Histology of the corpus luteum in the ewe: Changes during the estrous cycle and early pregnancy, and in response to some experimental treatments

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Abstract
A technique, developed for the objective histological evaluation of the corpus luteum of the sheep, was applied to normal cyclic and pregnant ewes as well as to ewes subjected to various experimental treatments. Gross histological regression of the corpus luteum progressed rapidly between days 15 and 17 of the estrous cycle. Similar regressive changes did not occur in pregnant ewes; up to day 40, the oldest stage studied, corpora lutea of early pregnancy were similar in appearance to those of mid‐cycle.Stresses such as high environmental temperature and acute undernutrition, which may lead to embryo mortality, had no significant effect on corpus luteum histology. Exogenous progesterone slightly depressed the size and weight of the corpus luteum but was without significant effect on its histology. Exogenous cortisol, and more particularly corticotropin, were sometimes associated with marked histological regression of the corpus luteum, although in ewes treated with these hormones there was no significant reduction in embryo survival at the time of autopsy.The observations do not support the suggestion that corpus luteum failure is a frequent cause of embryonic death in the ewe. The technique presented is considered sufficiently sensitive to detect quite small changes in the histological appearance of the corpus luteum, although the relationship of these changes to progesterone secretion is not known. However, the dramatic regression observed between days 15 and 17 of the estrous cycle fits the known pattern of progesterone secretion at this time.