Overview: Human Reproduction in NEET
Human Reproduction contributes 5–7 questions in NEET Biology. Hormonal control of the menstrual cycle, spermatogenesis vs oogenesis, fertilisation events, and early embryonic development are high-priority topics.
1. Male Reproductive System
Structure and Pathway of Sperm
Testes
- Located in scrotal sac outside abdominal cavity — 2–3°C below body temperature
- Each testis contains ~250 lobules; each lobule has 1–3 seminiferous tubules
- Two cell types in testes:
- Sertoli cells: Nourish and support developing sperm; form blood-testis barrier; produce inhibin (inhibits FSH)
- Leydig cells (interstitial cells): Between seminiferous tubules; produce testosterone (stimulated by LH)
Accessory Glands (contribute to semen)
| Gland | Volume Contribution | Main Secretion |
|---|---|---|
| Seminal vesicle | ~60% of semen | Fructose (energy for sperm), prostaglandins |
| Prostate gland | ~25–30% of semen | Alkaline fluid (neutralises vaginal acidity) |
| Bulbourethral gland (Cowper's gland) | Small volume | Mucus (lubricates urethra) |
Semen composition: Sperm (~5%) + seminal fluid (95%). Average volume = 2–5 mL per ejaculate.
Spermatogenesis (Sperm Formation)
Duration: ~74 days in humans
Sperm structure:
- Head: contains haploid nucleus (DNA) + acrosome (enzyme-filled cap)
- Midpiece: contains mitochondria (energy for movement)
- Tail (flagellum): propulsion
Hormonal Control of Spermatogenesis
- GnRH (hypothalamus) → stimulates anterior pituitary
- FSH → Sertoli cells → spermatogenesis
- LH → Leydig cells → testosterone secretion → spermatogenesis
- Testosterone also provides feedback to hypothalamus/pituitary (negative feedback)
2. Female Reproductive System
Organs and Functions
| Organ | Function |
|---|---|
| Ovaries | Produce eggs and hormones (estrogen, progesterone) |
| Fallopian tubes (oviducts) | Transport egg; site of fertilisation (ampullary region) |
| Uterus | Site of implantation and foetal development |
| Cervix | Lower uterus; produces mucus (consistency changes with cycle) |
| Vagina | Birth canal; receives semen |
Uterine Wall Layers
- Perimetrium: Outer covering
- Myometrium: Thick smooth muscle layer (contractions during labour)
- Endometrium: Inner lining — shed during menstruation; thickens for implantation
Oogenesis (Egg Formation)
Key difference from spermatogenesis: produces 1 egg + 3 polar bodies (not 4 equal cells)
Memory aid: Oocyte pauses twice — prophase I (years!) and metaphase II (until fertilisation or degeneration).
Comparison: Spermatogenesis vs Oogenesis
| Feature | Spermatogenesis | Oogenesis |
|---|---|---|
| Start | Puberty | Foetal life (prophase I arrest) |
| Products | 4 sperm per cell | 1 egg + 3 polar bodies |
| Meiosis completion | Both divisions completed before sperm formed | Meiosis II completed only at fertilisation |
| Duration | Continuous throughout adult life | One egg per month, depletes with age |
| Location | Seminiferous tubules | Ovarian follicles |
3. Menstrual Cycle (28-Day Cycle)
Phase-by-Phase Breakdown
Phase 1: Menstruation (Days 1–5)
- Corpus luteum degenerates → progesterone and estrogen levels fall sharply
- Endometrium loses blood supply → sheds
- ~50–80 mL blood lost; menstrual flow contains endometrial cells, blood, mucus
Phase 2: Follicular Phase (Days 1–13) — overlaps with menstruation
- FSH from anterior pituitary → several follicles begin developing in ovary
- Dominant follicle (Graafian follicle) matures → secretes increasing amounts of estrogen
- Estrogen → endometrium proliferates and thickens (proliferative phase)
- Rising estrogen → positive feedback → LH surge
Phase 3: Ovulation (Day 14)
- LH surge (peak at day 13–14) → Graafian follicle ruptures
- Secondary oocyte released from ovary → enters fallopian tube
- Remnant follicle → corpus luteum forms (yellow body)
Phase 4: Luteal Phase (Days 15–28)
- Corpus luteum secretes progesterone (and some estrogen)
- Progesterone → endometrium becomes secretory (glands enlarge, vascular)
- If no fertilisation: corpus luteum degenerates after ~10 days
- Progesterone falls → menstruation begins → cycle restarts
Hormonal Summary
| Hormone | Source | Effect in Cycle |
|---|---|---|
| GnRH | Hypothalamus | Stimulates FSH and LH release |
| FSH | Anterior pituitary | Follicle development |
| LH | Anterior pituitary | Ovulation trigger; corpus luteum maintenance |
| Estrogen | Graafian follicle | Endometrial proliferation; LH surge trigger |
| Progesterone | Corpus luteum | Endometrial secretory phase; inhibits FSH/LH |
4. Fertilisation and Implantation
Journey of the Sperm
1. Sperm deposited in vagina (~200–300 million sperm) 2. Swim through cervix and uterus → enter fallopian tube 3. Capacitation: Sperm undergo changes in female tract (~6 hours) that give them ability to fertilise 4. Meet secondary oocyte in ampullary region of fallopian tube 5. Sperm reach ovum within 30 minutes to several hours; viable for ~48–72 hours
Fertilisation Events
Polyspermy prevention: The cortical reaction releases cortical granule contents into the perivitelline space, modifying the zona pellucida so no more sperm can penetrate. This ensures diploid (not triploid or polyploid) offspring.
Early Embryonic Development (Cleavage)
Blastocyst Structure
- Trophoblast: Outer layer of cells → becomes placenta, secretes HCG
- Inner Cell Mass (ICM) / Embryoblast: Will develop into the embryo
Implantation
- Trophoblast cells invade endometrium (endometrial wall)
- HCG (Human Chorionic Gonadotropin) secreted immediately by trophoblast
- HCG maintains corpus luteum → continued progesterone production → endometrium maintained
- HCG detected in urine/blood from ~8–10 days after fertilisation → basis of pregnancy test
5. Embryonic Development
Gastrulation (Week 3)
Formation of three primary germ layers:
| Germ Layer | Derivatives |
|---|---|
| Ectoderm | Skin, nervous system, sense organs, hair, nails |
| Mesoderm | Muscles, skeleton, kidneys, blood, gonads, heart |
| Endoderm | Lining of gut, respiratory tract, liver, pancreas, bladder |
Memory aid: "Skin and nerves are ecto, muscles and bones are meso, gut is endo."
Placenta
The placenta is a disc-shaped organ formed from trophoblast + maternal endometrium.
Functions: 1. Exchange: Oxygen and nutrients from mother to foetus; CO₂ and waste from foetus to mother 2. Hormonal: Produces HCG (early pregnancy), progesterone, estrogen, hPL (human placental lactogen) 3. Protective: Barrier against some pathogens (not all — rubella, HIV, CMV can cross) 4. Immunological: Produces immunoglobulins for foetal immunity
Note: Foetal and maternal blood do NOT directly mix in the placenta — exchange occurs across thin membranes (chorionic villi surrounded by maternal blood in intervillous spaces).
Developmental Milestones
| Time | Development |
|---|---|
| 4 weeks | Heart starts beating; neural tube forms |
| 6 weeks | Limb buds visible; foetal heartbeat on ultrasound |
| 8 weeks | Major organ systems forming; embryo called foetus from this point |
| 12 weeks | All major organs formed; fingers, toes, external genitalia visible |
| 20 weeks | Movements felt by mother (quickening); lanugo (fine hair) visible |
| 24 weeks | Lungs developing; eyes open |
| 40 weeks | Full term; birth (parturition) |
Parturition: Birth triggered by complex hormonal interactions — estrogen rises, oxytocin from posterior pituitary stimulates uterine contractions, prostaglandins amplify contractions.
6. Reproductive Health
Contraceptive Methods
Natural Methods:
- Rhythm method (calendar-based): avoid intercourse near ovulation (day 10–17)
- Lactational amenorrhoea: exclusive breastfeeding inhibits ovulation (effective for ~6 months post-partum)
- Withdrawal (coitus interruptus): least reliable
Barrier Methods:
- Condom (male/female): also protects against STIs
- Diaphragm/cervical cap: covers cervix; used with spermicide
- Spermicides: chemicals that kill sperm
Hormonal Methods:
- Combined oral contraceptive pill: estrogen + progestin; prevents ovulation, thickens cervical mucus
- Mini-pill (progestin only): thickens cervical mucus, may inhibit implantation
- Injections (Depo-Provera): progestin injection every 3 months
- Hormonal implants: subdermal implant releasing progestin
Intrauterine Devices (IUDs):
- Non-medicated IUDs (Lippes loop): physical barrier effect
- Copper IUDs (Cu-T): copper ions are spermicidal; also prevent implantation
- Hormonal IUDs (Mirena): releases progestin locally
Surgical Methods (Permanent):
- Vasectomy: vas deferens cut and tied in males
- Tubectomy (tubal ligation): fallopian tubes cut and tied in females
Infertility and Assisted Reproductive Technologies (ART)
| Technique | Full Form | Process |
|---|---|---|
| IVF | In Vitro Fertilisation | Eggs and sperm combined outside body; embryo transferred to uterus ("test tube baby") |
| ET | Embryo Transfer | Transfer of in-vitro fertilised embryo to uterus |
| GIFT | Gamete Intrafallopian Transfer | Egg and sperm placed directly into fallopian tube |
| ZIFT | Zygote Intrafallopian Transfer | Fertilised egg (zygote) placed in fallopian tube |
| AI | Artificial Insemination | Sperm injected into uterus or cervix |
| ICSI | Intracytoplasmic Sperm Injection | Single sperm injected directly into egg |
Sexually Transmitted Infections (STIs)
| STI | Causative Agent | Key Feature |
|---|---|---|
| AIDS | HIV (Human Immunodeficiency Virus) | Destroys CD4+ T cells; no cure |
| Gonorrhoea | Neisseria gonorrhoeae (bacteria) | Treatable with antibiotics |
| Syphilis | Treponema pallidum (bacteria) | Chancre → systemic disease if untreated |
| Chlamydia | Chlamydia trachomatis | Most common bacterial STI; often asymptomatic |
| Genital herpes | Herpes Simplex Virus 2 | Lifelong; recurrent outbreaks |
| HPV | Human Papillomavirus | Causes genital warts; linked to cervical cancer |
Board Examination Questions
1-Mark Questions
Q1: Name the site of fertilisation in the female reproductive tract. Ampullary region of the fallopian tube (oviduct).
Q2: What is the function of the acrosome? The acrosome is a cap-like structure on the sperm head filled with hydrolytic enzymes (hyaluronidase, acrosin). During fertilisation, the acrosome reaction releases these enzymes to digest the zona pellucida and allow sperm to penetrate the egg.
Q3: Name the hormone responsible for ovulation. LH (Luteinising Hormone) — specifically the LH surge triggers ovulation.
Q4: What is colostrum? Colostrum is the first milk secreted by mammary glands in the first few days after delivery. It is rich in immunoglobulins (especially IgA) and provides passive immunity to the newborn.
Q5: Define amenorrhoea. Absence of menstruation. Can be primary (menstruation never started) or secondary (menstruation stops after previously being normal). Physiological amenorrhoea occurs during pregnancy and breastfeeding.
2-Mark Questions
5-Mark Questions
NEET Focus Points
1. Scrotal temperature: 2–3°C below body temperature — reason for scrotal location 2. Spermatogenesis end-products: 4 sperm from 1 spermatogonium; oogenesis gives 1 egg + 3 polar bodies 3. LH surge = ovulation trigger (not FSH, not estrogen directly) 4. Corpus luteum: Makes progesterone; maintained by HCG in early pregnancy 5. Fertilisation site: Ampullary region of fallopian tube (not uterus, not ovary) 6. HCG: Produced by trophoblast; basis of pregnancy test; maintains corpus luteum 7. ART methods: IVF, GIFT, ZIFT — know the differences