Notes on Menstrual Cycle free essay sample

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. We will write a custom essay sample on Notes on Menstrual Cycle or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page 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, releasing the secondary oocyte. ? Following ovulation, during the luteal phase of the ovarian cycle, LH stimulates the transformation of the follicular tissue left behind in the ovary to form the corpus luteum, a glandular structure (See figure(c)). Under continued timulation by LH during this phase of the ovarian cycle, the corpus luteum secretes progesterone and estrogen (see figure(d)). As the levels of progesterone and estrogen rise, the combination of these hormones exerts negative feedback on the hypothalamus and pituitary, inhibiting the secretion of LH and FSH. Near the end of the luteal phase, the corpus luteum disintegrates, causing the concentrations of estrogen a nd progesterone to decline sharply. The dropping levels of ovarian hormones liberate the hypothalamus and pituitary form the inhibitory effects of these hormones. The pituitary can then begin to secrete enough FSH to stimulate the growth of new follicles in the ovary, initiating the next ovarian cycle. The Uterine Cycle The hormones secreted by the ovaries –estrogen and progesterone have a major effect on the uterus. Estrogen secreted in increasing amounts by growing follicles signals the endometrium to thicken. In this way, the follicular phase of the ovarian cycle is coordinated with proliferative phase of the uterine cycle (see figure (e)). Before ovulation, the uterus is already being prepared for a possible embryo. After ovulation, ? estrogen and progesterone secreted by corpus luteum stimulate continued development and maintenance of the endometrium, including enlargement of arteries and growth of endometrial glands. These glands secrete a nutrient fluid that can sustain an early embryo even before it actually implants in the uterine lining. Thus, the luteal phase of the ovarian cycle is coordinated with what is called the secretory phase of the uterine cycle. The rapid drop in the level of ovarian hormones when the corpus luteum disintegrates causes spasms of the arteries in the uterine lining that deprive it of blood. The upper two-thirds of the endometrium disintegrates, resulting in menstruation the menstrual flow phase of the uterine and the beginning of a new cycle. By convention, the first day of menstruation is designated day 1 of the uterine (and ovarian) cycle.