The Entire Menstrual Cycle Is Driven by Changes in the Release

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The Answer is 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 stimulating their 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 affect LH secretion. These changes are responsible for the selection of a dominant follicle that is able to maintain 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 to LH. In addition, estrogen induces the expression of progesterone receptors on endometrial cells. In the late follicular phase, there is a switch in the response of cells to LH. Rather than acting through a negative feedback system, LH begins to act through a positive feedback system where increases in LH will further increase LH release from the anterior pituitary. The expression of LH receptors on the granulosa cells is partly responsible for stimulating changes in the dominant follicle that include: secretion of paracrines that stimulate oocyte meiotic progression, a decrease in estrogen secretion from granulosa cells, an initiation of progesterone secretion by granulosa cells, secretion of enzymes and paracrines that degrade the follicular wall, and the differentiation of granulosa and theca cells into the corpus luteum. During the early luteal phase, the secretion of estrogen from the corpus luteum is not enough to prevent the fall in estrogen concentration. This decrease in estrogen removes the stimulus for LH secretion, which ends the surge. However, progesterone concentrations begin to rise. The estrogen and progesterone concentrations rise as the corpus luteum matures. The rise in progesterone not only suppresses LH secretion but prevents the estrogen -induced increase in LH as well as decreasing plasma FSH concentrations.

The Entire Menstrual Cycle Is Driven by Changes in the Release

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