Human Reproduction
Male and female reproductive systems, gametogenesis.
Male reproductive system
Testis, spermatogenesis, accessory glands.
Female reproductive system
Ovary, oogenesis, menstrual cycle.
The menstrual cycle is a 28-day (average) cycle in females of reproductive age (~12 to 50), regulated by hormones from the hypothalamus, pituitary, and ovaries.
Four phases:
1. Menstrual phase (Days 1–5). Shedding of the uterine endometrium → bleeding. Estrogen and progesterone are low.
2. Follicular phase (Days 1–13). FSH from anterior pituitary stimulates ovarian follicles to grow. The dominant follicle produces increasing estrogen. Endometrium thickens (proliferative phase of uterus).
3. Ovulation (~Day 14). Surge in LH (luteinizing hormone) triggers release of the egg (secondary oocyte) from the dominant follicle. Egg moves into the fallopian tube. Window for fertilization: ~24 hours.
4. Luteal phase (Days 15–28). The ruptured follicle becomes the corpus luteum, which secretes progesterone (and some estrogen). Progesterone keeps the endometrium thick and ready for implantation (secretory phase).
If no fertilization: corpus luteum degenerates (becomes corpus albicans) → progesterone drops → endometrium sheds → next cycle.
If fertilization: implantation in endometrium → embryo secretes hCG (human chorionic gonadotropin) → maintains corpus luteum → progesterone continues → pregnancy.
Hormone summary:
| Hormone | Source | Function |
|---|---|---|
| GnRH | Hypothalamus | Triggers FSH and LH release |
| FSH (Follicle-stimulating) | Anterior pituitary | Follicle development; estrogen synthesis |
| LH (Luteinizing) | Anterior pituitary | Ovulation trigger; corpus luteum formation |
| Estrogen | Ovarian follicle | Endometrium growth; secondary sexual characteristics |
| Progesterone | Corpus luteum (and placenta in pregnancy) | Maintains endometrium; suppresses ovulation |
| hCG | Trophoblast / placenta | Maintains corpus luteum in early pregnancy. Basis of home pregnancy tests (detects hCG in urine) |
Negative feedback loops:
- High estrogen + progesterone (luteal phase) → suppresses FSH and LH (combined oral contraceptive pill works on this principle).
- High estrogen at mid-cycle → positive feedback → LH surge → ovulation.
Common JEE/NEET facts:
- Mature egg released is at metaphase II (completes meiosis only after sperm enters).
- Sperm life: ~3–5 days. Egg life: ~24 hours. Fertilizable window per cycle: ~6 days.
- Menarche: first period (
12 yr). Menopause: cessation of cycles (45–55 yr).
Fertilisation and pregnancy
Implantation, placenta, parturition.