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es·trous cy·cle(es'trŭs sī'kĕl)
Synonym(s): oestrous cycle.
estrous cyclea reproductive cycle caused by the cyclic production of gonadotrophic hormones by the PITUITARY GLAND. It occurs in adult female mammals and is only complete when pregnancy does not occur. It lasts from 5 to 60 days, dependent on species.
Initially, follicle growth occurs in the ovary as a result of FOLLICLE-STIMULATING HORMONE being secreted by the anterior pituitary This results in increased OESTROGEN production and a thickening of the uterus lining. The build-up of oestrogen results in positive feed-back, causing the anterior pituitary to produce LUTEINIZING HORMONE which in turn brings about ovulation with the formation of a CORPUS LUTEUM, development of the uterine glands, secretion of PROGESTERONE by the corpus luteum (which inhibits FSH production) and a consequent decrease in oestrogen production. Fertilization may occur at this stage and if it does so, pregnancy results. If not, there is a regression of the corpus luteum and a feedback to the anterior pituitary to produce FSH as progesterone decreases. New follicles are formed, the lining of the uterus thins, menstruation (see MENSTRUAL CYCLE occurs in human females and some other primates, and there is a cessation of progesterone secretion and a continued decrease of oestrogen. The cycle then begins again as new follicles grow. Only during the initial period of the cycle, during which ovulation takes place, will the female of most mammalian species copulate (the period of ‘heat'or oestrus).