Relationships between Histological Signs of Atresia, Steroids in Follicular Fluid, and Gonadotropin Binding in Individual Bovine Antral Follicles during Postpartum Anovulation1

Two experiments were conducted to determine the relationship between histological signs of atresia, gonadotropin binding, and steroids in fluid of medium-sized bovine follicles during postpartum anestrus. In Experiment I, ovaries of 21 cows were removed on Days 7, 14, 28, 42, or 56 after parturition. In Experiment II, ovaries of 29 cows were removed between Days 20 and 30 postpartum after 48 or 96 h of either saline (0.9% NaCI, 5 ml) or luteinizing hormone-releasing hormone (LHRH; 500 ng/5 ml saline) injections given every 2 h via jugular cannulas. Two to 10 follicles, 4.0-7.9 mrn in diameter, were removed per pair of ovaries. Follicles were classified as normal, intermediate, atretic, or luteinized-atretic, depending on their micromorphology. In both Experiments I and II, follicles classified as normal had 50-80% lower (p O.lO) between normal and intermediate follicles. Equal proportions of normal and atretic medium-sized follicles were located on the ovaries bearing the corpus albicans from pregnancy (CAP). The data suggest that presence of the CAP and/or gravid uterine horn has no effect on the proportion of follicles 4.0-7.9 mm in diameter that undergo atresia. In addition, atresia of these medium-sized antral follicles is reflected initially by increased progesterone in fluid, then by decreased estradiol. Since gonadotropin binding was not related to the degree of atresia, other events may regulate the shifts in steroidogenesis observed during follicular atresia in postpartum cattle.