Human Reproduction

2.1 Core concepts

• Reproductive events in humans: gametogenesis (sperms in males, ovum in females) → insemination → fertilisation → zygote → blastocyst formation → implantation → gestation → parturition; sperm formation continues into old age but ovum formation ceases around 50 years (NCERT §intro, p. 26).
• The male reproductive system, located in the pelvis, comprises a pair of testes plus accessory ducts, glands and the external genitalia (penis) (NCERT §2.1, p. 26).
• Testes lie outside the abdominal cavity within the scrotum, which keeps testicular temperature 2–2.5 °C below normal body temperature — necessary for spermatogenesis; each adult testis is 4–5 cm long, 2–3 cm wide, and contains about 250 testicular lobules (NCERT §2.1, p. 27).
• Each lobule has 1–3 highly coiled seminiferous tubules lined by male germ cells (spermatogonia) and Sertoli cells (which provide nutrition); interstitial spaces outside the tubules contain Leydig cells that synthesise and secrete androgens (NCERT §2.1, p. 27).
• Male accessory ducts in order: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra → urethral meatus; the vas deferens loops over the urinary bladder and is joined by the duct from the seminal vesicle to form the ejaculatory duct (NCERT §2.1, p. 27).
• Male accessory glands: paired seminal vesicles, a prostate, and paired bulbourethral glands; their secretions form the seminal plasma rich in fructose, calcium and certain enzymes; bulbourethral secretions also lubricate the penis (NCERT §2.1, p. 28).
• The female reproductive system has a pair of ovaries, a pair of oviducts, a uterus, a cervix, a vagina, external genitalia and a pair of mammary glands (NCERT §2.2, p. 28).
• Each ovary is 2–4 cm long, connected to the pelvic wall and uterus by ligaments, covered by a thin epithelium enclosing the ovarian stroma (peripheral cortex + inner medulla) (NCERT §2.2, p. 28).
• Each fallopian tube is 10–12 cm long; parts in order from the ovary are: infundibulum (with fimbriae that collect the ovum) → ampulla (wider) → isthmus (narrow lumen, joins uterus) (NCERT §2.2, pp. 28–30).
• The uterus (womb), shaped like an inverted pear, opens into the vagina through the cervix; the cervical canal plus vagina forms the birth canal; the uterine wall has three layers — perimetrium (outer thin), myometrium (middle thick smooth muscle, contracts at delivery) and endometrium (inner glandular, undergoes cyclical changes) (NCERT §2.2, p. 30).
• Female external genitalia: mons pubis, labia majora, labia minora, hymen and clitoris; presence/absence of hymen is not a reliable indicator of virginity (NCERT §2.2, p. 30).
• Each mammary gland has 15–20 mammary lobes containing alveoli that secrete milk; alveoli → mammary tubules → mammary duct → mammary ampulla → lactiferous duct → nipple (NCERT §2.2, p. 31).
• Spermatogenesis: diploid spermatogonia (46 chromosomes) on the inner wall of seminiferous tubules multiply mitotically; some become primary spermatocytes that undergo meiosis-I (reduction) → two haploid secondary spermatocytes (23 chromosomes each) → meiosis-II → four haploid spermatids; spermatids transform into spermatozoa via spermiogenesis, and are released by spermiation (NCERT §2.3, pp. 31–32).
• Hormonal regulation of spermatogenesis: GnRH (hypothalamus) at puberty → LH and FSH (anterior pituitary); LH acts on Leydig cells → androgens → spermatogenesis; FSH acts on Sertoli cells → factors supporting spermiogenesis (NCERT §2.3, pp. 31–32).
• Sperm structure: head (haploid nucleus capped by enzyme-filled acrosome), neck, middle piece (numerous mitochondria — energy for tail movement), and tail (motility); a human ejaculate contains 200–300 million sperms, ≥60% must be normal in shape/size and ≥40% vigorously motile for normal fertility (NCERT §2.3, p. 32).
• Oogenesis begins in embryonic life — a couple of million oogonia form in each foetal ovary, no new oogonia after birth; oogonia enter prophase-I and arrest as primary oocytes inside primary follicles; many degenerate, leaving 60,000–80,000 primary follicles per ovary at puberty (NCERT §2.3, pp. 32–33).
• Follicle progression: primary follicle → secondary follicle (more granulosa layers + new theca) → tertiary follicle (fluid-filled antrum; theca splits into theca interna and theca externa); primary oocyte completes meiosis-I unequally → large haploid secondary oocyte + tiny first polar body; tertiary follicle matures into Graafian follicle, secondary oocyte gains a zona pellucida, then Graafian follicle ruptures to release ovum — ovulation (NCERT §2.3, p. 33).
• Menstrual cycle is the reproductive cycle of female primates; the first menstruation is menarche; cycle averages 28/29 days; one ovum is released per cycle; menopause occurs around age 50 (NCERT §2.4, pp. 33, 35).
• Four phases: menstrual phase (3–5 days, endometrium breaks down, occurs only if ovum is not fertilised) → follicular phase (follicles mature, endometrium regenerates, FSH/LH and estrogens rise) → ovulatory phase (LH surge at ~day 14 ruptures Graafian follicle → ovulation) → luteal phase (Graafian follicle remnants become corpus luteum, which secretes progesterone to maintain endometrium for implantation) (NCERT §2.4, pp. 33–35).
• During pregnancy, menstrual cycles stop; without fertilisation, the corpus luteum degenerates, the endometrium disintegrates and menstruation begins a new cycle (NCERT §2.4, p. 35).
• Fertilisation: motile sperms travel cervix → uterus → ampullary region of the fallopian tube; ovum and sperms must reach the ampulla simultaneously; on contact with the zona pellucida the sperm triggers a membrane block to additional sperms (only one sperm fertilises); acrosomal enzymes help the sperm penetrate the zona pellucida and plasma membrane; entry of the sperm induces the secondary oocyte to complete meiosis-II — producing the second polar body and a haploid ovum (ootid); haploid sperm and ovum nuclei fuse → diploid zygote (NCERT §2.5, pp. 35–36).
• Sex determination: human female is XX, male is XY; 50% of sperms carry X and 50% Y; XX zygote → female, XY zygote → male — sex of the baby is determined by the father (NCERT §2.5, p. 36).
• Cleavage: as the zygote moves through the isthmus toward the uterus, mitotic divisions produce 2, 4, 8, 16 blastomeres; the 8–16 blastomere embryo is the morula; morula transforms into a blastocyst as it enters the uterus (NCERT §2.5, pp. 36–37).
• Blastocyst → trophoblast (outer layer) + inner cell mass (becomes embryo); the trophoblast attaches to the endometrium, uterine cells then divide and cover the blastocyst — implantation, leading to pregnancy (NCERT §2.5, p. 37).
• After implantation, chorionic villi appear on the trophoblast and interdigitate with the uterine tissue to form the placenta — the structural and functional unit between foetus and mother; the placenta supplies O₂ and nutrients, removes CO₂ and wastes, and is connected to the foetus by the umbilical cord (NCERT §2.6, p. 37).
• Placenta as endocrine tissue produces hCG, hPL, estrogens and progestogens; relaxin is secreted later in pregnancy by the ovary; hCG, hPL and relaxin are produced only during pregnancy; estrogens, progestogens, cortisol, prolactin, thyroxine all rise in maternal blood (NCERT §2.6, p. 37).
• Embryonic development milestones: inner cell mass → ectoderm + endoderm + mesoderm (the three germ layers containing stem cells); month 1 — heart formed (audible by stethoscope); end of month 2 — limbs and digits; end of 12 weeks (first trimester) — major organ systems formed, external genitals well-developed; month 5 — first foetal movements and head hair; end of 24 weeks (second trimester) — body covered with fine hair, eyelids separate, eyelashes form; 9 months — fully developed foetus ready for delivery (NCERT §2.6, pp. 37–38).
• Parturition: gestation period ~9 months; vigorous uterine contractions expel the foetus; signals from the fully developed foetus and placenta trigger mild contractions (foetal ejection reflex) → release of oxytocin from the maternal pituitary → stronger uterine contractions → more oxytocin (positive feedback); placenta is expelled soon after the infant (NCERT §2.7, p. 38).
• Lactation: mammary glands differentiate during pregnancy and start producing milk towards the end of pregnancy; colostrum, the milk of the first few days, contains antibodies essential for the newborn's resistance; breast-feeding in early infancy is recommended (NCERT §2.7, p. 38).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 2.1(a) & (b) Male pelvis and male reproductive system showing testes, ducts, glands and external genitalia (pp. 26–27).
• Figure 2.2 Sectional view of a seminiferous tubule showing spermatogonia, Sertoli cells and Leydig cells in the interstitium (p. 28).
• Figure 2.3(a) & (b) Female pelvis and female reproductive system showing ovary, oviduct parts (infundibulum, ampulla, isthmus), uterus, cervix, vagina (pp. 29).
• Figure 2.4 Sectional view of a mammary gland — alveoli → mammary tubules → mammary duct → mammary ampulla → lactiferous duct → nipple (p. 30).
• Figure 2.5 Enlarged seminiferous tubule showing the spermatogenic series (p. 31).
• Figure 2.6 Structure of a sperm — head (with acrosome and haploid nucleus), neck, middle piece (mitochondria), tail (p. 32).
• Figure 2.7 Sectional view of the ovary showing primary, secondary, tertiary and Graafian follicles, corpus luteum (p. 33).
• Figure 2.8(a) & (b) Schematic of spermatogenesis (four spermatids per primary spermatocyte) and oogenesis (one ovum + polar bodies) (p. 33).
• Figure 2.9 Events of the menstrual cycle correlated with ovarian and pituitary hormone levels and endometrial changes (p. 34).
• Figure 2.10 Ovum surrounded by several sperms at the zona pellucida (p. 35).
• Figure 2.11 Transport of ovum, fertilisation in the ampulla, cleavage and passage of the growing embryo (zygote → 2/4/8/16 blastomeres → morula → blastocyst → implantation) (p. 36).
• Figure 2.12 Human foetus within the uterus showing placenta and umbilical cord (p. 37).

2.4 Common confusions / NTA trap points

• Sertoli vs Leydig cells. Sertoli cells are inside the seminiferous tubule and nourish germ cells (FSH target); Leydig cells are in the interstitium and secrete androgens (LH target). NTA loves to swap these.
• Follicle terminology vs oocyte stage. The Graafian follicle releases a secondary oocyte, not an ovum in the strict sense — meiosis-II completes only on sperm entry, producing the ootid + second polar body.
• Site of fertilisation. Fertilisation occurs in the ampullary region of the fallopian tube, not in the uterus, ovary, or isthmus.
• Hormone–phase mapping. Progesterone dominates the luteal phase (corpus luteum); estrogens rise in the follicular phase; the LH surge defines the ovulatory phase. hCG/hPL/relaxin are pregnancy-only hormones.
• Cleavage vs ordinary mitosis. Cleavage divisions in the isthmus produce 2, 4, 8, 16 blastomeres without an increase in overall embryo size — the morula has 8–16 cells, the blastocyst has trophoblast + inner cell mass.
• Number of sperms. A normal ejaculate is 200–300 million sperms; at least 60% with normal shape/size and 40% vigorously motile for normal fertility — common distractors flip these percentages.