Growth of small follicles and concentrations of FSH during the equine oestrous cycle.

The temporal associations between increases in FSH and growth of small follicles (2-20 mm) were studied during one oestrous cycle (ovulation to ovulation) in 15 horse mares. Follicular diameters were obtained ultrasonically. For each day, follicles were combined for both ovaries, grouped from largest to smallest (excluding dominant follicles), and divided into three to five tiers of six follicles for each mare (for example: tier 1, six largest follicles; tier 2, next six largest follicles). A significant increase in mean diameters followed by a significant decrease was used to define a follicular wave within each of the tiers for each mare. Day of wave emergence was defined by the lowest mean preceding the increase. Follicular waves were detected in all tiers in each mare. The number of detected waves per interovulatory interval was greater (P < 0.05) for tier 5 (2.8 +/- 0.4) than for tier 1 (1.1 +/- 0.1) and tier 2 (1.9 +/- 0.3). A primary follicular wave (wave giving rise to the dominant follicle that ovulated during the subsequent oestrus) was identified in tier 1 in all mares. Composite profiles were constructed by normalizing each tier for each mare to the mean day of emergence of the primary wave for all mares, as detected in tier 1. The composite follicular profiles for the five tiers were approximately parallel; the follicles for tiers 2-5 emerged on the mean day of emergence of the primary wave (6 days after ovulation). The range of follicular means on the common day of wave emergence was 3 mm (tier 5) to 12 mm (tier 1).(ABSTRACT TRUNCATED AT 250 WORDS)