Ovulation and Fertilization

Follicular Development & LH Surge - Eggcellent Race

• Follicular Growth (FSH-driven): Primordial → Primary → Secondary (antral) → Graafian (dominant).
  • Granulosa cells: Convert androgens to estrogen (aromatase).
  • Theca cells: Produce androgens (LH-dependent).
• LH Surge: Triggered by sustained ↑ estrogen (positive feedback from dominant follicle).
• Key Actions of LH Surge:
  • Oocyte completes Meiosis I (becomes secondary oocyte).
  • Triggers ovulation (follicle rupture).
  • Luteinization of follicular cells → Corpus Luteum.

⭐ The LH surge, crucial for ovulation, typically begins 24-36 hours before oocyte release, with the oocyte being extruded approximately 10-12 hours after the LH peak.

Ovulation & Corpus Luteum - Pop & Progesterone

• Ovulation:
  • Trigger: LH surge (~10-12 hrs post-peak).
  • Mechanism: ↑ Proteolytic enzymes (e.g., collagenase) → follicular rupture.
  • Oocyte released: Secondary oocyte (metaphase II).
• Corpus Luteum (CL):
  • Formed from follicle remnants (granulosa & theca cells).
  • Secretes: Progesterone (dominant), estrogen, inhibin A.
  • Lifespan: ~14 days (no fertilization) → corpus albicans.
  • Pregnancy: Rescued by hCG; persists ~12 weeks.
• Progesterone Actions:
  • Endometrium: Secretory changes for implantation.
  • Myometrium: ↓ contractility.
  • Cervical mucus: Thick, scanty.
  • Thermogenic: ↑ basal body temp (~0.5°C).
  • Inhibits GnRH, LH, FSH. 📌 Progesterone Promotes Pregnancy.

⭐ The corpus luteum, formed post-ovulation, secretes progesterone which is vital for endometrial preparation and maintenance of early pregnancy; its lifespan is ~14 days without hCG stimulation.

Sperm Capacitation & Acrosome Rxn - Swimmers' Gauntlet

• Sperm Capacitation: Final maturation in female reproductive tract (~5-7 hrs).

  • Key changes:
    • Removal of cholesterol & glycoproteins from sperm head.
    • ↑ Membrane fluidity & Ca²⁺ influx.
    • Hyperactivated motility (whiplash tail movements).
  • Prepares sperm for acrosome reaction & fertilization.

• Acrosome Reaction: Triggered by sperm binding to Zona Pellucida (ZP3 glycoprotein).

  • Process:
    • Fusion of sperm's plasma membrane with outer acrosomal membrane.
    • Release of acrosomal enzymes: Hyaluronidase (digests corona radiata), Acrosin (digests zona pellucida).
  • Enables sperm to penetrate zona pellucida.

⭐ Sperm capacitation, occurring in the female reproductive tract, involves removal of cholesterol and glycoproteins from the sperm head, leading to hyperactivated motility and enabling the acrosome reaction.

Fertilization & Polyspermy Block - Zygote Genesis

• Fertilization Steps:
  • Sperm: Capacitation & acrosome reaction enable penetration.
  • Pathway: Corona radiata → Zona Pellucida (ZP3 binding) → oocyte membrane fusion.
• Polyspermy Block: Critical to prevent aneuploidy.
  • Fast Block: Oocyte membrane depolarization (transient).
  • Slow Block (Zona Reaction): Cortical granule release → ZP hardening.

  ⭐ The cortical reaction, triggered by sperm-oocyte fusion, results in the release of cortical granules, causing the zona pellucida to harden (zona reaction), thus providing the primary block to polyspermy.

• Zygote Genesis:
  • Oocyte: Completes Meiosis II post-sperm entry.
  • Pronuclei: Male & female pronuclei form.
  • Syngamy: Fusion of pronuclei → diploid zygote.

High‑Yield Points - ⚡ Biggest Takeaways

• LH surge is the primary trigger for ovulation.
• Ovulation occurs ~10-12 hours after LH peak or 24-36 hours after LH surge onset.
• Corpus luteum secretes progesterone, vital for endometrial support.
• Fertilization most commonly occurs in the ampulla of the fallopian tube.
• Capacitation and acrosome reaction are essential for sperm to fertilize the oocyte.
• Oocyte completes Meiosis II only after sperm penetration.
• Zona reaction and cortical reaction block polyspermy.