Human Reproduction — Revision Notes
Class 12 Biology. Male and female systems, gametogenesis, the menstrual cycle, fertilisation and pregnancy — the exam-relevant key points.
1. Male Reproductive System
- A pair of testes lie in the scrotum (temperature ~2–2.5 °C below body temperature, needed for spermatogenesis).
- Each testis has ~250 compartments (testicular lobules) with seminiferous tubules lined by germ cells and Sertoli cells (nourish sperm).
- Leydig cells (interstitial cells) secrete testosterone.
- Accessory glands: seminal vesicles, prostate, bulbourethral glands — together contribute the seminal plasma.
2. Female Reproductive System
- A pair of ovaries produce ova and hormones (estrogen, progesterone).
- Oviducts (fallopian tubes) → uterus (womb) → cervix → vagina.
- The uterine wall has three layers: perimetrium, myometrium (muscular) and endometrium (undergoes cyclic changes).
3. Gametogenesis
Spermatogenesis
- Begins at puberty in seminiferous tubules; spermatogonia (2n) → primary spermatocyte → (meiosis) → spermatids → spermatozoa.
- Stimulated by FSH (on Sertoli cells) and LH (on Leydig cells → testosterone).
Oogenesis
- Begins in the foetus; the oogonia form primary oocytes which are arrested in prophase-I until puberty.
- Each meiotic division gives one ovum + polar bodies. Meiosis-II completes only after fertilisation.
🔑 A sperm is much smaller than an ovum; one primary spermatocyte → 4 sperms, but one primary oocyte → 1 ovum.
4. Menstrual Cycle
- Average cycle length ~28 days; occurs in reproductive-age females (menarche to menopause).
- Menstrual phase (day 1–5): endometrium breaks down.
- Follicular / proliferative phase (day 6–13): FSH stimulates follicle growth; estrogen rebuilds endometrium.
- Ovulation (~day 14): a surge of LH releases the secondary oocyte.
- Luteal / secretory phase (day 15–28): corpus luteum secretes progesterone to maintain endometrium; if no fertilisation, it degenerates and menstruation follows.
🔑 The LH surge triggers ovulation. Progesterone from the corpus luteum maintains the endometrium.
5. Fertilisation & Pregnancy
- Fertilisation normally occurs in the ampulla of the fallopian tube.
- Fusion of sperm and ovum forms a diploid zygote; the acrosome and cortical reaction ensure monospermy.
- The zygote undergoes cleavage → morula → blastocyst, which implants in the endometrium.
- The placenta connects the foetus to the mother for nutrition, gas exchange and waste removal; it also secretes hormones (hCG, hPL, estrogen, progesterone).
- hCG maintains the corpus luteum in early pregnancy (basis of pregnancy tests). Gestation ~9 months; birth (parturition) is triggered by oxytocin.
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