Menstrual Cycle Physiology: Ovarian Cycle – Follicular Phase Explained
Early Follicular Phase (Days 0-5)
• Granulosa cells secrete activin, a hormone that 1) Incr FSH production by pituitary, 2) Incr FSH receptors on granulosa cells, and 3) Decr theca cell androgen production.
• This effect limits conversion to estrogen, preventing estrogen from negatively feeding back on anterior pituitary which would reduce FSH production.
• Resulting high FSH level stimulates follicle growth.
• By Day 5, the dominant follicle (the one most responsive to FSH) will be selected to continue growing (mechanism for this "selection" unclear, but this follicle is also the first to switch from activin to inhibin, and that inhibin decr the FSH such that other follicles are not as stimulated).
Late Follicular Phase (Days 5-14)
• Granulosa cells of dominant follicle switch to secreting inhibin instead of activin. Inhibin 1) Decr FSH production by pituitary, and 2) helps LH Incr theca cell production of androgens (leading to Incr conversion to estrogen)
• The Incr in estrogen during this time:
1. Incr FSH receptors on the follicle
2. Negatively feeds back on pituitary to reduce, not stop, FSH production (dominant follicle still grows because of greater number of FSH receptors)
3. Incr number of LH receptors on granulosa cells of the follicle, preparing for ovulation
1) Inhibin and activin are named for their effect on the anterior pituitary's FSH production.
2) Timely balance in steroid secretion between theca and granulosa cells is key to normal follicle growth. Disruption of this balance can lead to menstrual cycle disorders. I.e. early estrogen secretion -> inhibits FSH secretion too early -> lack of follicle growth (this is one mechanism behind Polycystic Ovarian Syndrome, PCOS).
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