Influence of Stage of Cycle, Corpus Luteum Location, Follicle Size, and Number of Large Follicles on Estradiol-17β Concentrations in Bovine Follicles

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Abstract
Concentrations of estradiol-17.beta. in follicular fluid were correlated to follicular size, stage of estrous cycle, location of corpus luteum, and presence of large follicles. Paired ovaries were obtained from 481 nonpregnant cows at slaughter and follicles were classified as ipsilateral or contralateral to the corpus luteum. Follicular fluid estradiol-17.beta. concentrations from 2494 small, 1485 medium, and 396 large follicles were quantified by radioimmunoassay. Stage of estrous cycle was estimated by visual examination of the corpus luteum. Follicles in stage 1 of the estrous cycle (d 1 to 4) had the highest estradiol-17.beta. concentration and the smallest mean follicular diameter. Location of follicles relative to the corpus luteum had no influence on estradiol-17.beta. concentrations. As follicular size increased, concentration of estradiol-17.beta. also increased. The presence of a single large follicle did not affect the concentration of estradiol-17.beta. in medium or small follicles. In contrast, if multiple large follicles occurred in the same cow, concentrations of estradiol-17.beta. were significantly lower in medium but not small follicles.