Human Reproduction

(a) Sperm is produced in: Seminiferous tubules (located in the testes) (b) Ovum is released from the: Graafian follicle (of the ovary) — this process is called ovulation, triggered by LH surge, typically on day 14 of a 28-day cycle (c) Placenta is the connection between: Foetus (developing embryo) and Mother (uterine wall / endometrium) • Placenta allows exchange of nutrients, oxygen, and waste between maternal and foetal blood without direct mixing of blood (d) LH surge in mid-cycle triggers: Ovulation (release of the ovum from the mature Graafian follicle) • The LH (Luteinising Hormone) surge on approximately day 14 causes the mature Graafian follicle to rupture and release the secondary oocyte

The male reproductive system consists of the following organs: Primary sex organs: • Testes (paired): Located in scrotal sac outside abdomen (temperature ~2°C lower for spermatogenesis). Each testis has lobules containing seminiferous tubules where spermatogenesis occurs. Interstitial cells (Leydig cells) between tubules secrete testosterone. Accessory ducts: • Rete testis → Vasa efferentia → Epididymis (comma-shaped, site of sperm maturation and storage) → Vas deferens (carries sperm to ejaculatory duct) → Ejaculatory duct → Urethra Accessory glands: • Seminal vesicles: Secrete fructose-rich fluid (60% of semen volume) — energy for sperm • Prostate gland: Secretes slightly alkaline fluid (neutralises vaginal acidity) • Bulbourethral glands (Cowper's glands): Secrete mucus for lubrication External genitalia: • Penis: Delivers semen/urine; contains erectile tissue (corpora cavernosa) • Glans penis: Covered by prepuce (foreskin) Note: Semen = spermatozoa + secretions from accessory glands. Normal ejaculate: 2–5 mL, 200–300 million sperm.

Spermatogenesis: Process of formation of spermatozoa (sperm) from spermatogonia in the testes. Steps: 1. Spermatogonia (2n, diploid stem cells) in seminiferous tubules 2. Mitotic divisions → increase in number of spermatogonia 3. Primary spermatocytes (2n) form from spermatogonia 4. Primary spermatocytes undergo Meiosis I → Two secondary spermatocytes (n, haploid) 5. Secondary spermatocytes undergo Meiosis II → Four spermatids (n, haploid) 6. Spermatids are rounded, non-motile — they transform into spermatozoa 7. This transformation of spermatids → spermatozoa is called spermiogenesis Spermiogenesis vs Spermatogenesis: Spermatogenesis: • The entire process of sperm formation from spermatogonia to spermatozoa • Involves both mitosis and meiosis • Results in 4 spermatids from each primary spermatocyte • Broader term encompassing all stages Spermiogenesis: • Only the final step — morphological transformation of spermatid → spermatozoon • Does NOT involve cell division • Involves: condensation of nucleus, formation of acrosome (from Golgi), development of flagellum (from centriole), loss of most cytoplasm • Spermatids become streamlined, motile spermatozoa • Narrow term — just the differentiation/maturation step After spermiogenesis, sperm are released from Sertoli cells (nurse cells) into the lumen of seminiferous tubules — this is called spermiation.

Spermiogenesis: • The process of transformation of non-motile, rounded spermatids into morphologically mature, motile spermatozoa • Does NOT involve cell division • Changes during spermiogenesis: (a) Golgi complex → acrosome (cap over nucleus, contains hydrolytic enzymes for penetrating egg) (b) Centrioles → axoneme (flagellum) for motility (c) Nucleus becomes condensed and elongated (d) Mitochondria aggregate in middle piece (for ATP production) (e) Most cytoplasm is shed as residual body • Result: slender, motile spermatozoon with head, neck, middle piece, and tail Spermiation: • The process of release of mature spermatozoa from Sertoli cells (nurse cells) into the lumen of the seminiferous tubules • Sertoli cells support, nourish, and protect developing sperm cells • During spermiation, the cytoplasmic connections between Sertoli cells and sperm are broken • Released sperm are transported to the epididymis where they are stored and undergo final functional maturation In summary: Spermiogenesis = spermatid → spermatozoon (morphological change); Spermiation = release of spermatozoa into the tubule lumen.

Structure of human ovum (secondary oocyte): The human ovum is a large, non-motile cell (~100–120 μm diameter). Layers from outside to inside: 1. Corona radiata: • Outermost layer • Multiple layers of follicular cells (granulosa cells) held together by hyaluronic acid • Provides protection and initial contact for sperm 2. Zona pellucida: • Thick, transparent glycoprotein coat • Contains ZP3 protein — acts as species-specific sperm receptor • Site of acrosome reaction and block to polyspermy 3. Vitelline membrane: • Plasma membrane of the ovum • After fertilisation, it becomes fertilisation membrane (prevents polyspermy) 4. Cytoplasm (ooplasm): • Contains organelles, yolk (vitellus), and various inclusions • Yolk provides nutrition for developing embryo • Contains maternal mRNA and proteins 5. Nucleus (germinal vesicle in primary oocyte): • In secondary oocyte (released at ovulation), meiosis I is complete • Arrested at metaphase II — completes only upon fertilisation • Large, vesicular nucleus 6. Polar body: • First polar body formed after meiosis I — lies between zona pellucida and vitelline membrane • Second polar body forms only after fertilisation

(a) Corpus luteum: • Yellow glandular body formed from ruptured Graafian follicle after ovulation • Secretes progesterone (and some oestrogen) • Progesterone maintains uterine endometrium for implantation and pregnancy • If fertilisation occurs: corpus luteum persists (maintained by HCG from embryo) • If no fertilisation: degenerates → corpus albicans → progesterone falls → menstruation (b) Endometrium: • Glandular inner lining of the uterus • Undergoes cyclic changes during menstrual cycle • Proliferates under oestrogen; becomes secretory under progesterone • Site of implantation of blastocyst • Shed during menstruation if fertilisation does not occur (c) Acrosome: • Cap-like structure covering anterior portion of sperm head • Contains hydrolytic enzymes (acrosin, hyaluronidase) • During fertilisation, acrosome reaction releases these enzymes • Enzymes digest zona pellucida and corona radiata, enabling sperm penetration into ovum (d) Sperm tail (flagellum): • Provides motility to sperm • Axoneme structure: 9+2 arrangement of microtubules • Driven by ATP produced by mitochondria in the middle piece • Enables sperm to swim through female reproductive tract to reach egg (e) Fimbriae: • Finger-like projections at the open end of the fallopian tube (oviduct), near the ovary • Sweep the released ovum into the fallopian tube during ovulation • Ciliated — cilia help move the egg into and along the tube • Without fimbriae, egg would not enter the oviduct efficiently

Fertilisation in humans (occurs in the ampullary region of the fallopian tube): Step 1 — Capacitation: • Sperm undergo changes in the female reproductive tract → become capable of fertilising the egg • Changes in membrane fluidity and motility pattern Step 2 — Contact with egg: • Sperm swim through uterus and fallopian tube • Sperm contact the secondary oocyte (arrested at Metaphase II) • Sperm first contacts the corona radiata Step 3 — Acrosome reaction: • Sperm binds to ZP3 glycoprotein on zona pellucida • Triggers acrosome reaction: acrosomal enzymes (hyaluronidase, acrosin) are released • Enzymes digest corona radiata and zona pellucida • Sperm penetrates zona pellucida Step 4 — Sperm-egg membrane fusion: • Sperm plasma membrane fuses with egg plasma membrane (vitelline membrane) • Sperm nucleus enters egg cytoplasm Step 5 — Cortical reaction (block to polyspermy): • Cortical granules beneath egg membrane fuse with plasma membrane • Release contents into perivitelline space → zona pellucida hardens (zona reaction) • This prevents other sperm from entering Step 6 — Completion of meiosis II: • Sperm entry activates the egg • Secondary oocyte completes meiosis II → forms mature ovum (n) + second polar body Step 7 — Pronucleus formation and syngamy: • Sperm nucleus → male pronucleus (n) • Egg nucleus → female pronucleus (n) • Both pronuclei fuse → zygote (2n) — this is syngamy • First cleavage division begins soon after

Major events in human embryonic development: 1. Fertilisation (Day 0): • Sperm + Egg → Zygote (2n) in the fallopian tube 2. Cleavage (Days 1–3): • Rapid mitotic divisions of zygote (without cell growth) • Zygote → 2-cell → 4-cell → 8-cell → 16-cell stage (morula) • Cells called blastomeres • Zona pellucida remains intact 3. Morula (Days 3–4): • 16-cell solid ball • Still in fallopian tube, moving toward uterus 4. Blastocyst (Days 5–6): • Morula develops into blastocyst • Fluid-filled cavity (blastocoel) forms • Inner cell mass (embryoblast): forms embryo • Outer layer (trophoblast): forms placenta and foetal membranes • Zona pellucida dissolves (hatching) 5. Implantation (Days 6–10): • Blastocyst adheres to and embeds in endometrium • Trophoblast cells proliferate and invade uterine wall • HCG is secreted by trophoblast — maintains corpus luteum 6. Gastrulation (Week 2–3): • Inner cell mass differentiates into three germ layers: – Ectoderm (outer): skin, nervous system – Mesoderm (middle): muscles, bones, heart, blood – Endoderm (inner): gut lining, lungs, liver 7. Organogenesis (Weeks 3–8): • Organ rudiments form from germ layers • Neural tube forms → brain and spinal cord • Heart begins beating (~Day 22) • Limb buds appear 8. Foetal period (Week 9 – birth): • Growth and maturation of organs • Human gestation: ~280 days (40 weeks) from last menstrual period