The female reproductive system is primarily regulated by five hormones including estrogen, progesterone, gonadotropin releasing hormone, follicle stimulating hormone, and luteinizing hormone. These hormones play a role in one or more stages of development and function of the female reproductive system.
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Ovaries produce three varieties of estrogen called estradiol, estrone, and estriol. Estrogens play a role in development of secondary sex characteristics that appear at puberty. Estradiol plays a role in breast development and promotes fat distribution to breasts, hips and legs.
Estrogens are produced by follicles within the ovary and help regulate the menstrual cycle. Estrogen promotes the rapid growth of cells lining the uterus called the endometrium in preparation for possible implantation and pregnancy. At the end of pregnancy, high levels of estrogen induce the ovaries to produce oxytocin which stimulates uterine contractions.
Estrogen also has non-reproductive functions. Estrogen prevents abnormal blood clotting which maintains heart health. In addition, estrogen helps bones retain calcium which keeps them strong. The natural decline of estrogen with aging often requires medical supplementation with exogenous estrogen to prevent declining heart health and osteoporosis.
Progesterone is also produced by the ovaries and levels fluctuate during the menstrual cycle. After ovulation, the follicle collapses and reorganizes itself into a new steroid secreting gland called the corpus luteum. The corpus luteum produces progesterone. Progesterone maintains the health of the uterine lining during pregnancy.
If pregnancy occurs, the implanting embryo produces human chorionic gonadotropin, or hCG, which keeps the corpus luteum functional so that it can continue secreting progesterone to support the pregnancy until the placenta is developed enough to take over progesterone production. If there is no pregnancy, the corpus luteum dies, progesterone levels fall and the uterine lining is shed during menses.
Gonadotropin Releasing Hormone
The monthly hormonal changes and development of a mature egg that occur with the monthly cycle are initiated in the brain by secretion of gonadotropin releasing hormone, or GnRH. GnRH in turn stimulates the production of follicle stimulating hormone, or FSH and luteinizing hormone, or LH which locally regulate follicular function in the ovary.
FSH and LH
FSH stimulates the follicles within the ovaries to produce increasing amounts of estrogen and small amounts of progesterone during the first two weeks of the menstrual cycle. Under the influence of FSH, multiple follicles grow until one or two follicles become dominant, secreting the most estrogen. As estrogen levels rise in the ovary, they are also released back into the blood system and return to the brain where estrogen signals the brain to stop producing FSH. The rising estrogen levels also signal another gland in the brain, the pituitary gland, to release luteinizing horomone which signals the dominant follicle to release its egg for possible fertilization.