Human Reproduction

Humans reproduce sexually and are viviparous; NCERT's order of events is gametogenesis → insemination → fertilisation → implantation → gestation → parturition. The male system, in the pelvis, has a pair of testes, accessory ducts, accessory glands and the external genitalia. The testes lie outside the abdominal cavity in the scrotum, which keeps them 2–2.5°C below body temperature, as spermatogenesis requires. Each adult testis is oval, about 4–5 cm by 2–3 cm, with about 250 testicular lobules, each holding one to three coiled seminiferous tubules where sperms form.

A tubule is lined inside by spermatogonia (germ cells) and Sertoli cells (which nourish them). The interstitial spaces outside hold small blood vessels and Leydig (interstitial) cells, which secrete androgens.

The accessory ducts run rete testis → vasa efferentia → epididymis → vas deferens: the tubules open into the vasa efferentia through the rete testis; the vasa efferentia open into the epididymis on the posterior surface of the testis; the epididymis leads to the vas deferens, which loops over the urinary bladder, receives the duct of the seminal vesicle and opens into the urethra as the ejaculatory duct. The ducts store and transport sperms. The urethra runs from the bladder through the penis to the urethral meatus; the enlarged glans penis is covered by the foreskin.

The accessory glands, paired seminal vesicles, one prostate and paired bulbourethral glands, secrete the seminal plasma, rich in fructose, calcium and enzymes; the bulbourethral secretion also lubricates the penis.

The female system has a pair of ovaries, a pair of oviducts (fallopian tubes), the uterus, cervix, vagina and the external genitalia, integrated with the pair of mammary glands.

The ovaries are the primary female sex organs, producing the ovum and steroid ovarian hormones. Each is 2–4 cm long, held by ligaments, and covered by a thin epithelium enclosing the ovarian stroma: a peripheral cortex and an inner medulla.

Oviducts, uterus and vagina are the accessory ducts. Each oviduct is 10–12 cm long: the funnel-shaped infundibulum near the ovary bears finger-like fimbriae that collect the ovum after ovulation, then the wider ampulla, then the narrow isthmus that joins the uterus. The single inverted-pear uterus opens into the vagina through the cervix; the cervical canal plus the vagina is the birth canal. The uterine wall has an outer thin membranous perimetrium, a middle thick smooth-muscle myometrium that contracts strongly at delivery, and an inner glandular endometrium that changes cyclically in the menstrual cycle.

External genitalia: mons pubis, labia majora, labia minora, hymen and clitoris. The hymen can be torn by a fall, a tampon or sports and may persist after coitus, so it is not a reliable indicator of virginity.

Each mammary gland has 15–20 mammary lobes of milk-secreting alveoli, opening into mammary tubules → mammary duct → mammary ampulla → lactiferous duct.

In the testis the spermatogonia on the inside wall of the seminiferous tubule multiply by mitosis; each is diploid (46 chromosomes). Some, now primary spermatocytes, periodically undergo meiosis: meiosis I (reduction division) gives two equal haploid secondary spermatocytes (23 chromosomes) and meiosis II gives four equal haploid spermatids. Spermatids become spermatozoa by spermiogenesis, a change of form without division; the sperm heads stay embedded in Sertoli cells until the sperms are released from the tubule by spermiation.

Hormonal control: at puberty the hypothalamus sharply increases GnRH, which makes the anterior pituitary secrete LH and FSH. LH acts on the Leydig cells, which secrete androgens, and androgens stimulate spermatogenesis. FSH acts on the Sertoli cells, which secrete factors that help spermiogenesis.

The sperm has a head, neck, middle piece and tail inside a plasma membrane. The head carries an elongated haploid nucleus capped by the acrosome, filled with enzymes that help fertilisation; the middle piece holds numerous mitochondria that power the tail, whose movement gives the motility fertilisation needs. A man ejaculates about 200–300 million sperms; for normal fertility at least 60% must be of normal shape and size and at least 40% vigorously motile. Secretions of the epididymis, vas deferens, seminal vesicle and prostate are essential for sperm maturation and motility; seminal plasma plus sperms is semen.

Oogenesis begins during embryonic development: a couple of million oogonia form in each foetal ovary, and none are added after birth. They enter prophase I of meiosis and are arrested there as primary oocytes; each, wrapped in a layer of granulosa cells, is a primary follicle. Most degenerate before puberty, leaving 60,000–80,000 per ovary.

From puberty, each cycle, a primary follicle gains more granulosa layers and a theca (secondary follicle), then a fluid-filled antrum (tertiary follicle) with theca interna and theca externa. Now the primary oocyte grows and completes meiosis I, an unequal division: a large haploid secondary oocyte keeping the bulk of the nutrient-rich cytoplasm, and a tiny first polar body. The tertiary follicle matures into the Graafian follicle, the secondary oocyte forms the zona pellucida around itself, and the follicle ruptures to release the secondary oocyte (ovum): ovulation. Meiosis II is completed only if a sperm enters.

Contrast with spermatogenesis: oogenesis starts in the foetus and has long arrests, spermatogenesis starts at puberty and is continuous; one primary oocyte gives one ovum plus polar bodies (unequal divisions), one primary spermatocyte gives four sperms (equal divisions); oogenesis ends at menopause, spermatogenesis continues into old age.

The reproductive cycle of female primates is the menstrual cycle. The first menstruation at puberty is menarche; cycles stop at menopause, around 50 years. Menstruation repeats every 28/29 days on average, with one ovum released mid-cycle.

Menstrual phase (days 1–5): the endometrial lining and its blood vessels break down and leave through the vagina; the flow lasts 3–5 days. It occurs only if the ovum was not fertilised, so a missed period may indicate pregnancy, though stress or poor health can also cause it. LH and FSH are low.

Follicular phase (days 6–13): primary follicles grow into a mature Graafian follicle while the endometrium regenerates by proliferation (hence proliferative phase). LH and FSH rise gradually, stimulating follicular development and secretion of estrogens by the growing follicles.

Ovulatory phase (about day 14): LH and FSH peak; the rapid rise of LH to its maximum, the LH surge, ruptures the Graafian follicle and releases the ovum.

Luteal phase (days 15–28): the remains of the follicle become the corpus luteum, which secretes large amounts of progesterone to maintain the endometrium for implantation. Without fertilisation it degenerates, the endometrium disintegrates and menstruation begins a new cycle; in pregnancy all events of the cycle stop. Because ovulation is mid-cycle, avoiding coitus from about day 10 to 17 is a natural contraceptive measure.

Release of semen into the vagina during coitus is insemination. Sperms swim through the cervix and uterus to the ampullary-isthmic junction of the fallopian tube, to which the ovum is also transported; fertilisation happens only if both arrive there simultaneously, which is why not every coitus leads to pregnancy.

A sperm touches the zona pellucida and induces changes in the membrane that block additional sperms, so only one sperm fertilises the ovum. Acrosome secretions help it enter through the zona pellucida and plasma membrane. Sperm entry induces completion of meiosis II of the secondary oocyte, an unequal division giving a second polar body and a haploid ovum (ootid); the haploid nuclei fuse into the diploid zygote.

Sex determination: female XX, male XY. Every ovum carries X; 50% of sperms carry X and 50% Y. XX becomes a girl, XY a boy, so the father decides the sex.

Cleavage (mitosis) begins as the zygote moves through the isthmus, forming 2, 4, 8, 16 blastomeres; the 8–16-cell morula keeps dividing and becomes the blastocyst as it moves into the uterus: an outer trophoblast and an inner cell mass. The trophoblast attaches to the endometrium, the inner cell mass becomes the embryo, and uterine cells divide rapidly to cover the blastocyst so that it is embedded in the endometrium: implantation, which leads to pregnancy.

After implantation, finger-like chorionic villi appear on the trophoblast, surrounded by uterine tissue and maternal blood; villi and uterine tissue interdigitate to form the placenta, a structural and functional unit between foetus and mother. It supplies oxygen and nutrients to the embryo and removes carbon dioxide and wastes; the umbilical cord connects it to the embryo and carries substances both ways.

The placenta is also an endocrine tissue, producing hCG (human chorionic gonadotropin), hPL (human placental lactogen), estrogens and progestogens; in later pregnancy the ovary secretes relaxin. hCG, hPL and relaxin are produced only during pregnancy. Estrogens, progestogens, cortisol, prolactin and thyroxine also rise several-fold.

Right after implantation the inner cell mass forms an outer ectoderm, an inner endoderm and, between them, a mesoderm; these three layers give rise to all adult tissues. The inner cell mass contains stem cells able to form all tissues and organs.

Timeline (1-mark items): after one month the heart forms (its sound through a stethoscope is the first sign of the foetus); by the end of the second month limbs and digits; by 12 weeks (first trimester) most major organ systems, with limbs and external genital organs well developed; in the fifth month first movements and hair on the head; by 24 weeks (second trimester) fine body hair, separated eyelids and eyelashes; at nine months the foetus is fully developed.

Pregnancy lasts about nine months, the gestation period. Vigorous uterine contractions then expel the foetus: parturition, induced by a complex neuroendocrine mechanism. Signals from the fully developed foetus and the placenta induce mild uterine contractions, the foetal ejection reflex, which triggers release of oxytocin from the maternal pituitary. Oxytocin makes the uterine muscle contract more strongly, which stimulates further oxytocin secretion; this stimulatory reflex continues with stronger and stronger contractions until the baby is expelled through the birth canal, followed soon after by the placenta. It is neuroendocrine because a neural reflex starts it and a hormone (oxytocin, from the mother's pituitary, not the placenta) drives it in a positive loop.

Lactation: the mammary glands differentiate during pregnancy and start producing milk towards its end. The milk of the first few days, colostrum, contains several antibodies absolutely essential for developing resistance in the newborn, which is why doctors recommend breast-feeding in the initial period.