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The Menstrual Cycle
The menstrual cycle is a series of events that takes place in women who have reached puberty. It lasts approximately 28 days and its purpose is to prepare the woman for a possible pregnancy. If fertilization does not take place during this period, then the thickened endometrium will break down and slough off along with the secondary oocyte in a process called menstruation (characterized by bleeding and cramps). The menstrual cycle can be broken down into three stages - the follicular phase, ovulation and the luteal phase. During the follicular phase, the hypothalamus releases the gonadotropin releasing hormone (GnRH) that moves down to the anterior pituitary gland and stimulates it to release the luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Both FSH and LH move down to the ovaries and stimulate the development of the immature follicle into the secondary follicle. As the follicle develops, granulosa cells in the follicle begin to release estrogens, which initiates the thickening of the endometrium. The rise in estrogen also causes a sharp rise in LH (LH surge) as well as rise in FSH. This in turn causes ovulation to take place. During ovulation, the secondary follicle ruptures, thereby releasing the secondary oocyte into the peritoneal cavity and then into the fallopian tube. The LH rise also causes the remaining portion of the follicle inside the ovary to develop into the corpus luteum. The corpus luteum begins producing estrogen but also produces another hormone called progesterone. Progesterone maintains the thickening of the endometrium, inhibits the uterus from contracting and inhibits another follicle from maturing. The rise in progesterone and estrogen creates a negative feedback loop that causes a decrease in concentration of GnRH and therefore a decrease in the levels of LH and FSH. Since the LH is needed to maintain the corpus luteum, the decrease in LH causes the corpus luteum to deteriorate into the corpus albicans, which stops releasing progesterone. Less progesterone means that the endometrium will stop thickening and will begin to break down, initiating the process of menstruation. As the concentration of progesterone and estrogen fall, the cycle restarts itself and repeats as described above. This process will continue until the woman reaches menopause.
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