The Ovarian Cycle The cycle begins with the release from the hypothalamus of gonadotrophin – releasing hormone (GnRH), which stimulates the pituitary to secrete small amounts of Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH). The FSH stimulates follicle growth, aided by LH and the cells of the growing follicles start to make estrogen. Notice in figure(d) that there is a slow rise in the amount of estrogen secreted during most of the follicular phase, the part of the ovarian cycle during which follicles are growing and oocytes maturing. The low levels of estrogen inhibit secretion of the pituitary hormones, keeping the levels of FSH and LH relatively low. The levels of FSH and LH, however, shoot up sharply when the secretion of estrogen by the growing follicle begins to rise steeply. Whereas a low level of estrogen inhibits the secretion of pituitary gonadotrophins, a high concentration has the opposite effect: It stimulates the secretion of gonadotrophins by acting of the hypothalamus to increase its output of GnRH. The figure(b) shows steep increases in FSH and LH levels that occur soon after the increase in the concentration of estrogen indicated in figure(d). the effect is greater for LH because the high concentration of estrogen also increases the sensitivity of LH-releasing cells in the pituitary to GnRH. By now, the follicles can respond more strongly to LH because more of their cells have receptors for this hormone. The increase in LH concentration caused by increased estrogen secretion from the growing follicle is an example of positive feedback. The LH induces the final maturation of the follicle. The maturing follicle develops an internal fluidfilled cavity and grows very large, forming a bulge near the surface of the ovary. The follicular phase ends , about a day after the LH surge, with ovulation: The follicle and adjacent wall of the ovary rupture,...
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