Control of Ovarian Function in Domestic Animals

Abstract

Plasma hormone levels during the estrous cycle of the cow, ewe, and sow have been measured, and the patterns of secretion of estrogens, progesterone, and luteinizing hormone during the cycle have been related to ovarian changes and other information concerning the cycle for each species. Peripheral plasma progesterone and LH levels are generally inversely related during the cycle in each species, and it seems clear that progesterone exerts a negative feedback on LH secretion in all three species, at least insofar as the cyclic release of preovulatory amounts of LH is concerned. Peak plasma progesterone levels are highest in the sow, lowest in the ewe, and intermediate in the cow. Plasma LH levels at estrus are highest in the ewe, lowest in the sow, and intermediate in the cow. Sharp peaks in plasma LH occur at the onset of estrus in the cow, and a few hours after the onset of estrus in the ewe and sow; these peaks are of about 6–8 hr duration. LH exerts a luteotrophic action on the corpora lutea of all three species, and verylow levels of LH secretion appear capable of maintaining the corpus luteum in the ewe and cow. There is no good evidence that prolactin is luteotrophic in any of these species. Three peaks of plasma estrogen levels are seen in the ewe and the cow and these appear related to periods of accelerated follicle growth. One peak occurs early in the cycle and before plasma progesterone levels rise appreciably and another occurs during the luteal phase just prior to corpus luteum regression. The third peak occurs after plasma progesterone levels decline and is associated with growth of the ovulating follicle. The luteal phase estrogen peak has not been found in the sow. The rapid rise in blood estrogens after the corpus luteum regresses facilitates the preovulatorysurge of LH in all three species. Cyclical regression of the corpus luteum in all three speciesappears to be under local control of the adjacent horn of the uterus. Exogenous estrogens are luteolytic in the cow and ewe, but luteotrophic in the sow. The ovaries of all three species contain very poorly developed interstitial tissue probably because of the neaily complete dedifftrentiation of the thecal cells during atresia. Thus, these animals lack an important source ofsteroid hormones present in the lodents and certain other species.