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Understanding Female Sexual Development and Menstrual Cycle

May 20, 2025

Lecture Notes: Female Sexual Development and the Menstrual Cycle

Introduction

  • The lecture focuses on female sexual development, specifically the menstrual cycle.
  • Part of General Biology Chapter 2.
  • Importance for MCAT preparation.

Ovaries and Gonadotropins

  • Ovaries are the functional sexual organs in females, derived from embryonic structures similar to testes.
  • Controlled by FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone).
    • Both are collectively known as gonadotropins, acting as messages to the gonads.

Role of FSH and LH

  • In men:
    • FSH stimulates spermatogenesis.
    • LH stimulates testosterone production.
  • In women:
    • FSH stimulates estrogen production.
    • LH stimulates progesterone later in the cycle.

GnRH and Hormonal Regulation

  • GnRH (Gonadotropin-Releasing Hormone) from the hypothalamus controls the release of LH and FSH from the anterior pituitary.
  • FSH targets the ovaries to release estrogen.

Overview of Female Reproductive Anatomy

  • Components:
    • Uterus, Fallopian tubes, Ovaries, Cervix, Vagina.
  • Purpose: Prepare environment in uterus for embryo implantation.

The Menstrual Cycle

  • Cycle Length: 28-30 days.
  • Day 1: Start of menstrual bleeding due to hormone level drop.
    • Leads to shedding of the uterine lining.
    • Drop in LH and FSH turns off GnRH.

Phases of the Cycle

  1. Follicular Phase
    • Increase in GnRH -> Increase in FSH and LH -> Increase in estrogen.
    • Estrogen thickens and vascularizes endometrium.
    • Estrogen initially inhibits GnRH but continues to rise.
  2. Ovulation
    • Peak estrogen switches feedback to positive -> Spike in GnRH -> Spike in FSH & LH.
    • LH Spike triggers ovulation (rupture of the follicle, release of ovum).
    • Associated abdominal pain known as mittelschmerz.
  3. Luteal Phase
    • LH turns ruptured follicle into corpus luteum, which secretes progesterone.
    • Progesterone maintains uterine lining.

Post-Ovulation Pathways

  • No Implantation:
    • Corpus luteum becomes insensitive to LH -> Drop in progesterone -> Shedding of uterine lining.
    • Restarts cycle with new rise in GnRH.
  • Implantation:
    • Embryo secretes beta HCG, maintains progesterone production until placenta formation.

Menopause

  • As women age, ovaries become less sensitive to FSH and LH.
  • Results in reduced estrogen, progesterone, and cessation of menstruation.
  • Increase in FSH and LH due to minimal response from ovaries.

Conclusion

  • Recapitulation of the menstrual cycle and hormonal regulation.
  • Importance of maintaining uterine lining through progesterone.

Note: Understanding the hormonal feedback mechanisms and phases of the menstrual cycle is crucial for grasping female sexual development and its implications in biology.

Full transcript