Reproductive Physiology

HPO Axis & GnRH - Command Center

• Hypothalamus: Releases Gonadotropin-Releasing Hormone (GnRH).
  • GnRH secretion: Pulsatile (critical).
    • ↑ Frequency/Amplitude → Favors LH.
    • ↓ Frequency/Amplitude → Favors FSH.
• Anterior Pituitary: GnRH acts on gonadotrophs.
  • Secretes: Follicle Stimulating Hormone (FSH), Luteinizing Hormone (LH).
• Ovaries: FSH & LH target.
  • Functions: Folliculogenesis, ovulation, steroidogenesis (Estrogen, Progesterone), peptides (Inhibin B ↓FSH, Activin ↑FSH).
• Feedback Loops:
  • Negative: Estrogen (low), Progesterone, Inhibin B (on FSH).
  • Positive: Estrogen (sustained high) → LH surge (ovulation).

⭐ Continuous GnRH administration suppresses FSH/LH (receptor downregulation); therapeutic use (GnRH agonists for fibroids, endometriosis).

Ovarian Cycle - Follicle Fiesta

Averages 28 days (range 21-35 days). Key phases: Follicular, Ovulation, Luteal, driven by HPO axis.

• Follicular Phase (Variable; ~Days 1-14):
  • ↑FSH: initiates follicle recruitment & growth (Primordial → Preantral → Antral → Graafian).
  • Dominant follicle selected; produces high Estrogen.
  • Estrogen: builds endometrium; late positive feedback triggers LH surge.
• Ovulation (~Day 14):
  • LH surge (due to sustained high Estrogen) triggers oocyte release from Graafian follicle.
  • 📌 Ovulation by LH (Oh Lord!).
• Luteal Phase (Fixed; ~14 days):
  • Corpus Luteum (CL) forms post-ovulation, secretes Progesterone (dominant) & some Estrogen.
  • Progesterone: prepares secretory endometrium for implantation.
  • No fertilization: CL regresses to Corpus Albicans; ↓hormones cause menstruation.

⭐ The LH surge precedes ovulation by 24-36 hours; ovulation occurs 10-12 hours after LH peak.

Endometrial Cycle - Uterine Updates

• Cyclical endometrial changes for implantation; mirrors ovarian cycle.
• Phases (approx. 28-day cycle):
  • Menstrual (Days 1-5):
    • Stratum functionalis sheds (↓ progesterone & estrogen).
    • Spiral artery constriction.
  • Proliferative (Days 6-14): Estrogen-driven (from follicles).
    • Endometrial growth (glands, stroma, arteries).
    • Thickness ↑ ~3-5 mm.
    • Cervical mucus: thin, watery, alkaline (ferning).
  • Secretory (Days 15-28): Progesterone-driven (corpus luteum). Endometrium receptive.
    • Glands tortuous, secrete glycogen.
    • Spiral arteries coil. Stroma edematous.
    • Thickness ↑ ~5-7 mm, vascular.
    • Implantation window: Days 20-24 (LH+6 to LH+10).
    • Cervical mucus: thick, scanty, acidic.

⭐ Decidualization: Progesterone-induced stromal changes for implantation & placentation, starts mid-secretory phase.

Key Hormones & Gametes - Vital Players

• HPG Axis:
  • Hypothalamus: GnRH (pulsatile).
  • Ant. Pituitary:
    • FSH: Follicle growth (ovary), Sertoli cell function (testis).
    • LH: Ovulation, CL formation; Leydig (testosterone) & Theca (androgen) stimulation.
• Gonadal Hormones & Roles:
  • Ovary:
    • Estrogen (E2): Endometrial proliferation, 2° sexual traits.
    • Progesterone: Secretory endometrium, pregnancy support.
    • Inhibin: ↓FSH.
    • AMH: Ovarian reserve marker.
  • Testis:
    • Testosterone: Spermatogenesis, 2° sexual traits.
    • Inhibin: ↓FSH.
• Gametes:
  • Oogenesis: Oogonia → 1° oocyte (Prophase I) → 2° oocyte (Metaphase II until fertilization). ~400 ova ovulated.
  • Spermatogenesis: Spermatogonia → Spermatozoa. Continuous (~74 days). Sertoli (nourish), Leydig (testosterone). 📌 Sertoli Supports Sperm.

⭐ LH surge is the most reliable indicator of impending ovulation, occurring 24-36 hours prior.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypothalamic GnRH pulses drive pituitary FSH/LH release.
• FSH stimulates follicular development & granulosa cell estrogen synthesis.
• Mid-cycle LH surge is crucial for ovulation & corpus luteum formation.
• Corpus luteum produces progesterone, vital for implantation & pregnancy maintenance.
• Estrogen proliferates endometrium; progesterone induces secretory changes for implantation.
• Fertilization most commonly occurs in the ampulla of the uterine tube.
• hCG, produced by syncytiotrophoblast, "rescues" corpus luteum post-conception.