|MadSci Network: General Biology|
Billy: The human menstrual cycle is controlled by hormones. There is a region in the brain called the hypothalamus. The hypothalamus produces a small hormone called "gonadotropin releasing hormone" (GnRH). GnRH travels to the pituitary gland (very close to the hypothalamus) in a special series of blood vessels called the hypothalamo-pituitary portal veins. Once GnRH arrives at the pituitary, it stimulates special cells to release two more hormones; 1) Luteinizing hormone (LH) and 2) Follicle-stimulating hormone (FSH). These pituitary hormones are released into the blood stream and are carried to the ovaries. Each hormone has specific target cells in the ovaries and together will cause the follicles containing the eggs to grow rapidly and to release the steroid hormone estradiol. This is called the follicular phase of the reproductive cycle. Once each month, LH is released from the pituitary gland at a much higher rate than any other time. Usually only one ovarian follicle will be ready to accept this higher amount of LH and will be caused to rupture or ovulate. The egg will be released from the follicle and will move into the Fallopian tubes of the uterus where it can be fertilized if spermatozoa are present. The cells of the follicle that stayed attached to the ovary after ovulation (called the corpus luteum) will be changed and will respond to LH stimulation by releasing the steroid hormone, progesterone. This is called the luteal phase of the reproductive cycle. The estradiol and progesterone will "feedback" on the hypothalamus and pituitary to ensure that LH and FSH are released in the proper amounts. The lining of the uterus (endometrium) will respond to the estradiol released during the follicular phase. There will be a thickening of the endometrium due to increases in blood vessels and in glands found there. This thickening prepares the uterus to accept the embryo if fertilization occurred and will be the site where the embryo grows and develops into a baby. Following ovulation, the levels of estradiol will decrease slightly and progesterone levels will dramatically increase. Progesterone will stimulate the endometrial glands to secrete and will also keep the uterine muscles from contracting. This will protect the developing baby during the early stages of pregnancy. If no fertilization had occurred and no embryo is present the menstrual cycle will continue because the corpus luteum will eventually stop producing progesterone. This will allow the pituitary to release more LH and FSH to begin the cycle again. The rapid drop in progesterone will also cause the endometrium to be sloughed off,leading to the menses or menstrual period, marking the end of one menstrual cycle and the beginning of the next menstrual cycle. I have included a link for a web site with diagrams and charts showing the levels of hormones for the entire cycle. http://www.endote xt.org/female/female3/female3.htm I hope this helps... Dr. David S. Mallory Professor of Biological Sciences Marshall University Huntington, WV
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