muscular and sebaceous base called areola. Internally each breast has 15 – 25 lobulated tubuloalveolar milk 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 colostrums. FUNCTIONING OF FEMALE REPRODUCTIVE SYSTEM Female reproductive system becomes operational at puberty. It is characterized by menarche or on-set of menstrual cycle. The 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 gonadotropin hormones. Menstrual cycle is also regulated by differing titres 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 secrete 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 result 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 with a periodicity of 28 days, right from menarche to menopause except during period of pregnancy. A single ovum is released roughly in the middle of cycle by one of the two ovaries. Endometrial lining shows cyclic proliferation. Gonadotropin and ovarian hormones also show cyclic increase and decrease. The cycle is characterized 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 desquamation of endometrium and menstrual flow/menses which continues for 3-5 days. It involves discharge of blood (a total of 45-100 ml), serous fluid, 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. Blood clotting does not occur due to presence of fibrinolysin. Menstrual phase is also considered the first day of manstural cycle. 2. Post-Menstrual/Follicular Phase. It lasts for 10 –12 days. On stimulation by GnRH, anterior pituitary secretes FSH which stimulates 6-12 ovarian follicles to undergo enlargement. Glanulosa cells undergo proliferation, secrete an oestrogen rich fluid in antrum. Follicular cells are stimulated to grow and develop FSH receptors. Estrogen and FSH promote development of LH receptors over granulosa cells. However, only one ovarian follicle continues growth. The others degenerate or undergo atresia. The nonfunctional follicles are also called artretic follicles. The growing ovarian follicle ultimately forms Graafian follicle. (a) Recovery Phase. It lasts for 2 days and brings about repair of ruptured blood vessels and reepithelialisation of mucous lining or endometrium of reproductive tract. (b) Proliferative Phase. The endometrial 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. Mucus lined channels develop in the cervical canal. 3. Fertility Phase/Ovulation. Production of FSH as well as LH increases. There is higher secretion of follicular fluid and oestrogen. Higher levels of oesterogen and LH cause explosive growth of Graafian follicle. The mature Graafian follicle rises to the surface of ovary and ruptures to release ovum. The phenomenon is called ovulation. Estrogen level now begins to fall. The ovum is drawn into fallopian tube. It is viable for two days when fertilization an occur. Ovulation 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 the ovurm. 4. Pre-Menstruation/Luteal/Secretory Phase. It lasts for 12 – 14 days. The phase is characterised by proliferation of empty ovarian follicle to form a yellow mass called corpus luteum under the influence of LH. 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 stages 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 about two days prior to completion of sexual cycle. LH level falls. Progesterone level is reduced. Reduced level of both progesterone and esterogen causes menses. GRAVID PHAE It is the phase of pregnancy when the uterus contains the developing embryo or foetus. The period between fertilization 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, fertilization, 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. 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 (Absent in human egg). The phenomenon is called vitellogenesis. In cooperation with follicular cells, the enlarged primary oocyte secretes glycoprotein 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 fertilized ovum. It produces a small secondary polar body. The primary polar body does not divide further in humans and most vertebrates. Hormonal Control of Oogenesis. GnRH or gonadotrophin releasing hormone is produced from hypothalamus in response to low titre of progesterone and oestrogen. GnRH activates adenohypophysis to secrete FSH (follicle stimulating hormone) and LH (lutenising hormone). FSH stimulates ovarian follicle to grow. The growing ovarian or Graafian follicle secretes hormone oestrogen. Oestrogen inhibits GnRH and FSH. In the presence of oestrogen and LH, the mature Graafian follicle rises to surface of ovary and bursts open to release ovum. The ruptured Graafian follicle is then converted into progesterone secreting corpus luteum. This inhibits release of LH. Significance. (i) It produces haploid ovum or egg. (ii) Oogenesis is specialized to produce large sized ova in which a lot of reserve food can be stored for future development. (iii) A single ovum is formed from an oogonium. (iv) It produces a progesterone producing corpus luteum , which is meant for proliferation of endometrium of uterus for receiving a fertilized egg, if fertilization occurs. Spermatogenesis Spermatogenesis is the process of formation of haploid functional spermatozoa from diploid germinal cells of seminiferous tubules. It starts during abdolescence (13 – 14 years of age) and continues therefore throughout life but declining markedly in old age. Lining layer of seminiferous tubules possesses primary germ cells an indifferent sustentacular cells that mature into nurse cells or Sertoli cells. Sertoli cells grow in size and develop a number of processes for supporting and nourishing spermatocytes and spermatids. (i) Multiplication Phase. Diploid primary germ cells or spermatogonial cells are small cells about 12 μm in diameter. At sexual maturity, these cells undergo repeated mitosis to form a number diploid spermatogonia. Spermatogonia get differentiated into two types, type A and type B. Type A spermatogona function as stem cells and produce more spermatogonia. Type B spermatogonia divide mitotically forming four spermatogonial generation before undergoing 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 spermetocyte undergoes meiosis I to form two haploid secondary spermatocytes. All the secondary spermatocytes derived from a single spermatogonium remainattached to one another. Secondary spermatocytes devide by meiosis II, each giving rise to two haploid spermatids. The spermatids become partially embedded in Sertoli cells for nourishment and support. (v) Spermiogenesis (spermateleosis, spermioteleosis). Spermiogenesis is differentiation of a spermatozoon from a spermatid. Golgi apparatus forms acrosome. Centriole divides into two.distal centriole form 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 release into lumen of semeniferous tubule for onward passage. Release of spermatozoa is called spermiation. About 64 days are required for the development of spermatozoa from spermatogonia. An adult human male manufactures 109-1010 sperms each day (upto 120 million ; Guyton and Hall, 2000). The sperms arestored for upto a month in epididymes and vasa deferentia. Nutrients from epithelium of epididymes are essential for maturation of sperms. Significance. (i) Spermatogenesis produces million of sperms so that at release a few are able to reach the ovum. (ii) It produces motile male gametes. Motility is essential for reaching upto ovum. (iii) Spermatogenesis produces haploid male gametes. (iv) Each sperm is provided with acrosome to penetrate the egg covering. Sperm Human sperm is dart-like flagellate structure of 60 μm lengh and maximum breadth of 3.5 um. It has four parts – head, neck, middle piece and tail. (i) Head. Knob-like but flat terminal part, 4-5um long and 2.5-3.5um broad. It has two component, acrosome and nucleus. Cytoplasm is nearly absent. Acrosome forms a covering over the anterior two thirds of head. It is derived from Golgi apparatus of sparmatid. It contains sperm lysins like hyaluronidase and proteolytic enzymes (e.g., corona penetrating enzyme, acrosin or zona lysine). Suface contains compatibility protein bindin for attaching to ZP3 and ZP2 receptors of egg. It was formerly called antifertilizin. Nucleus is dense mass of chromatin having some protamines. These may be one or more less dense areas called nuclear vacuoles. On the outside is present a double membrance head cap. It processes decapacitation factors which are removed by secretions in the female genital 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 microtubular band called manchette develops when the two centrioles begin to separate during spermiogenesis. (iii) Middle piece (Body). It is cylindrical part, 5-7um long and 1um 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 cytoplysm. Head, neck, middle piece and a part of tail are all covered on the outside by plasma membrane or sheath. (iv) Tail. It is narrow vibratile long part about 50 μm in length, with two regions, main and end piece. Main piece of tail is 0.5μm 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. Hormonal control of spermatogenesis Testosterone is essential for spermatogenesis. It is secreted by interstitial cells or Leydig cells under the control of LH or ICSH of pituitary. Pituitary gland also produces another hormone called FSH (follicle stimulating hormone). The hormone stimulate Sertoli cells to convert spermatids into sperms (spermiogenesis). Spermatogenesis is under control of growth hormone. 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). In case of excess activity, sertoli cells secrete glycoprotein inhibin which suppresses formation of FSH. Ovum Human egg or ovum is noncleidoic (without shell) and alecithal (absence of yolk), rounded female gamete having a diameter of about 100μm. The ovum possesses three coverings–inner plasma membrane, middle glycoprotein zona pellucida and outer cellular corona radiata with radially elongated scattered cells held in mucopolysaccharide (hyaluronic acid). Zonna pellucida carries compatibility receptor proteins collectively called fertilizin. In between plasma membrane and zona pellucida is perivetelline space in which one polar body is present 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 time of fertilization. Prior to it, the same is generally in the stage of metaphase II.Ectoplasm possesses mucopolysaccharide granules and microtubules. Mucopolysaccharide granules or cortical granules (Gulyas, 1980) are extruded after fertilization to modify both plasma membrane and zona pellucida for preventing entry of another sperm (Wassarman, 1990). Endoplasm has mitochondria, Golgi apparatus, ribosomes, RNA, fat drolets, glycogen particles and proteins. EGG COVERINGS (i) primary membranes. Membranes secreted by ovum, e.g., vitelline membrane. A perivitelline space curs between plasma membrane and vitelline membrane. (ii) Secondary Membranes. Membranes formed over the ovum by ells 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 IN HUMANS It is fusion of male and female gametes to form zygote. In human beings fertilization 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 demobilized or eaten but a number of the remain functional and undergo capacitation. That provides them the ability to fertilize an ovum. Capacitation requires 1-10 hours (usually 5-6hrs). It consists of three processes. (i) Neutralisation of inhibitory factors present in semen (ii) Weakening of covering membrane of acrosome head by dissolution of cholesterol.(iii) Entry of Ca2+ into sperms which changes sperm movement from undulations to whiplash motion. The activated sperms begin to pass into uterus and from there to oviducts. Viscous liquid secreted by female genital tract further chances sperm motility . 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 . 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 stimulated by prostaglandins(in male semen) and oxytocin (often formed in females). The movements are powerful to send the sperms into fallopian tubes within 5 minutes. After reaching an ovum one sperm comes to lie against it. It releases lysins from its acrosomal region. Hyalouronidase and corona penetrating enzymes separate as well as dissolve cells of corona radiata. The sperm head now reaches zona pellucida where receptor protein fertilizing (ZP3, ZP2) helps in attachment to specific protein (bindin or antifertilizin) or sperm. It is compatibility reaction (Lillie, 1919. (ii) Acrosome Reaction. In contact with zona pellucida, acrosome covering degenerates. The contained enzymes are released. Acrosin or zona lysi dissolves zona pellucida in the area of contact. (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 established contact with lateral surface of fertilization cone. It produces a weak depolarization and Ca2+ wave. 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. Cortical granules are extruded. They convert plasma membrane of egg in to fertilization membrane and deactivate sperm reception of zona pellucida. A perivitelline space is created between it and zona pellucida. This prevents entry of second sperm. (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 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 nucleocyteoplasmic 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. 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 balstocoel , is called blastula. It is of the following types: GASTRULATION Sum total of all the processes which convert a solid or hollow ball of cells or many layered disc of blastula into 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 divisioin 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. Balstopore 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 morphogeneis. 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 except part of urinary bladder, 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. 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. Yolk Sae. Membranous sac attached to embryo near allantois. 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. EMBRYO FORMATION OF HUMAN BEINGS 1. Morula Formation. 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. Phase of compaction ensues in 8-celled stage. 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 slowly due to feeble fluid produced by epithelial secretion and cilia. Ultimately it reaches uterus. It takes 4–6 days. Corona radiata dissolves away during this period. Blastulation. In uterus, the endometrial cells become full of nutrients which are also secreted into uterine cavity. The same is called uterine milk. As the young embryo reaches uterus, its outer cells begin to absorb nourishment and grow 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 balstocyst. It is equivalent to blastula of other animals. The size of balstocyst is roughly three times the size of morula from 0.1 mm to 0.3 mm. Trophoblst 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. Trophoblst cells is 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 o fertilisation. Trophoblast latter becomes two-layered, outer syncytiotrophoblast and inner cytotrophoblast. It secretes hCG (human chorionic gonadotrophin), forms villi for implantation and later on produces chorion, aminon and foetal part of placenta. Implantation. It begins in blastocyst stage after about a week of fertilization. Blastocyst creates a slit inzona pellucida partly due to its growth and partly by secreting trypsin-like enzyme. It comes out of the slit inthe form of letter 8. Breaking during this coming out causes development of identical twins. The exposed rophoblast or tropho-ectoderm comes in contact with endometrial lining in the region of embryonic pole Trophoblast cells secrete lytic enzymes, causes 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 endometriurm of uterus is called implantation. Implantation requires 3 – 4 days. It is completed by 9 – 10th day after fertilization. Implantation brings about nutrient enrichment and further enlargement of cells of endometrium or mucosal lining which is now called decidua. Decidua differentiates into three – decidua parietalis, deciduas 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, chorion, amnion and foetal part of placenta. Inner cells mass forms embryo. Blastocoel or segmentation cavity is fluid filled space which helps in rapid expansion of blastocyst or balstodermic vesicle. Gastrulation. The stage is characterized 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, They 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 form 2. 3. 4. 5. 6. 7. 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 does 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 Extraembryonic Mesoderm. Cells from trophoblast proliferate between endoderm and trophoblast at the abembryonic pole and between trophoblast and amnion at the embryonic or animal pole. They form extraembryonic 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 form primitive streak enter the space between ectoderm and endoderm and spread to all parts of embryonic disc. I is intraembryonic mesoderm. 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. Foetal Membranes. Four types of extra embryonic foetal membranes develop – amnion, chorion, allantois and yolk sac. Chorion is the outer foetal membrane which gives rise to villi, hence chorionic villi. In mammals the placenta is chorioallantoic being formed by both chorion and allantois. In human beings it is largely 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 nonmammals. It develops near the allantois. It is believed to form corpuscles till the liver of foetus becomes functional. Afterwards, it gradually shrinks and degenerate. Placenta. Foetus is attached to uterine wall by placenta. It is disc-shaped having a number of finger like chorionic villi have rich supply of blood capillaries. Uterine tissue around these villi have a number of bloosinuses 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. Imbilical cord is mainly formed of allantois and is covered by amniotic epithelium. 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 corticotropin, 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 accommodate developing foetus. 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 cmployed 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 causes 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 colostrums. It is rich in proteins, calories and antibodies. The antibodies provide passive immunity to the neonate. Actual milk secretion begins from third or fourth day. 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 breast are not emptied properly. It is an autocrine action where supply matches demand. - 248 – -