Patterns of growth and regression of ovarian follicles during the oestrous cycle and after hemiovariectomy in mares

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Abstract
Summary: To increase our understanding of the regulation of ovarian follicular development during the oestrous cycle, 13 mares were used in two experiments. In Experiment 1, the pattern of growth and regression of individual follicles, 15 mm or more in size, was monitored daily during 17 oestrous cycles by real‐time ultrasonography. Each examination was recorded on videotape to ensure accurate monitoring of individual follicles. Results indicated that the development of such follicles occurs in waves, with the most common pattern being one follicular wave per oestrous cycle (12 of 17 cycles). An alternative pattern of two follicular waves per cycle was observed in mares with (n=3) and without (n=2) dioestrous ovulations. The ovulatory follicle appeared earlier in the cycle in mares with one wave than in mares with two waves (Day 6.3 ± 0.9 vs Day 11.0 ± 1.2, respectively; p<0.01). Also, the ovulatory follicle was the largest follicle for a longer period of time in mares with one wave than in mares with two waves per cycle (9.5 ± 0.6 vs 5.8 ± 0.3 days, respectively; p<0.01). Mares with one follicular wave had shorter cycles than those with two waves per cycle (18.2 ± 0.6 vs 21.6 ± 0.5 days, p<0.001). In both cases, the interval between the appearance of the ovulatory follicle and its ovulation was, on average, 10 to 12 days. Overall, a negative correlation was found between the relative size of the ovulatory follicle at the beginning of the follicular phase and the length of the follicular phase (r=‐0.91, p<0.001). To study the control of follicular recruitment, patterns of follicular growth were monitored in nine mares following removal of the ovary bearing the presumptive ovulatory follicle (Experiment 2). Hemiovariectomies were performed between Day 14 of dioestrus and the fourth day of oestrus. Follicular dynamics were characterised by the development of one follicular wave that produced one to three ovulatory follicles in seven of nine mares. The ovulatory follicle reached 15 mm or more 6.3 ± 1.5 days following surgery and the interval from hemiovariectomy to ovulation was 13.7 ± 1.3 days. Two of nine mares hemiovariectomised on Day 14 failed to undergo complete luteolysis after the ovary bearing the ovulatory follicle, but not the corpus luteum, was excised. These mares had not ovulated by 30 days after surgery, although follicular waves producing ovulatory size follicles were observed. This study indicated that the development of ovarian follicles 15 mm or larger in the mare occurs in waves, with a more common pattern of one wave and an alternative pattern of two waves per oestrous cycle.