Distribution of visible follicles on the ovarian surface in ewes

Two studies were conducted to determine the distribution of follicles on the ovarian surface in ewes. In Exp. 1, the effect of the corpus luteum on the distribution of follicles was investigated on d 9, 12 and 14 postestrus in unilaterally ovulating ewes. Follicles (greater than or equal to 2 mm diameter) were inventoried by size and distance from the center of the corpus luteum or corpus albicans. There were more small follicles (less than or equal to 4 mm diameter) on d 9 than on d 12 and 14, Although the ovary bearing the corpus luteum had a greater number of large follicles, no positive relationship of the corpus luteum to the diameter of the closest follicle or the distance of the largest follicle was found. In Exp. 2, follicles were inventoried on ovaries from ewes during the estrous cycle, postpartum anestrus, seasonal anestrus and the peripuberal period. During the estrous cycle, two apparent waves of follicular growth were noted. In addition, during the luteal phase the luteal ovary consistently had a larger follicle than the nonluteal ovary. When estimates were made of the effects of size of follicle on distance of the follicle from the reference structure and of distance on size of follicle, presence of luteal tissue, and day of estrous cycle were found to influence the variances. In addition the interaction of these characteristics affected the variation of size or distance. Weighting the distribution of ovarian structures for relative ovarian mass allowed an examination of localization of structures to standardized ovarian areas. More follicles and corpora lutea were found in the middle of the ovary than on the ends. Although the numbers of follicles in a standardized area were correlated with ovarian mass, occurrence of corpora lutea in a standardized area was not significantly correlated with ovarian mass. Collectively, these results are indicative of a local effect by the corpus luteum on the diameter of the largest follicle and a systemic effect on the number of follicles. In addition, because specific regions appear to be more favorable for follicular activity and ovulation, a precise local effect of the corpus luteum is difficult to determine.