The Female Reproductive Organs
The female reproductive hormones arise from the hypothalamus, the anterior pituitary, and the ovaries. Although detectable amounts of the steroid hormone estrogen are present during fetal development, at puberty estrogen levels rise to initiate secondary sexual characteristics. Gonadotropin releasing hormone (GRH) is released by the hypothalamus to stimulate pituitary release of LH and FSH. LH and FSH propagate egg development in the ovaries. Eggs (ova) exist at various stages of development, and the maturation of one ovum takes about 28 days and is called the ovarian or menstrual cycle. The ova are contained within follicles which are support organs for ova maturation. About 450 of a female's 150,000 germ cells mature to leave the ovary. The hormones secreted by the ovary include estrogen, progesterone, and small amounts of testosterone.
As an ovum matures, rising estrogen levels stimulate additional LH and FSH release from the pituitary. Prior to ovulation, estrogen levels drop, and LH and FSH surge to cause the ovum to be released into the fallopian tube. The cells of the burst follicle begin to secrete progesterone and some estrogen. These hormones trigger thickening of the uterine lining, the endometrium, to prepare it for implantation should fertilization occur. The high progesterone and estrogen levels prevent LH and FSH from further secretion-thus hindering another ovum from developing. If fertilization does not occur, eight days after ovulation the endometrium deteriorates resulting in menstruation. The falling estrogen and progesterone levels which follow trigger LH and FSH, starting the cycle all over again.
Although estrogen and progesterone have major roles in the menstrual cycle, these hormones have receptors on a number of other body tissues. Estrogen has a protective effect on bone loss which can lead to osteoporosis. And progesterone, which is a competitor for androgen sites, blocks actions that would result from testosterone activation. Estrogen receptors have even been found in the forebrain indicating a role in female neuronal function or development.
Hormones related to pregnancy include human chorionic gonadotrophin (HCG), estrogen, human chorionic somatomammotrophin (HCS), and relaxin. HCG is released by the early embryo to signal implantation. Estrogen and HCS are secreted by the placenta. And relaxin is secreted by the ovaries as birth nears to relax the pelvic area in preparation for labor.
Science EncyclopediaScience & Philosophy: Heterodyne to Hydrazoic acidHormones - Mechanisms Of Action, The Hypothalamus, The Pituitary Gland, The Thyroid Gland, The Parathyroid Glands - Major hormones