Menstrual Cycle

Menstrual Cycle Overview - The Monthly Clock

• Menstrual Cycle: Rhythmic physiological changes in fertile females, preparing for potential pregnancy.
  • Menarche: Onset of menstruation.
  • Menopause: Cessation of menstruation.
  • Normal length: 28 ± 7 days.
• Two Interconnected Cycles:
  • Ovarian Cycle: Governs egg development. Key phases:
    • Follicular (maturation)
    • Ovulation (release)
    • Luteal (corpus luteum)
  • Uterine Cycle: Prepares uterine lining. Key phases:
    • Menstrual (shedding)
    • Proliferative (rebuilding)
    • Secretory (thickening)
  • 📌 Mnemonic (Ovarian phases): FOLlow Ovulation Luckily.

⭐ Day 1 of the cycle is the first day of menstruation.

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Hormonal Control - Hormone Harmony

• Hypothalamic-Pituitary-Ovarian (HPO) Axis: Master regulator.
  • Hypothalamus: Pulsatile Gonadotropin-Releasing Hormone (GnRH).
  • Anterior Pituitary: Follicle-Stimulating Hormone (FSH) & Luteinizing Hormone (LH).
  • Ovary: Estrogen (E2), Progesterone (P4), Inhibins (A & B).
• Key Hormone Actions:
  • FSH: Follicular development, ↑E2.
  • LH: Ovulation, corpus luteum (CL) formation & P4 secretion.
  • Estrogen (E2): Endometrial proliferation. Dual feedback:
    • Low E2: Negative feedback on FSH/LH.
    • Sustained high E2 (>200 pg/mL for >48 hrs): Positive feedback → LH surge.
  • Progesterone (P4): Secretory endometrium, maintains pregnancy. Negative feedback.
  • Inhibins: Suppress FSH (Inhibin B - follicular; Inhibin A - luteal).

⭐ The LH surge is the primary trigger for ovulation, occurring approximately 24-36 hours before follicle rupture.

Ovarian Cycle - Egg's Grand Show

1. Follicular Phase (FSH-driven, variable length):

• Recruitment: Primordial → Primary → Antral follicle.
• Selection: One dominant Graafian follicle (~20-25 mm) matures.
• FSH: Stimulates follicle growth, estrogen synthesis.
• Estrogen: ↑FSH receptors; high levels induce LH surge.
• Two-cell, two-gonadotropin:
  • Theca (LH): Androgens.
  • Granulosa (FSH): Androgens → Estradiol (aromatase).

2. Ovulation (~Day 14, LH surge-triggered):

• LH surge (from ↑estrogen): Oocyte meiosis I completion, stigma formation, oocyte release.

3. Luteal Phase (Progesterone-dominant, fixed ~14 days):

• Corpus Luteum (CL) forms from ruptured follicle.
• CL secretes: Progesterone (dominant), Estrogen.

  ⭐ Corpus luteum is the main source of progesterone in the luteal phase, vital for endometrial receptivity and early pregnancy.

• Fate:
  • No pregnancy: CL degenerates → Corpus Albicans (↓hormones → menses).
  • Pregnancy: hCG maintains CL.

Ovarian Cycle Flow:

Uterine Cycle - Womb's Wallpaper Reno

• Menstrual Phase (Days 1-5):
  • Shedding of stratum functionalis due to ischemia.
  • Trigger: ↓ Progesterone & Estrogen → spiral artery constriction.
  • Endometrial thickness: ~1-2 mm.
• Proliferative Phase (Days 6-14): "Rebuilding the lining"
  • Driven by ↑ Estrogen (ovarian follicles).
  • Regeneration & growth of stratum functionalis.
  • Glands: straight, tubular; Arteries: spiral, elongate.
  • Endometrial thickness: up to ~8-10 mm.
• Secretory Phase (Days 15-28): "Preparing for implantation"
  • Driven by ↑ Progesterone (corpus luteum).
  • Differentiation: glands become tortuous, secrete glycogen-rich fluid.
  • ↑ Vascularity, edema; spiral arteries become highly coiled.
  • Maximal thickness: ~12-16 mm.
  • 📌 Mnemonic: Secretory phase = Soft, Spongy, Swollen.

⭐ Pinopodes, small protrusions on endometrial cells, appear during the implantation window (mid-secretory, days 20-24), crucial for blastocyst adhesion.

High-Yield Points - ⚡ Biggest Takeaways

• FSH initiates follicular growth; LH surge is crucial for ovulation.
• Estrogen (follicular) drives proliferative phase; Progesterone (luteal) drives secretory phase.
• Corpus luteum secretes progesterone; its regression causes menstruation.
• Ovulation occurs ~14 days before menses, triggered by LH surge.
• LH surge is the best hormonal marker for predicting ovulation.
• Menstruation signifies endometrial shedding due to progesterone withdrawal.
• Mid-cycle estrogen peak causes positive feedback for LH surge.