Hormonal Regulation of Ovarian Cycle

• childhood ovaries secrete small amounts of estrogens ------>

• block hypothalamic release of GnRH
• (no FSH or LH secreted from anterior pituitary)
  

• early puberty hypothalamus less INHIBITED by estrogens ----->

• gradually more GnRH is released ------>
• gradually more FSH and LH released
  

• late puberty GnRH released more cyclicly ----->

• FSH and LH released cyclicly ----->
• menarche - first menstrual period (bleeding)
  

2. Hormonal Interactions During Ovarian Cycle
   

• GnRH, LH, FSH and their Role in Estrogen Level
  

1. increasing release of GnRH from hypothalamus -----> increased release of FSH and LH from ant. pituitary ----->
   

• FSH stimulates growth/division of follicle cells
• LH stimulates development of thecal cells
  

• LH stimulates thecal cells to produce androgens
• FSH stimulates granulosa to convert to estrogens
  

2. temporary NEGATIVE FEEDBACK - these low levels of estrogens briefly inhibits more release of FSH and LH (although it is still being produced and stored!)
   
3. temporary POSITIVE FEEDBACK - these low levels of estrogens briefly enhance FSH stimulation of granulosa
   

• Higher Estrogen Leads to Actions of LH
  

1. estrogen levels eventually get higher and higher
2. suddenly, high estrogen creates POSITIVE FEEDBACK to brain increasing GnRH, FSH, LH levels
3. LH and FSH levels rise very quickly (MIDCYCLE)
4. high LH level increases fluid level in antrum, and rupture/release of the secondary oocyte
5. estrogen level drops immediately after ovulation, but soon begins to rise again
6. LH stimulates "corpus luteum" to secrete progesterone to prepare the endometrium for implantation and pregnancy.
   

• Continuation Depends on Corpus Luteum and Progesterone
  

1. progesterone and estrogen levels rise, leading to a NEGATIVE FEEDBACK on LH and FSH from anterior pituitary
   
2. LH levels decline, corpus luteum starts to degenerate, and less progesterone released

• PREGNANCY -------> placenta releases Human Chorionic Gonadotropin, (maintaining progesterone levels for pregnancy)

Category: Physiology Notes