Human Reproduction Class 12 Notes मानव जनन — कक्षा 12 जीव विज्ञान अध्याय 2

Revise these first (prerequisites)

• Cell division: mitosis and meiosis (Class 11 level)
• What a hormone is and how glands work (basic idea)
• Gametes (sperm/ovum) are haploid, the zygote is diploid
• Basic organ systems of the human body

1. The complete journey of human reproduction

   Human reproduction occurs in a few steps: formation of gametes (gametogenesis), transfer of sperm into the female tract (insemination), fertilisation, attachment of the embryo to the uterus (implantation), pregnancy (gestation) and birth of the baby (parturition / प्रसव). If you understand each step separately, the chapter will feel easy.

2. Male system: factory + pipeline

   The testis (वृषण) is the sperm-producing factory, located outside the body in the scrotum because sperm formation needs a temperature 2–2.5°C lower than body temperature. Sperms travel out through ducts (the pipeline), and glands (seminal vesicle, prostate, bulbourethral) provide fluid for them.

3. Female system: ovary, oviduct, uterus

   The ovary (अंडाशय) produces the ovum. The oviduct / fallopian tube is the passage where fertilisation takes place. The uterus (गर्भाशय) is the 'home' where the baby grows for 9 months — its inner lining, the endometrium, is prepared every month.

4. Menstrual cycle = monthly preparation

   Roughly every 28/29 days, the female body releases an ovum and prepares the uterus for pregnancy. If fertilisation does not occur, the prepared endometrium layer breaks down and comes out — this is called menstruation (आर्तव). It is a normal, healthy process.

5. From fertilisation to baby

   The sperm and ovum meet in the ampullary region of the fallopian tube → zygote. The zygote divides to form a ball-like structure (morula → blastocyst), which attaches to the uterine wall (implantation). Then the placenta (अपरा) forms, which supplies food and oxygen from the mother to the baby.

6. Birth and milk

   At the end of pregnancy, strong contractions of the uterus lead to the birth of the baby; the hormone oxytocin plays a major role in this. The milk produced in the first few days after birth, colostrum, is rich in antibodies (IgA) and acts like the baby's first 'vaccine'.

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As per NCERT 2026-27 — every point you need for the board exam.

1. Steps of human reproduction

   Gametogenesis → insemination → fertilisation → cleavage + blastocyst → implantation → gestation → parturition. Humans are viviparous.

2. Male reproductive system

   Testes lie in the scrotum (testis temperature is ~2–2.5°C lower than body temperature). Each testis has ~250 lobules, each lobule has 1–3 seminiferous tubules. Tubules contain spermatogonia + Sertoli cells (nutrition). Between the tubules are Leydig cells → androgens (testosterone).

3. Male ducts & glands

   Rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra. Glands: seminal vesicles (pair), prostate, bulbourethral (pair).

4. Female reproductive system

   Ovaries (estrogen, progesterone), oviduct (infundibulum, fimbriae, ampulla — site of fertilisation, isthmus), uterus (perimetrium, myometrium, endometrium), cervix, vagina. Mammary glands are also female accessory organs.

5. Spermatogenesis (शुक्राणुजनन)

   Begins at puberty. Spermatogonia (2n) → primary spermatocyte (2n) → meiosis I → secondary spermatocytes (n) → meiosis II → spermatids (n) → spermiogenesis → spermatozoa. 1 primary spermatocyte → 4 sperms.

6. Hormones (male)

   GnRH from the hypothalamus → LH and FSH from the pituitary. LH → Leydig cells (androgens). FSH → Sertoli cells → help in spermiogenesis.

7. Sperm structure

   Head (acrosome + nucleus), neck, middle piece (mitochondria = energy), tail. One ejaculation contains 200–300 million sperms; for normal fertility, ~60% must have normal shape and at least 40% must show vigorous motility.

8. Oogenesis (अंडजनन)

   Begins during foetal life itself — oogonia are formed, and primary oocytes get arrested in prophase of meiosis I. Primary follicle → secondary → tertiary (antrum) → Graafian follicle. Secondary oocyte + 1st polar body. Ovulation occurs at the secondary oocyte stage; meiosis II is completed upon sperm entry.

9. Menstrual cycle (आर्तव चक्र)

   ~28 days. Menstrual phase (3–5 days) → follicular/proliferative phase (FSH, LH increase; estrogen) → ovulation on the 14th day (LH surge) → luteal/secretory phase (corpus luteum → progesterone, endometrium prepared). If fertilisation does not occur, the corpus luteum degenerates → menstruation. Menarche = first menstruation; menopause at ~50 years.

10. Fertilisation & implantation

    In the ampullary region, sperm + ovum → zygote. The sperm decides the sex (X or Y). Cleavage → morula (8–16 cells) → blastocyst (trophoblast + inner cell mass). The trophoblast attaches to the endometrium = implantation → pregnancy.

11. Placenta

    Chorionic villi + uterine tissue. Exchange of nutrition, O₂ and wastes. Hormones: hCG, hPL, estrogens, progestogens; relaxin from the ovary in late pregnancy. Inner cell mass → ectoderm, mesoderm, endoderm (stem cells).

12. Embryo development

    1 month: heart. End of 2nd month: limbs and digits. By 12 weeks (1st trimester): major organ systems. 5th month: first movements, hair on the head. 24 weeks: fine hair on the body, eyelids separate. Pregnancy lasts ~9 months.

13. Parturition & lactation

    Foetal ejection reflex → oxytocin → uterine contractions → birth of the baby. The milk produced in the first few days = colostrum — rich in antibodies (IgA), essential for the newborn's immunity.

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. Hormonal control of spermatogenesis

   At puberty, GnRH from the hypothalamus increases → LH and FSH from the pituitary. LH acts on Leydig cells to stimulate production of androgens (testosterone), which stimulate spermatogenesis. FSH acts on Sertoli cells and causes release of factors that help in spermiogenesis. In other words, one hormone signals the 'factory owner' and the other signals the 'support staff'.

2. How many gametes from one meiocyte?

   After meiosis, one primary spermatocyte forms 4 sperm. But one primary oocyte forms only 1 ovum; the rest become polar bodies — due to unequal cytoplasmic division, the ovum gets more cytoplasm and food. So 100 primary spermatocytes → 400 sperm, but 100 primary oocytes → only 100 ova.

3. LH surge and ovulation

   In the follicular phase, the Graafian follicle secretes estrogen. In the middle of the cycle (~day 14), both LH and FSH peak; the rapid LH surge ruptures the Graafian follicle and releases the ovum (secondary oocyte). The remaining follicle then becomes the corpus luteum, which secretes progesterone that maintains the endometrium. If fertilisation does not occur, the corpus luteum degenerates → progesterone falls → menstruation.

4. 'Pause' points of oogenesis

   Oogenesis begins during the embryonic stage itself; primary oocytes are arrested in prophase-I. The secondary oocyte released at ovulation is in meiosis-II, and meiosis-II is completed only when a sperm enters it — forming the ovum and the second polar body. Hence, 'the true ovum is formed only after fertilisation' — a favourite board exam concept.

5. Placenta — supply line + endocrine gland

   The placenta is formed by chorionic villi (embryo's part) and uterine tissue (mother's part), and is connected to the embryo by the umbilical cord. It supplies O₂ and nutrients and removes CO₂ and wastes. It also produces hormones — hCG, hPL, estrogens, progestogens; in the later stage of pregnancy, relaxin is also secreted by the ovary. These hormones increase only during pregnancy.

6. Inner cell mass = stem cells

   The outer layer of the blastocyst, the trophoblast, attaches to the uterus, and the inner cell mass forms the embryo. The inner cell mass contains stem cells that can give rise to any tissue/organ; these form the ectoderm, mesoderm and endoderm.

Common confusions — cleared up

1. Spermatogenesis vs Spermiogenesis

   Spermatogenesis = the whole process (spermatogonia → sperm). Spermiogenesis = only the final step, in which a spermatid changes into a sperm; their release is called spermiation.

2. In ovulation: ovum or secondary oocyte?

   In ovulation, it is actually the secondary oocyte that is released; the mature ovum forms only after sperm entry, when meiosis-II is completed.

3. Leydig vs Sertoli cells

   Leydig (interstitial) cells produce androgens (under LH). Sertoli cells provide nourishment to germ cells (under FSH).

4. Corpus luteum vs Graafian follicle

   The Graafian follicle is the mature follicle before ovulation (estrogen). After ovulation, the remaining part becomes the corpus luteum (progesterone).

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