HISTOLOGICAL POPULATIONS AND ATRESIA OF OVARIAN FOLLICLES IN POSTPARTUM CATTLE TREATED WITH AN AGONIST OF GONADOTROPIN-RELEASING-HORMONE

Surface and histological changes of ovarian follicles and degree of at resia were characterized 3 and 6 d after a treatment with a GnRH agoni st (buserelin) in postpartum beef cows. Sixteen cyclic cows on d 7 of their estrous cycle and eight acyclic cows were used. On d 0 of the ex perimental period, eight cyclic cows were injected with 2 mt of physio logical saline and the remaining eight cyclic cows and the acyclic cow s were injected i.m. with 8 mu g of buserelin. Ovaries were collected 3 or 6 d later (n = 4 cows per group per day). All follicles (> 1.57 m m) were observed microscopically, measured using routine histological techniques, and classified as either nonatretic or atretic, with early or late atresia, and grouped into Class 1, 1.57 to 3.67 mm, Class 2, 3.68 to 8.56 mm, or Class 3, > 8.56 mm. Buserelin induced formation of new corpora lutea in all cows but one (acyclic cow). Class I follicle s decreased on d 3 but increased on d 6 (day x treatment, P < .004), w hereas those of Class 2 increased (P < .07) on both d 3 and 6 after bu serelin treatment. However, within Class 2 follicles, we detected an i ncrease on d 3 and 6 in the number (P < .01) and proportion (P < .07) of atretic follicles, an increase in the number of follicles in late a tresia on d 6 (day x treatment, P < .03), and a decrease in the number (P < .06) and proportion(P < .03) of nonatretic follicles on d 6 (day x treatment). In acyclic cows, the numbers of Class 2 atretic follicle s on d 6 (day x treatment, P < .04) and of Class II nonatretic follicl es on d 3 and 6 (P < .07) were less than in cyclic cows. These results indicate that buserelin induced ovulation and increased the turnover of Class 2 follicles from the pool of Class 1 follicles but limited gr owth to large follicles by increasing atresia. Acyclic and cyclic cows had similar responses to buserelin for large follicles but not for fo llicles of Classes 1 or 2.