Reproductive Physiology

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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).

HPG Axis: Male and Female

⭐ 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.

Ovarian Follicle Development and Hormonal Regulation

⭐ 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. Menstrual cycle phases, histology, and hormones

⭐ 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. Male and Female HPG Axis

⭐ 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.

Practice Questions: Reproductive Physiology

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Hormone predominantly secreted after 14 days that acts on the endometrium is?

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Flashcards: Reproductive Physiology

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Ovulation follows the _____, which occurs due to a burst of estradiol

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Ovulation follows the _____, which occurs due to a burst of estradiol

LH surge

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