Preovulatory follicular events in sheep

To investigate the number, size and fate of the follicles greater than 2 mm in diameter involved in the differentiation of the preovulatory follicle in-vivo monitoring of growth and regression of individual follicles was performed at different times after a PG-induced follicular phase. In Exp. 1, the 3 largest follicles of each ovary were labelled with ink in the peripheral stroma at 0, 4, 8, 12, 24 or 48 h after a PG injection. At a second laparotomy, it was assessed which follicles had ovulated. In Exp. 2, a similar procedure was applied in 12 ewes which underwent repeated laparotomies at 6, 30 and 54 h after PG and after ovulation. At all times, there was a huge variability in the size at the time of ink labelling of the follicles that finally ovulated ('ovulatory' follicles). A single measurement of follicle size is therefore a poor prediction of the ovulatory follicles. However, a significant reduction in the proportion of non-ovulatory follicles within the size range of ovulatory follicles occurred at 12-24 h after PG, together with a change in follicle hierarchy at 8-24 h after PG. The results of repeated laparotomies showed that only 60% of follicles greater than 2 mm at 6 h after PG had grown by 30 h. Selection occurred between 30 and 54 h after PG as indicated by a significant drop in the number of growing follicles to a level at 54 h that equalled the ovulation rate. The growth rate of the ovulatory follicles was 1.4 and 0.4 mm per day between 6 and 30 h and 30 and 54 h after PG respectively. The shrinkage rate of non-ovulatory follicles was 1.6 mm/day.