Human Reproduction

The male reproductive system includes a pair of testes housed in the scrotal sac (2–3°C below body temperature, essential for spermatogenesis), accessory ducts, glands, and external genitalia.

Testis structure: Each testis has ~250 compartments with seminiferous tubules lined by spermatogonia and Sertoli cells. Leydig cells (interstitial cells) produce androgens.

Sperm pathway: Seminiferous tubules → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.

Accessory glands:

• Seminal vesicles — secrete fructose-rich fluid; provides energy for sperm.
• Prostate gland — secretes slightly alkaline fluid that activates sperm.
• Bulbourethral (Cowper's) glands — secrete lubricating fluid that neutralises urethral acidity.

The female reproductive system includes a pair of ovaries, a pair of oviducts (fallopian tubes), the uterus, cervix, vagina, and external genitalia.

Oviduct regions: Infundibulum (fimbriated funnel, collects ovum) → Ampulla (widest, site of fertilisation) → Isthmus (narrow, joins uterus).

Uterus layers:

• Perimetrium — outer epithelial layer.
• Myometrium — thick smooth muscle layer; contracts during parturition.
• Endometrium — inner glandular layer; undergoes cyclic changes; implantation occurs here.

Spermatogenesis — starts at puberty in the seminiferous tubules:

Spermatogonia (2n) → Primary spermatocyte → Secondary spermatocytes (n) → Spermatids → Spermatozoa (by spermiogenesis).

• Each primary spermatocyte → 4 functional sperms.
• Sertoli cells nourish developing germ cells and secrete inhibin (inhibits FSH).
• LH stimulates Leydig cells → testosterone.

Oogenesis — begins during foetal development:

Oogonium → Primary oocyte (arrested at Prophase I at birth) → Resumes at puberty → Secondary oocyte (arrested at Metaphase II) → Ovulation → Fertilisation → Oocyte completes meiosis II → Ovum + second polar body.

• Each primary oocyte → 1 functional ovum + 3 polar bodies.
• FSH stimulates follicle development; LH surge triggers ovulation.

The menstrual cycle is ~28 days in humans. It involves coordinated ovarian and uterine changes under hormonal control.

• Menstrual phase (Day 1–5) — endometrium sheds due to low oestrogen and progesterone.
• Follicular/Proliferative phase (Day 6–13) — rising FSH stimulates follicle growth; oestrogen rises; endometrium proliferates.
• Ovulation (Day 14) — LH surge triggers release of secondary oocyte from Graafian follicle.
• Luteal/Secretory phase (Day 15–28) — ruptured follicle forms corpus luteum; progesterone (+ oestrogen) peak; endometrium becomes secretory; if no fertilisation, corpus luteum degenerates → hormone fall → menstruation.

Fertilisation: Sperm penetrates secondary oocyte → acrosomal reaction → corona radiata and zona pellucida penetrated → completes meiosis II → zygote (2n).

After fertilisation, the zygote undergoes cleavage to form a blastocyst. The inner cell mass (embryoblast) forms the embryo; the outer layer (trophoblast) attaches to the endometrium — this is implantation (~7th day).

Placenta: Formed by embryonic and maternal tissues; facilitates exchange of nutrients, oxygen, and wastes. Secretes human chorionic gonadotropin (hCG) — the basis of pregnancy tests; also secretes oestrogen, progesterone, relaxin, and human placental lactogen.

Embryonic development milestones:

• 4 weeks — heart formation.
• 8 weeks — limbs, digits recognisable; embryo becomes foetus.
• ~5 months — foetal movements felt.
• 9 months — full term gestation.

Parturition is triggered by oxytocin (Ferguson reflex). Lactation: Colostrum (early milk) is rich in IgA antibodies and provides passive immunity to the newborn.