Chapter 2 — Human Reproduction

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

This chapter examines human sexual reproduction and viviparity. It details the anatomy and function of the male reproductive system—testes, accessory ducts, and accessory glands—and the female reproductive system—ovaries, oviducts, uterus, and mammary glands. The chapter explains gametogenesis: spermatogenesis produces haploid sperms via meiosis starting at puberty, while oogenesis produces a mature ovum from primary oocytes. The menstrual cycle, lasting 28-29 days on average, consists of menstrual, follicular, and luteal phases regulated by pituitary and ovarian hormones. During fertilisation in the ampullary region of the fallopian tube, a sperm fuses with an ovum to form a diploid zygote; the sex is determined by the sperm's X or Y chromosome. The zygote undergoes cleavage to form a blastocyst, which implants in the endometrium. Over nine months of pregnancy, the embryo develops via three germ layers—ectoderm, mesoderm, endoderm—and forms major organ systems. Parturition is induced by neuroendocrine signals causing uterine contractions via oxytocin. Lactation—milk production and colostrum secretion—supports newborn care.

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.