Human Reproduction — Short Notes
Humans are viviparous and reproduce sexually with distinct reproductive events: gametogenesis, insemination, fertilisation, implantation, gestation, and parturition.
Male Reproductive System
- Testes — in scrotum (temp ~2–2.5°C < body); each has ~250 lobules, each with 1–3 seminiferous tubules.
- Seminiferous tubule wall: germ cells (spermatogonia) + Sertoli cells (nurse cells).
- Leydig cells in interstitial tissue secrete testosterone.
- Accessory ducts: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.
- Accessory glands: seminal vesicles, prostate, bulbourethral (Cowper's) glands — together secrete seminal plasma.
Female Reproductive System
- Ovaries — each has 1–2 lakh primary follicles at birth (rest degenerate).
- Fallopian tubes = oviducts: infundibulum → ampulla → isthmus → uterus.
- Uterus wall: perimetrium (outer) · myometrium (smooth muscle, thick) · endometrium (glandular, sheds during menstruation).
- Cervix connects uterus to vagina; cervix + vagina = birth canal.
- Mammary glands — modified sweat glands; each has 15–20 mammary lobes.
Gametogenesis
Spermatogenesis (in seminiferous tubules)
- Spermatogonia (2n) → primary spermatocytes (2n) → meiosis I → 2 secondary spermatocytes (n) → meiosis II → 4 spermatids (n) → spermiogenesis → 4 sperms.
- Starts at puberty under GnRH (hypothalamus) → FSH & LH (anterior pituitary). LH stimulates Leydig cells → testosterone. FSH acts on Sertoli cells → factors for spermiogenesis.
- Sperm structure: head (nucleus + acrosome), neck, middle piece (mitochondria — energy), tail (flagellum).
- Semen = sperm + seminal plasma; 200–300 million sperms per ejaculation; must have >60% normal morphology and >40% motility for fertility.
Oogenesis (in ovaries)
- Begins during embryonic (foetal) development — millions of oogonia form; enter meiosis I & arrest at prophase I → primary oocytes.
- ~1–2 lakh primary follicles at birth; ~60,000–80,000 by puberty.
- Each menstrual cycle, some follicles grow → primary → secondary → tertiary → Graafian follicle.
- Primary oocyte completes meiosis I → secondary oocyte + 1st polar body.
- Secondary oocyte starts meiosis II and arrests at metaphase II; completes meiosis II only after fertilisation.
- Ovulation = release of secondary oocyte from Graafian follicle.
Menstrual Cycle (~28 days)
- Menstrual phase (Days 1–5): endometrium sheds if no fertilisation.
- Follicular / proliferative phase (Days 6–13): follicles grow (FSH); endometrium regenerates (oestrogen).
- Ovulation (Day ~14): LH surge → Graafian follicle ruptures.
- Luteal / secretory phase (Days 15–28): corpus luteum secretes progesterone; endometrium becomes secretory; if no fertilisation, corpus luteum degenerates → menstruation.
- Menarche — first menstruation (puberty). Menopause — cessation (~45–50 years).
Fertilisation & Implantation
- Occurs in the ampullary–isthmic junction of oviduct.
- Sperm penetrates zona pellucida via acrosomal enzymes → cortical reaction blocks polyspermy.
- Meiosis II of secondary oocyte completes → ovum + 2nd polar body.
- Sperm nucleus + ovum nucleus → zygote (2n).
- Zygote undergoes cleavage → morula (8, 16 blastomeres) → blastocyst (has trophoblast + inner cell mass) as it moves to uterus.
- Implantation: blastocyst embeds in the endometrium (~day 7).
Pregnancy & Placenta
- Placenta forms from trophoblastic villi + maternal blood sinuses.
- Functions: nutrition, respiratory gas exchange, excretion, and endocrine (hCG, hPL, oestrogen, progesterone; later relaxin).
- hCG — detected in urine → pregnancy test.
- Gestation ~9 months (280 days from LMP). Heartbeat detectable by end of month 1; limbs and digits by 3 months; hair by 5; largely developed by 6.
Parturition & Lactation
- Signal from foetus + placenta → foetal ejection reflex → oxytocin from posterior pituitary → uterine contractions → intense positive feedback → birth.
- Colostrum (first milk, first few days) — rich in antibodies (IgA); critical for infant immunity.
- Lactation maintained by prolactin; milk ejection by oxytocin.
Summary
- Gamete production in humans is meiosis-based; spermatogenesis is continuous, oogenesis pauses at prophase I and only completes on fertilisation.
- Menstrual cycle is orchestrated by FSH, LH, oestrogen, progesterone; ovulation depends on the LH surge.
- Fertilisation happens in the oviduct; the blastocyst implants in the endometrium; the placenta supports the foetus for 9 months.
Human Reproduction — Short Notes
Humans are viviparous and reproduce sexually with distinct reproductive events: gametogenesis, insemination, fertilisation, implantation, gestation, and parturition.
Male Reproductive System
- Testes — in scrotum (temp ~2–2.5°C < body); each has ~250 lobules, each with 1–3 seminiferous tubules.
- Seminiferous tubule wall: germ cells (spermatogonia) + Sertoli cells (nurse cells).
- Leydig cells in interstitial tissue secrete testosterone.
- Accessory ducts: rete testis → vasa efferentia → epididymis → vas deferens → ejaculatory duct → urethra.
- Accessory glands: seminal vesicles, prostate, bulbourethral (Cowper's) glands — together secrete seminal plasma.
Female Reproductive System
- Ovaries — each has 1–2 lakh primary follicles at birth (rest degenerate).
- Fallopian tubes = oviducts: infundibulum → ampulla → isthmus → uterus.
- Uterus wall: perimetrium (outer) · myometrium (smooth muscle, thick) · endometrium (glandular, sheds during menstruation).
- Cervix connects uterus to vagina; cervix + vagina = birth canal.
- Mammary glands — modified sweat glands; each has 15–20 mammary lobes.
Gametogenesis
Spermatogenesis (in seminiferous tubules)
- Spermatogonia (2n) → primary spermatocytes (2n) → meiosis I → 2 secondary spermatocytes (n) → meiosis II → 4 spermatids (n) → spermiogenesis → 4 sperms.
- Starts at puberty under GnRH (hypothalamus) → FSH & LH (anterior pituitary). LH stimulates Leydig cells → testosterone. FSH acts on Sertoli cells → factors for spermiogenesis.
- Sperm structure: head (nucleus + acrosome), neck, middle piece (mitochondria — energy), tail (flagellum).
- Semen = sperm + seminal plasma; 200–300 million sperms per ejaculation; must have >60% normal morphology and >40% motility for fertility.
Oogenesis (in ovaries)
- Begins during embryonic (foetal) development — millions of oogonia form; enter meiosis I & arrest at prophase I → primary oocytes.
- ~1–2 lakh primary follicles at birth; ~60,000–80,000 by puberty.
- Each menstrual cycle, some follicles grow → primary → secondary → tertiary → Graafian follicle.
- Primary oocyte completes meiosis I → secondary oocyte + 1st polar body.
- Secondary oocyte starts meiosis II and arrests at metaphase II; completes meiosis II only after fertilisation.
- Ovulation = release of secondary oocyte from Graafian follicle.
Menstrual Cycle (~28 days)
- Menstrual phase (Days 1–5): endometrium sheds if no fertilisation.
- Follicular / proliferative phase (Days 6–13): follicles grow (FSH); endometrium regenerates (oestrogen).
- Ovulation (Day ~14): LH surge → Graafian follicle ruptures.
- Luteal / secretory phase (Days 15–28): corpus luteum secretes progesterone; endometrium becomes secretory; if no fertilisation, corpus luteum degenerates → menstruation.
- Menarche — first menstruation (puberty). Menopause — cessation (~45–50 years).
Fertilisation & Implantation
- Occurs in the ampullary–isthmic junction of oviduct.
- Sperm penetrates zona pellucida via acrosomal enzymes → cortical reaction blocks polyspermy.
- Meiosis II of secondary oocyte completes → ovum + 2nd polar body.
- Sperm nucleus + ovum nucleus → zygote (2n).
- Zygote undergoes cleavage → morula (8, 16 blastomeres) → blastocyst (has trophoblast + inner cell mass) as it moves to uterus.
- Implantation: blastocyst embeds in the endometrium (~day 7).
Pregnancy & Placenta
- Placenta forms from trophoblastic villi + maternal blood sinuses.
- Functions: nutrition, respiratory gas exchange, excretion, and endocrine (hCG, hPL, oestrogen, progesterone; later relaxin).
- hCG — detected in urine → pregnancy test.
- Gestation ~9 months (280 days from LMP). Heartbeat detectable by end of month 1; limbs and digits by 3 months; hair by 5; largely developed by 6.
Parturition & Lactation
- Signal from foetus + placenta → foetal ejection reflex → oxytocin from posterior pituitary → uterine contractions → intense positive feedback → birth.
- Colostrum (first milk, first few days) — rich in antibodies (IgA); critical for infant immunity.
- Lactation maintained by prolactin; milk ejection by oxytocin.
Summary
- Gamete production in humans is meiosis-based; spermatogenesis is continuous, oogenesis pauses at prophase I and only completes on fertilisation.
- Menstrual cycle is orchestrated by FSH, LH, oestrogen, progesterone; ovulation depends on the LH surge.
- Fertilisation happens in the oviduct; the blastocyst implants in the endometrium; the placenta supports the foetus for 9 months.