any one of three glycoprotein (see protein
released by either the anterior pituitary gland
or the placenta (the organ in which maternal and fetal blood exchange nutrients and waste products) that have various effects upon the ovaries
and testes (see testis
). Gonadotropin-releasing hormone (Gn-RH) is produced and released from the hypothalamus. Gn-RH release stimulates the secretion of both follicle-stimulating hormone (FSH) and lutenizing hormone (LH) from the pituitary gland. Gn-RH is a peptide composed of ten amino acids which are synthesized in the hypothalamus (that portion of the brain nearest the pituitary). This hormone travels in the bloodstream to the anterior pituitary, where it causes the release of the gonadotropic hormones. The hormones FSH and LH inhibit the amount of Gn-RH released by a mechanism called "negative feedback." In the female, FSH causes an increase in the weight of the ovaries and encourages the growth of Graafian follicles (containing maturing eggs); in males, FSH produces spermatogenesis in the testes. In females, secretion of LH is associated with the maturation of the follicles, the manifestation of heat (or estrus), and the release of the egg from the follicle, which is transformed into a corpus luteum. In males, LH stimulates the testes to release testosterone. Sex hormones released from the ovaries and testes eventually reach the hypothalamus and help to regulate the hormonal cycle. Human chorionic gonadotropin (HCG), produced in the placenta, helps to maintain pregnancy once a fetus begins to develop. It appears in the urine in approximately the first week after the first missed menstrual period, and is the basis for two kinds of pregnancy tests; in the Ascheim-Zondek testm it causes the ovaries of an immature female rat or mouse to gain weight and produce ripened follicles, and in the Friedman test, it stimulates female rabbits to ovulate.
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