Ovarian follicular response of mares to an equine pituitary extract after suppression of follicular development

Abstract

Follicular population dynamics were monitored in 36 mares by diagnostic ultrasonography. The objective was to identify the day of physiological selection of the ovulatory follicle in mares in which follicular development was first suppressed with an estrogen-progesterone regimen. The hypothesis was tested that the selection process involves selection for an ovulatory follicle and concurrent selection against the other follicles in the cohort from which the ovulatory follicle arose. The experimental design was based on the consideration that mares treated with equine pituitary extract (EPE) before the time of physiological selection would exhibit a superstimulatory response whereas mares given EPE after the time of selection would not. One group of mares was untreated ( n = 12) and four groups ( n = 6) were given EPE when the diameter of the largest follicle attained 15 mm, 20 mm, 25 mm, and 30 mm after suppression of follicular development with estradiol-17β and progesterone. The mares were examined until ovulation or the diameter of the largest follicle attained 35 mm. Diameters of the largest and second largest follicles and number of follicles in 6 to 10 mm, 11 to 15 mm, 16 to 20 mm, 21 to 25 mm, 26 to 30 mm, and > 30 mm diameter categories were determined. There was a main effect of day ( P < 0.0001) for diameters of the two largest follicles. The group effect and the group by day interaction were significant for diameter of the second largest follicle; the interaction appeared due primarily to increased diameter during the later portion of the experimental period in the 15-mm, 20-mm, and 25-mm groups. Mean diameter of the second largest follicle on the day that the largest follicle reached 35 mm was greater ( P < 0.05) for the 15-mm (32.5 ± 0.9 mm), 20-mm (33.2 ± 1.1 mm), and 25-mm groups (30.8 ± 1.8 mm) than for the untreated (20.1 ± 1.5 mm) and 30-mm groups (22.5 ± 2.4 mm). There was a main effect of group ( P < 0.08 to P < 0.01) and day ( P < 0.006 to P < 0.0001) for all diameter categories. There were more follicles > 30 mm ( P < 0.05) on the day that the largest follicle reached 35 mm in the 15-mm (2.5 ± 0.6) and 20-mm (2.2 ± 0.6) groups than for the untreated (0.8 ± 0.2) and > 30-mm groups (0.8 ± 0.3). The 25-mm group was intermediate (1.2 ± 0.4). In the 15-mm and 20-mm groups versus the 30-mm group there were more ( P < 0.02) follicles 21 to 25 mm and 26 to 30 mm several days before ovulation or the day that the diameter of the largest follicle reached 35 mm. Administration of EPE when the diameter of the largest follicle attained 15 mm, 20 mm, or 25 mm resulted in a superstimulatory response, indicating that selection for an ovulatory follicle and against other follicles had not yet occurred. By the time that the largest follicle reached 30 mm, the ovulatory follicle had already been selected as indicated by the lack of a superstimulatory response. Results indicated that selection of the ovulatory follicle occurred when the diameter of the largest follicle reached between 25 mm and 30 mm and supported the hypothesis that at approximately the time of physiological selection of the ovulatory follicle other follicles in the cohort are committed to atresia.