Follicle stimulating hormones (FSH) trigger several of the follicles in the ovary to develop which is known as the follicular phase. Usually, only one follicle will make it to maturity. As the follicle develops, it secretes estrogen which stimulates the thickening of the endometrium in the menstrual cycle as well as small amounts of progesterone. As the follicle matures, it migrates to the surface of the ovary. The fimbriae of the fallopian tube stretch across the top of the ovary to catch the emerging oocyte. The remnants of the follicle remain on the ovary and form the corpus luteum, which marks the beginning of the luteal phase.
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'Follicle Stimulating Hormones (fsh)'
The corpus luteum secretes large amounts of progesterone and small amounts of estrogen. The progesterone causes the endometrium to continue to thicken and become more vascular, preparing it for pregnancy. High levels of progesterone and estrogen also inhibit the pituitary gland from producing FSH and LH, so no other follicles develop. If fertilization does not occur, the corpus luteum degenerates into inactive scar tissue which is known as the corpus albicans. Estrogen and progesterone levels decrease, causing the endometrium to shed, resulting in menstruation.
When there is a decline in ovarian hormones, the pituitary gland is no longer inhibited and so follicle stimulating hormone (FSH) levels begin to rise and the menstrual cycle begins. The first phase of the menstrual cycle is the menstrual phase which is the first day of noticeable vaginal bleeding which can last from three to five days. Menstruation is a result of the endometrium shedding its functional layer which is otherwise known as the stratum functionalis. The proliferative phase is when menstruation ceases which occurs at around day five of the cycle where only the base layer otherwise known as the stratum basalis remains in the uterus. At about day six, rising levels of estrogen which is secreted by the ovaries stimulates the repair of the base layer as well as the growth of blood vessels. During this stage, the endometrium thickens to two to three mm.
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In the secretory phase, after ovulation which is about day 14, increased progesterone from the corpus luteum causes the functional layer to thicken even more, this time as a result of secretion and fluid accumulation. During this phase, the endometrium develops into a bed about five to six mm thick, which around the right size for a fertilized ovum. During the premenstrual phase, if fertilization does not occur, the corpus luteum atrophies and progesterone levels decrease. Blood vessels nourishing the endometrium constrict which interrupts blood flow. The endometrium becomes ischemic and necrotic, causing it to slough off the uterine wall. This forms the menstrual flow.
