Uterine Cycle - for the implanted zygote. Secretory Phase...

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Uterine Cycle The cyclic changes that occur in the uterus prepare the uterus for implantation and development of the fertilized egg. It consists of the buildup of the endometrium through most of the sexual cycle, followed by degradation and discharge if pregnancy does not occur. There are four uterine stages: the menstrual phase , the proliferative phase , the secretory phase , and the premenstrual phase . (See Figure 4) Figure 4 Proliferative Phase During the proliferative phase , increasing amounts of estrogen secreted from the developing follicle stimulates the mitotic cell division of the stratum basalis layer of the endometrium. These cells differentiate and mature into stratum functionalis cells and replace the tissue that was lost during the previous menstrual cycle. Estrogen also stimulates significant angiogenesis which increases the blood supply to provide energy and raw materials for cell growth and nourishment
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Unformatted text preview: for the implanted zygote. Secretory Phase After ovulation, the uterus enters the secretory phase . The corpus luteum secretes progesterone, which stimulates the thickening of the stratum functionalis. This thickening is the result of increased glycogen-rich fluids secreted into the interstitial space. Premenstrual and Menstrual Phases If implantation does not occur, the corpus luteum degrades and progesterone levels decrease. Without progesterone, the arteries that supply the stratum functionalis constrict, resulting in endometrial ischemia ; without blood, the tissue dies. These events occur premenstrually . During menstruation , the necrotic endometrial tissues fall away from the wall of the uterus and accumulated fluids, blood and tissues (and the unfertilized egg) are discharged through the vagina. The menstrual fluid contains fibrinolysin, which prevents the blood from clotting....
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