Overview
This lecture explains the hormonal regulation of the female reproductive system, focusing on coordination between the ovarian and uterine cycles and the roles of key hormones in ovulation, menstruation, and pregnancy.
Coordination of Ovarian and Uterine Cycles
- The ovarian cycle (egg development and ovulation) must be synchronized with the uterine cycle (endometrial changes).
- Menstruation and ovulation are kept separate to optimize conditions for fertilization and implantation.
- The central nervous system, specifically the hypothalamus and pituitary gland, plays a key regulatory role.
Follicle Structure and Hormonal Roles
- Each ovarian follicle contains thecal cells (outer layer) and granulosa cells (inner layer).
- Thecal cells secrete androgens, which granulosa cells convert into estrogens.
- Estrogens promote endometrium growth and provide positive feedback to granulosa cells.
Hypothalamic-Pituitary-Ovarian Axis
- The hypothalamus releases gonadotropin-releasing hormone (GnRH), stimulating the anterior pituitary.
- The anterior pituitary secretes follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
- FSH stimulates follicle (and granulosa cell) growth; LH targets thecal cells and triggers ovulation.
Hormonal Control Through the Cycle
- Early follicular phase: FSH promotes follicle growth; estrogens increase, thickening the endometrium.
- Late follicular phase: Estrogens peak, stimulating hypothalamus and pituitary to increase FSH and LH.
- Progesterone, produced by granulosa cells, maintains and further develops the endometrium during the luteal phase.
- LH surge induces ovulation (release of the egg).
Luteal Phase and Feedback Loops
- After ovulation, the corpus luteum forms and secretes estrogen and progesterone.
- In the luteal phase, estrogen and progesterone negatively feedback to suppress GnRH, FSH, and LH.
- If no fertilization occurs, the corpus luteum degrades, hormone levels drop, and menstruation begins.
- If fertilization occurs, the corpus luteum is maintained to keep progesterone and estrogen high, preventing new follicle development.
Effects of Birth Control Pills
- Birth control pills mimic pregnancy hormone levels, inhibiting FSH and LH release and preventing follicle development and ovulation.
- Progesterone in the pill helps maintain the endometrium and reduce menstrual cramping.
Key Terms & Definitions
- Follicle — the ovarian structure surrounding and nurturing the egg.
- Granulosa cells — follicle cells converting androgens to estrogens, supporting egg development.
- Thecal cells — outer follicle cells that secrete androgens.
- FSH (Follicle-Stimulating Hormone) — promotes follicle and granulosa cell growth.
- LH (Luteinizing Hormone) — triggers ovulation and acts on thecal cells.
- GnRH (Gonadotropin-Releasing Hormone) — released by the hypothalamus to stimulate FSH and LH secretion.
- Corpus luteum — post-ovulation structure secreting progesterone and estrogen.
- Endometrium — the inner lining of the uterus that thickens to support pregnancy.
- Progesterone — hormone maintaining the endometrium and inhibiting new follicle growth.
Action Items / Next Steps
- Review the stages of the ovarian and uterine cycles and the role of each hormone.
- Prepare diagrams illustrating hormonal feedback loops.
- Read textbook chapter on hormonal control of reproduction for next class.