Human Reproduction Class 12 Notes

Male Reproductive System

The male reproductive system consists of a pair of testes, accessory ducts (rete testis → vasa efferentia → epididymis → vas deferens), glands (seminal vesicles, prostate, Cowper's bulbourethral glands) and the penis. The testes are located in the scrotum outside the abdominal cavity to maintain a temperature 2–2.5°C below body temperature, which is essential for spermatogenesis.

Spermatogenesis

Spermatogenesis begins at puberty and continues throughout life. Spermatogonia (2n) multiply by mitosis; some differentiate into primary spermatocytes (2n). Each primary spermatocyte undergoes meiosis I to form two secondary spermatocytes (n), which each undergo meiosis II to form four spermatids (n). These spermatids transform into sperms (spermiogenesis) — a process of condensation of nucleus, formation of acrosome, and development of a middle piece with mitochondria and a flagellum.

Female Reproductive System

The female reproductive system includes a pair of ovaries, oviducts (fallopian tubes), uterus, cervix and vagina. The oviduct has three parts: infundibulum (with finger-like fimbriae that catch the ovum), ampulla (the usual site of fertilization) and isthmus. The uterus is lined by the endometrium, which undergoes cyclic changes during the menstrual cycle.

Oogenesis and the Menstrual Cycle

Phases of the Menstrual Cycle (28 days)

• Menstrual phase (days 1–5): Endometrium breaks down and is shed as menstrual blood.
• Follicular / proliferative phase (days 6–13): FSH stimulates follicle growth; primary follicle → Graafian follicle. Endometrium regenerates. Estrogen from growing follicle peaks.
• Ovulation (~day 14): LH surge (triggered by high estrogen via positive feedback) causes rupture of the Graafian follicle and release of the secondary oocyte.
• Luteal / secretory phase (days 15–28): Ruptured follicle becomes the corpus luteum, secreting progesterone and estrogen. Endometrium thickens and becomes glandular. If no fertilization, corpus luteum degenerates → progesterone drops → menstruation.

Fertilization and Early Development

Sperms are deposited in the vagina and travel through the cervix and uterus to the oviduct. Capacitation (loss of cholesterol from sperm membrane in the female tract) is necessary for the sperm to acquire the ability to fertilize. When a sperm contacts the secondary oocyte, its acrosomal enzymes digest the zona pellucida. One sperm enters, and the cortical reaction (cortical granule release) blocks polyspermy.

• The zygote undergoes cleavage divisions as it travels down the oviduct → 2-cell → 4-cell → 8-cell → morula (solid ball of 16 cells, day 4).
• The morula develops a fluid-filled cavity → blastocyst (day 5–6). Outer layer is trophoblast (forms placenta); inner cell mass forms the embryo.
• Implantation: the blastocyst embeds into the vascularized endometrium around day 7 after fertilization. The trophoblast secretes hCG (human chorionic gonadotropin), which maintains the corpus luteum.

Parturition and Lactation

Parturition is the process of delivery of the fetus at the end of pregnancy (~280 days or 40 weeks from the last menstrual period). A neuroendocrine reflex involving oxytocin from the posterior pituitary causes strong uterine contractions (Ferguson reflex). Prostaglandins from the uterus enhance these contractions. Relaxin loosens the pelvic ligaments and dilates the cervix.

Lactation

Colostrum is the first milk secreted in the initial days after birth. It is rich in antibodies (especially IgA) and provides passive immunity to the newborn. Prolactin from the anterior pituitary stimulates milk production; oxytocin causes milk ejection (let-down reflex) during suckling. Breastfeeding also helps in post-partum recovery and creates mother-child bonding.

Solved Examples

Example: Explain why a woman with blocked fallopian tubes cannot conceive naturally. Which ART technique would you suggest and why?

Solution: Fertilization normally occurs in the ampulla of the fallopian tube. If the tubes are blocked, sperm cannot reach the ovulated oocyte. IVF-ET (In Vitro Fertilization — Embryo Transfer) is the appropriate technique: the oocyte is collected from the ovary, fertilized with sperm in the laboratory, and the resulting embryo is transferred into the uterus at the blastocyst stage, bypassing the blocked tubes entirely.

Example: A pregnant woman undergoes a routine checkup. The doctor says her hCG levels are normal. What does hCG indicate, and which cells produce it?

Solution: hCG (human chorionic gonadotropin) is produced by the trophoblast cells of the blastocyst after implantation. It is detected in urine and blood and is the basis of pregnancy test kits. Its main function is to maintain the corpus luteum so that it continues to produce progesterone, preventing menstruation and sustaining the pregnancy until the placenta is fully functional around the end of the first trimester.