oestrous cycle defined in 1951 yearoestrous cycle - oestrous (estrous) cycle;
oestrous cycle - Reproductive cycle of short duration (usually between five and sixty days according to species) occurring in sexually mature females of many species of mammal, in absence of pregnancy (and only during breeding season, where such occurs). Each cycle consists of a brief period (usually a day or two) of oestrus (estnis) or 'heat' at which time and at no other the female will copulate with a male, ovulation coinciding with this oestrus; and, preceding and succeeding oestrus, there are various changes throughout the body, particularly in the uterus, which may be regarded as preparations for pregnancy. The whole set of changes is controlled by hormones. In outline the phases of the cycle are typically (a) follicular with growth of Graafian follicles in ovary,, proliferation of lining of uterus (endo-metrium), increasing secretion of oestrogen by ovary; (b) owlation, with activation of mating reflexes; (c) luteal, with corpus luteum formation in ovary, great development of uterine glands, secretion of progesterone by corpora lutea, oestrogen secretion diminishing; (d) regression of corpus luteum, beginning of new follicular growth, return to unproliferated state of uterine lining, diminution of oestrogen and cessation of progesterone secretion. If fertilization occurs in phase (b) then phase (d) is omitted, and the cycle is suspended in the luteal phase for the duration of pregnancy. If there is no fertilization then the cycle usually immediately repeats, but in some species there is only one cycle per breeding season. Cycle depends on cyclical production of gonadotrophic hormones by pituitary gland, these hormones controlling changes in the ovary which in turn controls all the other changes by means of the cycles of oestrogen and progesterone production. There is considerable variation according to species in the characteristics of the cycle; the luteal phase may for instance be omitted (mouse); and another variant is the menstrual cycle.
near oestrous cycle in Knolik
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