Composition and characteristics of follicular waves during the bovine estrous cycle

Abstract

The characteristics of anovulatory and ovulatory follicular waves for 18 interovulatory intervals with two waves were studied in Holstein heifers. Daily ultrasonic monitoring of individually identified follicles was used. Waves were detectable retrospectively as a cohort of 4 to 6 mm follicles on a mean of day 0 (day of ovulation) for the anovulatory wave and day 10 for the ovulatory wave. For each wave, the follicles which became dominant versus subordinate did not differ in diameter on the first day of the wave, but the dominant follicle was significantly larger than the subordinates on the following day. On the average, the subordinates ceased growing 4.4 days after the origin of a wave. The dominant follicle of the anovulatory wave grew linearly (1.8 ± 0.1 mm/day) to an average of 15.8 ± 0.5 mm, remained static for a mean of 6 days and then regressed linearly (−1.0 ± 0.1 mm/day). The dominant ovulatory follicle grew slower ( P<0.0001) (linear slope, 1.2 ± 0.1 mm/day) than the dominant anovulatory follicle. The diameter of the ovulatory follicle on the day before ovulation (16.2 ± 0.4 mm) was not different from the diameter of the dominant anovulatory follicle during the static phase. The numbers of growing, static and regressing 4 to 6 mm identified follicles did not differ between anovulatory and ovulatory waves. Ninety-five percent of the growing identified follicles were assignable to a wave (follicles emerging within two days of each other) and each wave emerged during a consistent and narrow time period (anovulatory wave, days −1, 0, or 1; ovulatory wave, days 8, 9, 10, or 11). It was concluded, therefore, that the formation of waves was a well-controlled phenomenon. There was a consistent temporal relationship between emergence of the ovulatory wave and onset of regression of the dominant follicle of the anovulatory wave (length of interval from beginning of ovulatory wave to beginning of regression of anovulatory follicle, approximately 3 days). Perhaps, therefore, the mechanism that caused regression of the subordinate follicles of the ovulatory wave also caused regression of the large, static, dominant follicle of the anovulatory wave.