Histological populations and atresia of ovarian follicles in postpartum cattle treated with an agonist on gonadotropin-releasing hormone

Surface and histological changes of ovarian follicles and degree of atresia were characterized 3 and 6 d after a treatment with a GnRH agonist (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 experimental period, eight cyclic cows were injected with 2 mL of physiological saline and the remaining eight cyclic cows and the acyclic cows were injected i.m. with 8 micrograms of buserelin. Ovaries were collected 3 or 6 d later (n = 4 cows per group per day). All follicles (> 1.57 mm) 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 1 follicles decreased on d 3 but increased on d 6 (day X treatment, P < .004), whereas those of Class 2 increased (P < .07) on both d 3 and 6 after buserelin treatment. However, within Class 2 follicles, we detected an increase 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 atresia 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 follicles on d 6 (day X treatment, P < .04) and of Class 1 nonatretic follicles 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 growth to large follicles by increasing atresia. Acyclic and cyclic cows had similar responses to buserelin for large follicles but not for follicles of Classes 1 or 2.