Chapter 2 — Human Reproduction

Chapter 2 of the Class 12 Biology NCERT textbook, "Human Reproduction", covers the male and female reproductive systems, gametogenesis (spermatogenesis and oogenesis), the menstrual cycle, fertilisation, implantation, pregnancy, embryonic development, parturition, and lactation.

• Anatomy of the reproductive systems — The male system comprises testes, accessory ducts, and glands built to produce and deliver sperm, while the female system of ovaries, oviducts, uterus, and mammary glands is organised to produce ova, nurture an embryo, and feed the newborn.
• Gametogenesis: making sperm and ova — Spermatogenesis at puberty produces vast numbers of haploid sperms via meiosis, whereas oogenesis begins before birth and yields a single mature ovum, reflecting the very different investment each sex makes in a gamete.
• The hormone-regulated menstrual cycle — The roughly 28-day cycle progresses through menstrual, follicular, ovulatory, and luteal phases, orchestrated by pituitary and ovarian hormones to prepare a follicle for ovulation and the endometrium for a possible pregnancy.
• Fertilisation through pregnancy to birth — A sperm fuses with the ovum in the fallopian tube to form a zygote that cleaves into a blastocyst and implants. Over nine months the embryo builds organs from three germ layers, ending in oxytocin-driven parturition and lactation.

1. 01The male reproductive system includes paired testes (each 4–5 cm long) housed in the scrotum, which maintains a temperature 2–2.5°C lower than body temperature—essential for spermatogenesis.
2. 02Each testis contains about 250 testicular lobules, with seminiferous tubules lined by spermatogonia (male germ cells) and Sertoli cells; Leydig cells in interstitial spaces secrete androgens.
3. 03Spermatogenesis begins at puberty stimulated by GnRH, LH, and FSH; a single sperm consists of head (with acrosome), neck, middle piece (packed with mitochondria), and tail.
4. 04The human male ejaculates 200–300 million sperms, of which at least 60% must have normal shape and size and 40% must show vigorous motility for normal fertility.
5. 05The female reproductive system features paired ovaries (2–4 cm long) that produce the ovum and steroid hormones; the oviduct (10–12 cm) extends from the ovary to the uterus via the infundibulum and ampulla.
6. 06Oogenesis begins during embryonic development with millions of oogonia; by puberty only 60,000–80,000 primary follicles remain, declining further with age until menopause at approximately 50 years.
7. 07The menstrual cycle averages 28–29 days: the menstrual phase (3–5 days) sheds the endometrial lining; the follicular phase promotes follicle growth and estrogen secretion; ovulation occurs at mid-cycle (day 14) triggered by LH surge; the luteal phase forms the corpus luteum, which secretes progesterone to maintain the endometrium.
8. 08Fertilisation occurs when a sperm fuses with an ovum in the ampullary region, forming a diploid zygote; the sperm's sex chromosome (X or Y) determines the embryo's sex, not the mother's.
9. 09The zygote undergoes repeated mitotic division forming 2, 4, 8, 16 blastomeres to create a morula; this develops into a blastocyst consisting of trophoblast (outer layer) and inner cell mass, which implants in the uterine endometrium.
10. 10During the nine-month gestation period, the embryo differentiates into three primary germ layers—ectoderm, mesoderm, endoderm; stem cells within the inner cell mass give rise to all tissues and organs.
11. 11Parturition is induced by signals from the fully developed foetus and placenta triggering foetal ejection reflex, releasing oxytocin from the maternal pituitary, which causes increasingly stronger uterine contractions.
12. 12Lactation produces milk after pregnancy; colostrum—secreted during initial lactation days—contains antibodies essential for developing resistance in newborn babies.