Menstrual Cycle

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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.

Menstrual Cycle: Hormones, Cycles, Tempoka

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

Ovarian Cycle: Follicle, Ovulation, Corpus Luteum

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.

Endometrium Histology: Proliferative, Secretory, Menstrual

⭐ 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.

Practice Questions: Menstrual Cycle

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LH surge is associated with?

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Flashcards: Menstrual Cycle

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The _____ of the myocytes decreases appreciably in the post partum period.

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The _____ of the myocytes decreases appreciably in the post partum period.

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