Endocrine Pharmacology TOPIC
23. Physiology Overview: The Reproductive System
On this page: I. The Female Reproductive System II. The Male Reproductive System	Resources: This page

I. The Female Reproductive System     top

Figure 23.1 Feedback Control of Ovarian Hormone Production

1. Feedback Control of Ovarian Hormone Production
   • The pathway leading to the secretion of female steroid sex hormone begins in the hypothalamus, with secretion of the releasing hormone GnRH (gonadotropin-releasing hormone)
   • GnRH stimulates the pituitary to release FSH and LH
   • FSH and LH stimulate production of sex steroids, estradiol - the most common estrogen (E2 in Figure 23.1) and progesterone (P in Figure 23.1)
   • Negative and positive feedback by estradiol and progesterone controls FSH and LH production:
     • Low levels of E2 and P cause negative feedback to inhibit FSH and LH secretion
     • High levels of E2 and P stimulate FSH and LH secretion
   
    
2. The Menstrual Cycle
    

   Figure 23.2 The Menstral Cycle

   
    
   1. Follicular phase
      • The menstrual cycle begins as FSH stimulates development of an ovarian follicle. As this follicle matures, it begins producing a small but steadily rising amount of estradiol. This low level of estradiol causes:
        • Negative feedback: inhibition of FSH and LH secretion
        • Growth of the lining of the uterus
   2. Ovulation
      • The amount of estradiol secreted at mid-cycle has increased to the high level that stimulates a surge of FSH and LH release by the pituitary
      • The LH surge causes ovulation: the follicle ruptures and releases its egg near the opening of its fallopian tube
   3. Luteal Phase
      • Following the release of its egg, the follicle changes into a hormone-secreting structure, the corpus luteum, and secretes estradiol and progesterone in response to LH.
        • Estradiol and progesterone stimulate the continued development of the lining of uterus
      • If pregnancy does not occur, the corpus luteum eventually disintegrates and stops secreting estradiol and progesterone. The uterine lining sheds and menstruation occurs.
      • If pregnancy occurs, the placenta produces hCG (human chorionic gonadotropin) which maintains corpus luteum's secretion of progesterone
      • At 3 months gestation, the placenta takes over secretion of progesterone

II. The Male Reproductive System      top

1. Feedback Control of Testicular Hormone Production
   • As in the female reproductive system, GnRH stimulates the pituitary to release FSH and LH
   • FSH stimulates the cells of the seminiferous tubules to produce sperm.
   • LH stimulates the interstitial cells of the testes to produce androgens.
     • Androgens also stimulate the production of sperm
   • Androgens exert negative feedback on the production of GnRH, FSH, and LH.
   • FSH and LH also exert negative feedback on GnRH production.
   • In human males, these feedback loops keep androgen levels relatively stable.

Quick Question

Question: What would happen if a woman begins taking a regular, low dose of estradiol at the start of her menstrual cycle (when her own estradiol production is extremely low)?   CLICK HERE FOR ANSWER

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