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8) The entire menstrual cycle is driven by changes in the release of particular hormones. Describe the hormonal regulation of the menstrual cycle.Answer: During the early and mid-follicular phases, estrogen and progesterone concentrations are reduced by the loss of the corpus luteum. This removes the block on follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion by removing the progesterone-induced suppression. FSH stimulates follicular development by binding to granulosa cells and stimulatingtheir growth and proliferation. This also stimulates the differentiation of theca cells that respond to LH by secreting androgens. These androgens are taken up by the granulosa cells and converted into estrogens. As the menstrual cycle progresses into later follicular phase, the increases in estrogen and inhibin secretion from granulosa cells cause a decrease in FSH concentrations. LH secretion is maintained under these conditions because inhibin does not affectLH secretion. These changes are responsible for the selection of a dominant follicle that is able tomaintain its secretion of estrogen in the face of the decrease in FSH (which stimulates atresia). The increase in estrogen facilitates the changes in the uterus that are characteristic of the proliferative phase, which include promoting oocyte development and follicular growth. Estrogen induces LH receptor expression on granulosa cells that renders those cells responsive toLH. In addition, estrogen induces the expression of progesterone receptors on endometrial cells.
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