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Hormones in Menstrual Cycle
The menstrual cycle can be broken down into the pre-ovulatory phase and the post-ovulatory phase. During the pre-ovulatory phase (before ovulation takes place), the gonadotropin releasing hormone (GnRH) stimulates the anterior pituitary gland to release the luteinizing hormone (LH) and the follicle stimulating hormone (FSH). LH stimulates theca cells in the immature follicle to differentiate and proliferate. Theca cells are responsible for producing androgens and releasing those androgens to another type of cell called granulosa cell. FSH on the other hand stimulates the proliferation of granulosa cells, which use the androgens to form estrogens. Estrogens initiate the thickening of the endometrium and create a positive feedback loop on GnRH, FSH and LH. This in turn causes the LH surge (as well as a rise in FSH), which leads to the process of ovulation. During the post-ovulatory phase (after ovulation takes place), the LH causes the formation of the corpus luteum, which begins producing estrogen as well as progesterone. Progesterone inhibits the contraction of the uterus, inhibits another follicle from maturing and maintains the thickening of the endometrium. Both progesterone and estrogen now create a negative feedback loop on GnRH, LH and FSH. This means that a rise in progesterone causes less LH to be produced and this is precisely why the corpus luteum begins to degenerate into the corpus albicans (remember LH is needed to form and maintain the corpus luteum). Therefore, the corpus luteum eventually breaks down and stops releasing progesterone and estrogen. As progesterone and estrogen levels fall, the endometrium can no longer be maintained and begins to break down in a process called menstruation. A fall in progesterone also causes uterine contractions, which is what women experiencing during menstrual cramps.
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