progesterone (prōjĕs´tərōn´), female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle ruptures during the release of the egg cell. If fertilization does not take place, the secretion of progesterone decreases and menstruation occurs. If fertilization does occur, progesterone is secreted during pregnancy by the placenta and acts to prevent spontaneous abortion; the hormone also prepares the mammary glands for milk production. Progesterone is also synthesized from cholesterol in the cortex of the adrenal gland where it is a precursor for the synthesis of other steroids including testosterone. Synthetic compounds with progesteronelike activity have been developed that, along with estrogen, are used in oral contraceptives.

progesterone A hormone, produced primarily by the corpus luteum of the ovary but also by the placenta, that prepares the inner lining of the uterus for implantation of a fertilized egg cell. If implantation fails, the corpus luteum degenerates and progesterone production ceases accordingly. If implantation occurs, the corpus luteum continues to secrete progesterone, under the influence of luteinizing hormone and prolactin, for several months of pregnancy, by which time the placenta has taken over this function. During pregnancy, progesterone maintains the constitution of the uterus and prevents further release of eggs from the ovary. Small amounts of progesterone are produced by the testes. See also progestogen.

progesterone One of the progestogens, steroid sex hormones, which are synthesized in the initial steps in the biosynthetic pathway in the gonads that converts cholesterol to androgens and to oestrogens; or any natural or synthetic steroid having a progesterone-like action. Progesterone is the main progestogen produced by the ovaries, particularly in the second half of the menstrual cycle after ovulation has occurred and the empty follicle has formed a corpus luteum. They are also important hormones in pregnancy and secreted in increasing concentrations during gestation.

Saffron A. Whitehead

progesterone Steroid hormone secreted mainly by the corpus luteum of the mammalian ovary and by the placenta during pregnancy. Its principal function is to prepare and maintain the inner lining (endometrium) of the uterus for pregnancy. Synthetic progesterone is one of the main components of the contraceptive pill.

progesterone (proh-jest-er-ohn) n. a steroid hormone secreted by the corpus luteum of the ovary, the placenta, and also (in small amounts) by the adrenal cortex and testes. It is responsible for preparing the endometrium of the uterus for pregnancy.

pro·ges·ter·one / prōˈjestəˌrōn; prə-/ • n. Biochem. a steroid hormone released by the corpus luteum that stimulates the uterus to prepare for pregnancy.

progesterone A steroid hormone that is secreted mainly by the corpus luteum. It is responsible for the maintenance of pregnancy.

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