Human Reproduction — Long Notes

Humans reproduce sexually and are viviparous — the young develop inside the mother's body and are born live. Reproduction in humans involves gametogenesis (spermatogenesis and oogenesis), insemination, fertilisation, implantation, gestation, and parturition.

1. The Male Reproductive System

1.1 Testes and Their Environment

• Located outside the abdominal cavity in a pouch called the scrotum, which keeps the testicular temperature about 2–2.5°C lower than the core body temperature — essential for spermatogenesis.
• Each testis is oval, about 4–5 cm long × 2–3 cm wide. Inside it is divided into ~250 testicular lobules; each lobule contains 1–3 seminiferous tubules in which sperm are produced.

1.2 Seminiferous Tubule Structure

Cross-section shows two cell populations lining the tubule:

• Male germ cells (spermatogonia) at various stages of division → become sperms.
• Sertoli cells (nurse cells) — provide nutrition and structural support to germ cells; targets of FSH.

Between tubules is interstitial tissue containing Leydig cells (interstitial cells), which secrete androgens (mainly testosterone) under LH control. Also small blood vessels and immune cells.

1.3 Accessory Ducts

Sperm route from testis to outside: Testis → rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra → urethral meatus.

The vas deferens loops over the ureter and joins the duct of the seminal vesicle to form the ejaculatory duct, which opens into the urethra.

1.4 Accessory Glands

Three sets contribute to seminal fluid:

• Seminal vesicles (paired) — fructose-rich secretion (sperm energy) + prostaglandins.
• Prostate gland — alkaline milky fluid to neutralise vaginal acidity.
• Bulbourethral (Cowper's) glands — pre-ejaculatory mucus that lubricates.

Semen = sperms + these secretions. 200–300 million sperms per ejaculate are typical.

2. The Female Reproductive System

2.1 Ovaries

• Paired, almond-shaped, located either side of the lower abdomen.
• Each ovary at birth contains ~1–2 lakh (100,000–200,000) primary follicles, most of which will degenerate over life.
• Two functions: (i) produce the female gamete (ovum) and (ii) secrete oestrogen and (later) progesterone.

2.2 Oviducts (Fallopian Tubes)

From the ovary end inward: infundibulum (with fimbriae) → ampulla → isthmus → uterus. Fertilisation happens in the ampullary-isthmic junction.

2.3 Uterus

Pear-shaped, single, muscular. Wall layers (outer to inner):

• Perimetrium — external thin membrane.
• Myometrium — thick smooth muscle; contracts during labour.
• Endometrium — glandular lining that undergoes cyclic changes during the menstrual cycle; sheds during menstruation.

Below the uterus is the cervix, whose cavity is the cervical canal. The cervical canal + the vagina together form the birth canal.

2.4 External Genitalia and Mammary Glands

• External genitalia: mons pubis, labia majora, labia minora, hymen, clitoris.
• Mammary glands are paired structures of glandular tissue and fat. Each has 15–20 mammary lobes; lobes → lobules → alveoli (milk-secreting cells) → ducts → mammary ampulla → lactiferous duct.

3. Gametogenesis

3.1 Spermatogenesis

Begins at puberty and continues throughout life. Location: seminiferous tubules.

1. Spermatogonia (2n) divide by mitosis → some become primary spermatocytes (2n).
2. Primary spermatocyte undergoes meiosis I → 2 secondary spermatocytes (n).
3. Each secondary spermatocyte undergoes meiosis II → 2 spermatids (n).
4. So each primary spermatocyte → 4 haploid spermatids.
5. Spermiogenesis — spermatids transform into motile spermatozoa (compact head, mitochondrial midpiece, tail).
6. Spermiation — mature sperms detach into the lumen and travel to the epididymis for maturation.

Hormonal control: hypothalamus secretes GnRH → anterior pituitary → FSH & LH → LH acts on Leydig cells → testosterone → spermatogenesis; FSH acts on Sertoli cells → spermiogenesis support.

Sperm structure: head (nucleus with acrosome), neck, middle piece (spirally arranged mitochondria providing ATP), tail (flagellum for movement).

3.2 Oogenesis

Peculiar timing:

• Begins during embryonic development. Millions of oogonia form in the foetal ovary and enter meiosis I, arresting at prophase I — these are now primary oocytes. They are enclosed by a granulosa layer to form primary follicles.
• At birth: 1–2 lakh primary follicles. By puberty: ~60,000–80,000. Only a few hundred will ever ovulate.
• At puberty and each cycle: a few primary follicles resume; they grow through secondary → tertiary stages, developing a fluid-filled antrum, and finally the Graafian follicle stage.
• The primary oocyte completes meiosis I → secondary oocyte + first polar body. The secondary oocyte begins meiosis II but arrests at metaphase II.
• Ovulation — the Graafian follicle ruptures, releasing the secondary oocyte.
• Meiosis II is completed only when a sperm fertilises the oocyte → mature ovum + second polar body.

Net result per primary oocyte: 1 ovum + 3 polar bodies (compare: 1 primary spermatocyte → 4 sperms).

4. Menstrual Cycle

Reproductive cycle of ~28 days from puberty (menarche) to menopause (~45–50 years). Phases:

Days	Phase	Ovary	Uterus	Hormones
1–5	Menstrual	Old corpus luteum degenerates	Endometrium sheds → menstrual flow	Oestrogen & progesterone ↓
6–13	Follicular / Proliferative	Follicles grow → Graafian follicle	Endometrium regenerates	FSH ↑, oestrogen ↑
~14	Ovulation	Graafian follicle ruptures	Endometrium peak of proliferation	LH surge (also FSH peak)
15–28	Luteal / Secretory	Ruptured follicle → corpus luteum	Endometrium becomes secretory, ready for implantation	Progesterone ↑

• Menarche — first menstruation, marking puberty in females.
• Menopause — cessation of menstrual cycles, ~45–50 years.
• If fertilisation occurs, the corpus luteum persists (maintained by hCG) and the cycle pauses.

5. Fertilisation and Implantation

1. During sexual intercourse, ~200–300 million sperms are deposited in the vagina.
2. Sperms swim through the cervix, uterus, and up the oviduct. Only a few hundred reach the ampullary–isthmic junction.
3. On contact with the zona pellucida of the secondary oocyte, sperms release acrosomal enzymes that dissolve a path through it. One sperm penetrates.
4. The cortical reaction blocks polyspermy: the zona hardens.
5. Sperm entry triggers completion of meiosis II → mature ovum + second polar body.
6. The male and female pronuclei fuse → zygote (2n).

Cleavage and blastocyst:

• The zygote undergoes rapid mitotic divisions (cleavage) as it travels toward the uterus.
• 2 → 4 → 8 → 16 (morula) → blastocyst.
• A blastocyst has an outer trophoblast (will form placenta) and an inner inner cell mass (embryoblast) (will form embryo).

Implantation (~7 days after fertilisation): the trophoblast attaches to the endometrium, and the blastocyst embeds itself in the endometrial thickness. The endometrium proliferates around it to enclose it (decidua).

6. Pregnancy and Embryonic Development

6.1 The Placenta

After implantation, the trophoblast forms finger-like chorionic villi that project into the endometrium and become surrounded by maternal blood sinuses — this is the placenta, the structural and functional connection between the developing foetus and the mother.

Functions:

• Nutrition — glucose, amino acids, ions, fatty acids from mother to foetus.
• Respiratory — O₂ and CO₂ exchange.
• Excretion — foetal wastes to mother's blood.
• Endocrine — secretes hCG, hPL, oestrogen, progesterone, and later relaxin.
• Immunological barrier — restricts some but not all substances.

hCG (human chorionic gonadotropin) maintains the corpus luteum in the first trimester and is the hormone detected in urine pregnancy tests. hPL (human placental lactogen) alters maternal metabolism to favour foetal growth. Relaxin (secreted later) helps loosen pelvic ligaments in preparation for parturition.

The placenta connects to the foetus through the umbilical cord, containing two arteries and one vein.

6.2 Embryonic Development Milestones

• Fertilisation → implantation ~week 1.
• Month 1 — heart forms and starts beating.
• Month 2 — organogenesis begins.
• Month 3 — limbs and digits well-developed.
• Month 4 — external genitalia recognisable; movements felt.
• Month 5 — hair appears on head.
• Month 6 — eyelids separate; body largely developed.
• Month 9 — mature foetus, ready for birth.
• Duration of pregnancy (gestation): ~9 months (≈280 days from LMP).

7. Parturition (Childbirth)

Signals begin from the placenta and fully-developed foetus → the foetal ejection reflex → uterus and posterior pituitary → release of oxytocin → strong uterine contractions → stretching of the cervix increases oxytocin release → positive feedback loop → intense contractions → expulsion of the baby.

Stages: dilation → expulsion of the baby → expulsion of the placenta.

8. Lactation

• The mammary glands begin producing milk toward the end of pregnancy.
• Colostrum — the first fluid produced after delivery for a few days; yellowish; rich in antibodies (mainly IgA); provides passive immunity to the newborn.
• Milk production is maintained by prolactin (anterior pituitary); milk ejection on suckling is triggered by oxytocin (posterior pituitary).
• Breast feeding, especially during the first ~6 months, is strongly recommended for infant health.

Key take-aways

1. Human reproduction is a tightly hormone-coordinated sequence: GnRH → FSH/LH → gonadal steroids → gametes and secondary characters.
2. Spermatogenesis is continuous; oogenesis pauses twice — once as primary oocyte at prophase I (foetal → puberty) and once as secondary oocyte at metaphase II (until fertilisation).
3. The menstrual cycle sets the woman's monthly window for fertilisation, with LH surge → ovulation.
4. Fertilisation happens in the oviduct, followed by cleavage and implantation of the blastocyst; the placenta then supports the foetus for ~9 months, ending in an oxytocin-driven parturition.