Human Reproduction Class 12 Notes

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 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.

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.

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.

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 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.

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.

Exercises (NCERT)

1. 1
   Fill in the blanks: (a) Humans reproduce ______ (asexually/sexually). (b) Humans are ______ (oviparous/viviparous/ovoviviparous). (c) Fertilisation is ______ in humans (external/internal).
2. 2
   Name the hormones involved in the regulation of the menstrual cycle.
3. 3
   Draw a labelled diagram of the male reproductive system.
4. 4
   Draw a labelled diagram of the female reproductive system.
5. 5
   What is the significance of the scrotum being located outside the abdominal cavity?
6. 6
   Describe the structure of a sperm.
7. 7
   Differentiate between spermatogenesis and oogenesis.
8. 8
   What is meant by the menstrual cycle? Explain its phases.
9. 9
   Name the parts of the fallopian tube where fertilisation occurs.
10. 10
    Explain the process of implantation.
11. 11
    What is the role of the placenta during pregnancy?
12. 12
    Describe the three germ layers in the embryo.
13. 13
    What is parturition? Which hormones induce it?
14. 14
    What is lactation? Name the hormone responsible for it.
15. 15
    Why is colostrum important for a newborn?

Answers at a glance

1. 1
   Humans reproduce sexually; humans are viviparous; fertilisation is internal.
2. 2
   GnRH (hypothalamus), FSH and LH (pituitary), oestrogen and progesterone (ovary).
3. 3
   Male system: testes (in the scrotum), epididymis, vas deferens, seminal vesicles, prostate, Cowper's glands, urethra and penis.
4. 4
   Female system: ovaries, fallopian tubes (oviducts), uterus, cervix, vagina and external genitalia.
5. 5
   The scrotum keeps the testes about 2–2.5 °C below body temperature, which is essential for spermatogenesis.
6. 6
   A sperm has a head (nucleus + acrosome), a neck (centrioles), a middle piece (mitochondria for energy) and a tail (motility).
7. 7
   Spermatogenesis produces four haploid sperm from one spermatogonium; oogenesis produces one ovum and three polar bodies from one oogonium, with unequal divisions.
8. 8
   The menstrual cycle is a ~28-day cyclic change of the female reproductive tract; phases: menstrual, follicular (proliferative), ovulatory and luteal (secretory).
9. 9
   Fertilisation occurs in the ampullary-isthmic junction of the fallopian tube.
10. 10
    Implantation is the attachment of the blastocyst to the endometrium of the uterus.
11. 11
    The placenta provides nutrition and oxygen to the foetus and removes waste; it also secretes hCG, hPL, oestrogen and progesterone.
12. 12
    The three germ layers are ectoderm, mesoderm and endoderm.
13. 13
    Parturition is childbirth, induced by oxytocin (and relaxin); it is a neuroendocrine reflex.
14. 14
    Lactation is milk production by the mammary glands, controlled by the hormone prolactin.
15. 15
    Colostrum contains antibodies (IgA) that give the newborn passive immunity.

MCQ Questions

2 Mark Questions

1. 1
   Write the functions of the Sertoli and Leydig cells.
2. 2
   What is the significance of the acrosome in a sperm?
3. 3
   Name the hormones that regulate the menstrual cycle.
4. 4
   Define implantation and mention its site.
5. 5
   What is the function of colostrum?

3 Mark Questions

1. 1
   Describe the process of spermatogenesis.
2. 2
   Explain the phases of the menstrual cycle.
3. 3
   Describe the process of fertilisation in humans.
4. 4
   Explain the development of the embryo up to implantation.
5. 5
   What is the role of the placenta?

5 Mark Questions

1. 1
   Draw a labelled diagram of the human male reproductive system and describe its parts.
2. 2
   Draw a labelled diagram of the human female reproductive system and describe its parts.
3. 3
   Explain oogenesis and the changes that occur in the ovary during the menstrual cycle.