The Human Female Reproduction System—Part 2

Hormonal Feedback in the Female Reproductive System: Key Organs/Glands and the Hormones They Release

Hypothalamus

Gonadotropin-releasing hormone (GnRH)

Anterior pituitary

• Luteinizing hormone (LH)
• Follicle stimulating hormone (FSH)

Ovaries

• Estrogens
• Progesterone

Uterus

• Does not produce hormones.
• Is affected by estrogens and by progesterone.

Figure 43.14, Purves's Life: The Science of Biology, 7th Edition

Hormonal Control of Ovarian and Menstrual Cycles

• Hypothalamus
  • Continuously releases GnRH unless actively prevented from doing so by other hormones (most notably progesterone).
• Anterior pituitary
  • 1. FSH and LH are released into blood due to the influence of GnRH from the hypothalamus.
  • These hormones stimulate the follicle to grow.
• Developing follicle
  • 2. Estrogens are released due to positive influence of FSH and LH.
  • 3. Estrogen levels rise slowly.
    • Estrogen stimulates the growth of the endometrium of the uterus.
    • Throughout most of the cycle estrogen inhibits the release of gonadotropin by the hypothalamus and LH and FSH by the anterior pituitary.
• Day 12: Things Change!!
• On about day 12, estrogen levels get so high that estrogen's influences radically changes. This is non-intuitive and unexpected so don't get confused by it- just accept it! On day 12 estrogen switches roles and positively influences the hypothalamus and anterior pituitary, resulting in a surge of LH and FSH around day 12 of the cycle (positive feedback).

• Anterior pituitary

• 4. The LH surge causes several things (the FSH increase is still not well understood):
  • 5. Triggers the primary oocyte (diploid) to resume meiosis I, resulting in formation of the secondary oocyte (haploid).
  • 5. Stimulates final explosive growth of the follicle, resulting in ovulation.
  • 5. Transforms the follicle that remains after ovulation into the corpus luteum (hence the name- "luteinizing homone!")
• Corpus luteum
  • 6. Secretes estrogens and progesterone.
    • 7. These hormones inhibit the hypothalamus and the anterior pituitary, shutting down the release of FSH and LH.
      • Lower levels of FSH and LH prevent the development of more follicles.
    • The estrogen and progesterone stimulate further growth of the endometrium.
  • 8. Begins to disintegrate about one week after ovulation.
    • It is the architect of its own destruction, since it secretes progesterone, which inhibits the release of LH.
      • The corpus luteum can only persist in the presence of luteinizing hormone (LH)!
    • There is one other hormone that will prevent the corpus luteum from disintegrating, but that hormone is produced by the developing embryo.
  • 9. As a result of the disintegration of the corpus luteum, estrogen and progesterone levels plummet.
    • This causes the endometrium to "die."
      • Its blood and tissue are shed.

New Beginnings

• With the loss of the corpus luteum and the resulting dramatic decrease in progesterone and estrogen, the hypothalamus and anterior pituitary are no longer inhibited by the negative feedback.
• The hypothalmus thus resumes the spontaneous release of Gonadatropin Releasing Hormone (GnRH).
• This, in turn, stimulates the release of FSH and LH from the anterior pituitary, initiating the development of a new set of follicles.

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