Human Reproduction - Class 12 Biology

Class 12 Biology | Unit I — Reproduction

Chapter 3: Human Reproduction

Male & Female Reproductive Systems • Gametogenesis • Menstrual Cycle • Fertilisation • Pregnancy • Parturition

1. Male Reproductive System

1.1 Structure

Testes: Primary sex organs. Located in scrotum (outside abdominal cavity, 2–2.5°C below body temp for sperm production). Each testis has seminiferous tubules (site of sperm formation) + interstitial cells (Leydig cells) (produce testosterone).
Seminiferous tubules lined by Sertoli cells (nurse cells — provide nutrition to developing sperm) and spermatogonia (primary germ cells).
Rete testis → Vasa efferentia → Epididymis → Vas deferens → Ejaculatory duct → Urethra: pathway of sperm from testis to outside.
Accessory glands: Seminal vesicles (60% of semen, fructose + prostaglandins), Prostate gland (30%, citric acid, enzymes), Bulbourethral glands / Cowper's glands (5%, lubricating mucus, alkaline).
Semen: Sperm + secretions of accessory glands. Volume: 2–5 mL per ejaculation, ~200–300 million sperm.

1.2 Spermatogenesis

Spermatogenesis: Process of formation of spermatozoa from spermatogonia in seminiferous tubules. Begins at puberty under FSH and testosterone.

Spermatogonia (2n, type A and B) → [mitosis] → Primary spermatocytes (2n, 46 chromosomes) → [Meiosis I] → Secondary spermatocytes (n, 23 chromosomes) → [Meiosis II] → Spermatids (n, 23 chromosomes) → [Spermiogenesis] → Spermatozoa (sperm, n).

Spermiogenesis: Transformation of spermatids into spermatozoa (elongation, acrosome formation, flagellum development, cytoplasm reduction). Cytoplasm shed as residual body.

Spermiation: Release of mature sperm from Sertoli cells into lumen of seminiferous tubule.

1.3 Structure of Sperm

Head: Contains acrosome (derived from Golgi body; contains hydrolytic enzymes for penetrating egg) + nucleus (haploid, condensed DNA).
Middle piece: Contains numerous mitochondria (ATP for motility).
Tail: Long flagellum for motility. Contains axoneme (9+2 microtubule arrangement) surrounded by fibrous sheath.
Plasma membrane covers entire sperm.

⚠️ NEET Focus (2013, 2016, 2019, 2022): Sertoli cells = nurse cells (nourish sperm). Leydig cells = testosterone. Acrosome = from Golgi apparatus. Middle piece = mitochondria. Spermatogonia → Primary spermatocyte → Secondary spermatocyte → Spermatid → Sperm. Meiosis I = primary → secondary. Meiosis II = secondary → spermatid.

2. Female Reproductive System

2.1 Ovary

Two ovaries, 2–4 cm long. Primary female sex organs.
Outer cortex: contains follicles (at different stages). Inner medulla: connective tissue, blood vessels.
At birth: ~2 million primary follicles (primary oocytes). By puberty: ~40,000 remain. Lifetime: ~400 ovulations (1 per month for ~35 years).

2.2 Oogenesis

Oogenesis: Formation of mature female gametes (ova/eggs) from oogonia. Begins in fetal ovary; completed at fertilisation.

Oogonia (2n) → [mitosis, in fetal ovary] → Primary oocytes (2n) → [starts Meiosis I, arrested in prophase I until puberty] → at each cycle: Primary oocyte completes Meiosis I → Secondary oocyte (n) + 1st polar body (n). Secondary oocyte arrested in Meiosis II (metaphase II). Released at ovulation. Meiosis II completed only if fertilisation occurs → Ovum (n) + 2nd polar body (n).

2.3 Follicle Development

Stage	Description
Primordial follicle	Primary oocyte surrounded by single layer of flattened granulosa cells.
Primary follicle	Oocyte enlarges; granulosa cells become cuboidal/columnar (multilayered).
Secondary follicle	Theca cells form; zona pellucida appears (glycoprotein coat around oocyte). Antrum not yet formed.
Tertiary/Graafian follicle	Antrum (fluid-filled cavity) develops. Ovum pushed to one side = cumulus oophorus. Ready for ovulation.
Corpus luteum	Forms after ovulation from ruptured follicle cells. Produces progesterone (+ oestrogen). Maintains pregnancy. Degenerates if no fertilisation → progesterone drops → menstruation.

⚠️ NEET Focus: At birth: 2 million oocytes. At puberty: 40,000. Lifetime ovulations: ~400. Zona pellucida = glycoprotein coat around egg. Antrum = in Graafian follicle. Corpus luteum → progesterone. Primary oocyte arrested in prophase I; secondary oocyte in metaphase II.

3. Menstrual Cycle

Menstrual cycle: Cyclic changes in female reproductive system over ~28 days. First menstruation: menarche (puberty, ~12–13 yrs). Cycles stop at menopause (~48–50 yrs). Menarche to menopause = reproductive period.

Phase	Days (approx.)	Events	Hormones
Menstrual phase	Day 1–5	Uterine lining (endometrium) shed. Bleeding occurs. Corpus luteum degenerates; progesterone + oestrogen fall.	↓ Progesterone, ↓ oestrogen
Follicular/Proliferative phase	Day 6–13	FSH rises → follicle development → oestrogen secretion → endometrium thickens (rebuilds). Primary → Graafian follicle.	↑ FSH, ↑ oestrogen
Ovulation	Day 14	LH surge (LH peak) → Graafian follicle ruptures → secondary oocyte released. Corpus luteum forms.	↑↑ LH surge, ↑ FSH
Luteal/Secretory phase	Day 15–28	Corpus luteum secretes progesterone + oestrogen → further endometrium thickening (secretory, prepares for implantation). If no fertilisation → CL degenerates → progesterone drops → menstruation.	↑ Progesterone, ↑ oestrogen (then ↓ both)

⚠️ NEET Focus (2014, 2017, 2019, 2021, 2022): Ovulation on Day 14. Triggered by LH surge. Corpus luteum → progesterone. No pregnancy → CL degenerates → progesterone falls → menstruation. FSH stimulates follicular growth. Menarche ~12–13 yrs, menopause ~48–50 yrs. Cycle length = 28 days (average).

4. Fertilisation and Early Embryonic Development

4.1 Fertilisation

Secondary oocyte (arrested in metaphase II) is released at ovulation → picked up by fimbriae of fallopian tube.
Fertilisation occurs in ampullary–isthmic junction of Fallopian tube (usually).
Sperm: acrosomal enzymes (hyaluronidase, acrosin) penetrate corona radiata then zona pellucida.
Sperm-egg fusion (sperm head enters oocyte) → stimulates completion of Meiosis II of secondary oocyte → ovum + 2nd polar body formed.
Sperm and egg nuclei fuse → Zygote (2n). Cortical reaction: changes in zona pellucida prevent polyspermy.

4.2 Cleavage and Blastulation

Cleavage: Series of rapid mitotic divisions of zygote without cell growth (cells getting smaller). 2 → 4 → 8 → 16 cells. 16-cell stage = morula.
Morula continues dividing → blastocyst: hollow ball with inner cell mass (ICM / embryoblast) + outer layer trophoblast.
Trophoblast: forms placenta, chorion; attaches to endometrium.
ICM: forms the embryo proper (3 germ layers).
Implantation: Blastocyst embeds in uterine endometrium (~7 days after fertilisation). Trophoblast cells release enzymes, erode endometrium.

4.3 Gastrulation and Placenta

ICM → Gastrulation: 3 germ layers form: Ectoderm (skin, nervous system), Mesoderm (muscles, bones, blood), Endoderm (gut, lungs).
Placenta: disc-shaped organ connecting embryo to uterine wall via umbilical cord. Functions: exchange of gases, nutrients, solutes, waste between mother and embryo. Endocrine organ: produces hCG (human chorionic gonadotropin — maintains CL in early pregnancy), progesterone, oestrogen, human placental lactogen (hPL).
hCG detected in urine → basis of pregnancy test.

4.4 Parturition

Normal gestation period = 9 months (40 weeks / ~280 days) from last menstrual period. Actual development ~38 weeks from fertilisation.
Parturition triggered by complex neuroendocrine signals — oxytocin causes uterine contractions (positive feedback).
Baby delivered first (head), followed by placenta (afterbirth).
Lactation: Prolactin stimulates milk production. Oxytocin (milk ejection reflex). First milk = colostrum (rich in IgA antibodies — passive immunity to newborn).

⚠️ NEET Focus (2015, 2017, 2020, 2022): Fertilisation = ampullary-isthmic junction. 16-cell = morula. Blastocyst = trophoblast + ICM. Implantation at ~7 days. hCG = pregnancy test hormone (from trophoblast, maintains CL). Gestation = 9 months. Parturition hormone = oxytocin. Colostrum = first milk, rich in IgA.

🎓 Key NEET Questions

Q1. [NEET 2022] Which hormone triggers ovulation?

Answer LH (Luteinising Hormone) surge on Day 14 of the menstrual cycle triggers ovulation — rupture of the Graafian follicle and release of the secondary oocyte. High oestrogen (from mature Graafian follicle) triggers the LH surge (positive feedback on anterior pituitary).

Q2. [NEET 2021] The inner cell mass in the blastocyst develops into:

(a) Placenta (b) Trophoblast (c) Embryo proper (d) Chorion

Answer: (c) The Inner Cell Mass (ICM / embryoblast) develops into the embryo proper through gastrulation (3 primary germ layers: ectoderm, mesoderm, endoderm). The trophoblast develops into the placenta and extraembryonic membranes.

Q3. [NEET 2020] Which phase of oogenesis is completed only at fertilisation?

Answer The secondary oocyte is arrested in Metaphase II of Meiosis (Meiosis II) after ovulation. Meiosis II is completed only at fertilisation (when sperm fuses with the egg) → mature ovum + second polar body formed.

Q4. [NEET 2019] Corpus luteum produces:

(a) FSH (b) LH (c) Oestrogen only (d) Progesterone and oestrogen

Answer: (d) Corpus luteum (formed from ruptured Graafian follicle after ovulation) produces primarily progesterone (large amounts) and also oestrogen. Progesterone maintains endometrium for implantation and in early pregnancy until placenta takes over.

Q5. [NEET 2018] Where does fertilisation normally occur in humans?

Answer Fertilisation normally occurs in the ampullary–isthmic junction of the Fallopian tube (oviduct). The secondary oocyte is released at ovulation → swept by fimbriae into the fallopian tube → sperm travel from vagina through uterus into tube → fertilisation occurs in the ampullary region.

Q6. [NEET 2017] Acrosome of sperm is derived from which organelle?

(a) Nucleus (b) Golgi body (c) Mitochondria (d) Lysosome

Answer: (b) The acrosome is a cap-like structure covering the sperm head. It is derived from the Golgi body (Golgi apparatus) during spermiogenesis. It contains hydrolytic enzymes (hyaluronidase, acrosin) that help the sperm penetrate corona radiata and zona pellucida of the egg.

💡 Rapid Revision Checklist

Sertoli = nurse cells; Leydig = testosterone; Spermatogonia → 1° → 2° → spermatid → sperm (meiosis)
Acrosome = Golgi; Middle piece = mitochondria; Tail = flagellum
At birth: 2 million oocytes; Puberty: 40,000; Lifetime ~400 ovulations
Zona pellucida = glycoprotein. 1° oocyte arrested in prophase I; 2° in metaphase II (until fertilisation)
Ovulation = Day 14, triggered by LH surge. Corpus luteum → progesterone
Fertilisation: ampullary–isthmic junction. 16-cell = morula. Blastocyst = trophoblast + ICM
Implantation ~Day 7 after fertilisation. hCG = pregnancy hormone (urine test)
Parturition = oxytocin. Gestation = 9 months. Colostrum = IgA (passive immunity)

NCERT Solutions - Human Reproduction - Class 12

CLASS 12 BIOLOGY | NCERT SOLUTIONS

Chapter 3 — Human Reproduction

All NCERT Exercise Questions with Detailed Solutions

📋 Note: All questions from NCERT Class 12 Biology Chapter 3 Exercise. Hormone names, cycle phases, and embryonic stages as per NCERT.

NCERT Exercise Questions & Solutions

2 MarksQ1. Fill in the blanks: (a) Humans reproduce ___. (b) Humans are ___. (c) Fertilisation is ___ in humans. (d) Male and female gametes are ___. (e) Zygote is ___ in humans.

✓ Answer
(a) Humans reproduce sexually.
(b) Humans are viviparous (give birth to young ones; not egg-laying).
(c) Fertilisation is internal (occurs inside the female body, in the Fallopian tube).
(d) Male and female gametes are haploid (n = 23 chromosomes).
(e) Zygote is diploid (2n = 46 chromosomes; formed by fusion of haploid gametes).

5 MarksQ2. Draw a labelled diagram of the male reproductive system and briefly describe its functions.

✓ Answer
(Diagram as per NCERT textbook page)

Components and Functions:

Structure	Function
Testes (in scrotum)	Produce spermatozoa (spermatogenesis) and testosterone (Leydig cells). Scrotum = 2°C below body temp for optimal sperm production.
Epididymis	Storage and maturation of sperm. Sperm acquire motility and fertilising capacity here.
Vas deferens	Muscular tube that carries sperm from epididymis to ejaculatory duct during ejaculation.
Seminal vesicles	Secrete ~60% of semen volume; contain fructose (energy for sperm), prostaglandins, fibrinogen.
Prostate gland	Secretes ~30% of semen; citric acid, enzymes, zinc. Activates sperm and provides buffering.
Bulbourethral glands	Secrete lubricating alkaline mucus; pre-ejaculatory fluid. Neutralise acidic urine in urethra.
Urethra / Penis	Common passage for urine and semen. Penis delivers semen into female reproductive tract.

5 MarksQ3. Describe the menstrual cycle. How is it regulated by hormones?

✓ Answer
The menstrual cycle is the cyclic reproductive event in the female reproductive system occurring approximately every 28 days (from menarche to menopause).

Phase	Days	Key Events	Hormones
Menstrual	1–5	Endometrium shed; bleeding	↓ Progesterone, ↓ Oestrogen
Follicular	6–13	FSH → follicle grows → oestrogen → endometrium rebuilds	↑ FSH, ↑ Oestrogen
Ovulation	14	LH surge → Graafian follicle ruptures → oocyte released	↑↑ LH peak
Luteal	15–28	Corpus luteum → Progesterone → secretory endometrium; if no fertilisation: CL degenerates → menstruation	↑ Progesterone

Hormonal regulation:

GnRH from hypothalamus → stimulates anterior pituitary to release FSH and LH.
FSH → follicle development → oestrogen secretion.
Rising oestrogen → positive feedback → LH surge → ovulation.
After ovulation: corpus luteum secretes progesterone → maintains endometrium.
High progesterone → negative feedback → suppresses FSH/LH → no new follicle grows.
If no fertilisation: corpus luteum degenerates → progesterone falls → endometrium shed = menstruation.

5 MarksQ4. Describe the process of spermatogenesis. What are the hormones involved in regulation of spermatogenesis?

✓ Answer
Spermatogenesis: production of spermatozoa in the seminiferous tubules of testes. Begins at puberty, continues throughout life.

Steps:

Undifferentiated spermatogonia (2n) divide by mitosis → Type A (stem cells) + Type B (go on to differentiate).
Type B spermatogonia grow into primary spermatocytes (2n, 4n DNA after replication).
Meiosis I: Primary spermatocytes → secondary spermatocytes (n; 23 chromosomes, 2 chromatids).
Meiosis II: Secondary spermatocytes → spermatids (n; 23 single chromosomes).
Spermiogenesis: Spermatids differentiate into spermatozoa — elongation, acrosome formation (from Golgi), flagellum (9+2 axoneme), mitochondria concentrated in middle piece, cytoplasm reduced.
Spermiation: Mature sperm released from Sertoli cells into tubule lumen.

Hormonal regulation:

GnRH (hypothalamus) → FSH + LH release from anterior pituitary.
FSH: Stimulates Sertoli cells → produce androgen-binding protein (ABP) → concentrates testosterone in seminiferous tubules → supports spermatogenesis.
LH (ICSH): Stimulates Leydig (interstitial) cells → secrete testosterone → essential for spermatogenesis.

5 MarksQ5. Define spermiogenesis and spermiation.

✓ Answer
Spermiogenesis: The process of transformation/differentiation of spermatids (round, non-motile) into mature spermatozoa (elongated, motile). It involves:

Formation of acrosome (from Golgi apparatus).
Condensation of nucleus.
Development of flagellum (from centriole/axoneme 9+2 microtubule structure).
Migration of mitochondria to middle piece.
Shedding of excess cytoplasm as residual body.

Spermiation: The process by which mature spermatozoa are released from the Sertoli cells into the lumen (central space) of the seminiferous tubules. After spermiation, sperm are transported to the epididymis where they are stored and acquire full motility.

3 MarksQ6. What is the role of the placenta in human pregnancy?

✓ Answer
Placenta: A disc-shaped structure connecting the developing embryo/fetus to the uterine wall via the umbilical cord. A structural and functional unit formed by chorionic villi of embryo interdigitating with uterine tissues.

Functions:

Nutrition: Transports nutrients (glucose, amino acids, fatty acids) from maternal blood to fetal blood (via diffusion and active transport).
Gas exchange: O₂ from mother → fetus; CO₂ from fetus → mother.
Waste removal: Metabolic wastes (urea, uric acid) from fetus → maternal circulation.
Endocrine organ: Secretes hCG (human chorionic gonadotropin — maintains corpus luteum in early pregnancy), progesterone, oestrogen, and human placental lactogen (hPL).
Immune protection: Transfers maternal IgG antibodies to fetus (passive immunity).

Facts Capsule - Human Reproduction - Class 12

CLASS 12 BIOLOGY | NEET RAPID CAPSULE

Facts & High-Yield Points

Chapter 3 — Human Reproduction | 20 Key Facts for NEET

♂ Male System & Spermatogenesis

FACT #01 — Testis Cells

Sertoli cells: nurse/sustentacular cells, nourish developing sperm, secrete ABP (androgen-binding protein). Leydig (interstitial) cells: between tubules, secrete testosterone. LH stimulates Leydig; FSH stimulates Sertoli.

FACT #02 — Spermatogenesis Stages

Spermatogonia (2n) → [Mitosis] → Primary spermatocyte (2n) → [Meiosis I] → Secondary spermatocyte (n) → [Meiosis II] → Spermatid (n) → [Spermiogenesis] → Sperm (n).

FACT #03 — Sperm Structure

Head: acrosome (from Golgi, enzymes) + nucleus. Middle piece: mitochondria (ATP). Tail: flagellum (9+2 axoneme, motility). Acrosome = Golgi. Spermiogenesis: spermatid → sperm. Spermiation: release into tubule lumen.

FACT #04 — Accessory Glands

Seminal vesicles = 60% of semen (fructose, prostaglandins). Prostate = 30% (citric acid, enzymes, zinc). Bulbourethral (Cowper's) = lubricant, alkaline, pre-ejaculatory. Scrotum = 2°C below body temp. Sperm count = 200–300 million per ejaculation.

♀ Female System & Oogenesis

FACT #05 — Oocyte Numbers

At birth: 2 million primary oocytes. At puberty: 40,000. Lifetime ovulations: ~400. All primary oocytes arrested in prophase I of Meiosis I from fetal period until puberty.

FACT #06 — Oogenesis Arrests

Primary oocyte: arrested in Prophase I. Released from arrest each cycle → completes Meiosis I → secondary oocyte (n) + 1st polar body. Secondary oocyte: arrested in Metaphase II (released at ovulation). Meiosis II completed ONLY if fertilisation occurs.

FACT #07 — Follicle Stages

Primordial → Primary → Secondary (zona pellucida appears) → Graafian (antrum forms) → Ovulation → Corpus luteum (progesterone). Zona pellucida = glycoprotein coat. Antrum = hallmark of Graafian follicle.

📅 Menstrual Cycle

FACT #08 — Menstrual Cycle Master Table

Phase	Days	Events	Key Hormones
Menstrual	1–5	Endometrium shed, bleeding	↓ P4, ↓ Oestrogen
Follicular	6–13	Follicle grows; endometrium rebuilds	↑ FSH, ↑ Oestrogen
Ovulation	Day 14	Graafian follicle ruptures; oocyte released	↑↑ LH surge
Luteal	15–28	Corpus luteum; secretory endometrium	↑ Progesterone

Menarche (~12–13 yrs, first period). Menopause (~48–50 yrs). Cycle = 28 days (average). Ovulation triggered by LH surge.

🐱 Fertilisation & Development

FACT #09 — Fertilisation Site

Fertilisation occurs in ampullary-isthmic junction of Fallopian tube. Acrosome enzymes (hyaluronidase, acrosin) penetrate corona radiata then zona pellucida. Cortical reaction prevents polyspermy.

FACT #10 — Cleavage and Blastocyst

Zygote (2n) → cleavage (mitosis, cells smaller) → 2→4→8→16 cells = Morula → Blastocyst (hollow). Blastocyst = outer trophoblast (placenta, chorion) + inner ICM/embryoblast (embryo proper).

FACT #11 — Implantation and hCG

Implantation: blastocyst embeds in uterine endometrium ~7 days after fertilisation. hCG (human chorionic gonadotropin): secreted by trophoblast, maintains corpus luteum in early pregnancy. Basis of pregnancy test (urine hCG).

FACT #12 — Placenta and Parturition

Placenta: exchanges nutrients, gases, waste. Endocrine: hCG, progesterone, oestrogen, hPL. Gestation = 9 months. Parturition triggered by oxytocin (uterine contractions, positive feedback). Colostrum = first milk = rich in IgA (passive immunity).

🧠 Mnemonics — Remember Fast

Spermatogenesis: “PC-SS” Primary spermatocyte (2n) → [Meiosis I] → Secondary spermatocyte (n) → [Meiosis II] → Spermatid → [spermiogenesis] → Sperm. Sertoli = support. Leydig = testosterone.

Cycle: “MFOL” Menstrual (D1-5) → Follicular (D6-13, FSH/Oestrogen) → Ovulation (D14, LH surge) → Luteal (D15-28, Progesterone). No fertilisation → Menstruation restarts.

Oocyte Arrests: “P1 → M2” Primary oocyte arrested in Prophase I (from fetal life to puberty). Secondary oocyte arrested in Metaphase II (ovulation to fertilisation). Meiosis II completes only at fertilisation.

Development: “ZMB” Zygote → cleavage → Morula (16-cell) → Blastocyst (trophoblast + ICM) → Implantation (~Day 7). hCG = pregnancy hormone from trophoblast.

🔢 Critical Numbers — Never Forget

2 million — oocytes at birth 40,000 — oocytes at puberty ~400 — lifetime ovulations Day 14 — ovulation (LH surge) 28 days — menstrual cycle 16 cells — morula stage ~7 days — implantation after fertilisation 9 months — gestation period hCG — pregnancy hormone (urine test) IgA — in colostrum (passive immunity)

📱 Practice MCQs for this topic inside our App