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The Mcrtstruaf Cycfe

ENDOCRINE PHYSIOLOGY

Figurę 8.27 Menstrual Cycle_

The menstruaf cycle is drvided into threc phases: follicular, ovulatory, and luteal. The follicular phase bcgins during menses with the prolifer-ation of Ihe granulosa cells in a selected fołlicle. This is associated willi rising levels of estradiol and. to a lesser extent, progeslins, which feed back on iKith the hypothalamus and piluiłary to stimulate li.e., positive feed back) a surge in GnRI I secretion followed by peaks m luteinizing (U b and follicle-stimulating hormone (FSH) secretion. which thcn induce ovulation (see Figurę 8.28).

Following ovulation. the follicular cells transform into the corpus luteum and produce large amounts of progesterone and estradiol. During this luteal phase. the granulosa cells also produce inhibin. logether, progesterone, estradiol, and inhibin feed back on the pilu itary to suppress LH and TSI t secretion (see Figurę 8.28). In the absence of fertilization of the released egg, the corpus luteum regresses and menses begins.

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