Use of high-resolution transrectal ultrasonography to assess changes in numbers of small ovarian antral follicles and their relationships to the emergence of follicular waves in cyclic ewes

Abstract

Transrectal ovarian ultrasonographic studies have shown that, in cattle, follicular wave emergence is associated with a large increase in the number of small antral follicles (4–6 mm in diameter); an analogous association has not been found for small follicles (2–3 mm in diameter) in the ewe. In previous studies in ewes, accurate assessment of the number of follicles has been limited to follicles ≥2 or 3 mm in size. Newer, high-resolution equipment allowed us to identify follicles ≥0.4 mm and to quantify all antral follicles ≥1 mm in diameter in seven cyclic Western White Face ewes. This allowed us to expand the small follicle pool examined, from 1 to 4 follicles/day (2–3.5 mm in diameter) in earlier studies, to 8–18 follicles/day (1–3 mm in diameter). Total number of small follicles (≥1 and ≤3 mm in diameter) increased between Days −1 and 0 (Day 0=day of ovulation), and declined between Days 1 and 3 ( P<0.05). There were no significant changes in the number of small or medium (4 mm in diameter) follicles around days of follicle wave emergence (±2 days). The 1–3 follicles in the 2–3 mm size range, which constituted a follicle wave (i.e. grew to ≥5 mm in size before regression or ovulation), were the only small follicles to emerge in an orderly succession during the estrous cycle, approximately every 3–5 days. Thus, unlike in cattle, there is no apparent increase in numbers of small follicles at follicle wave emergence in cyclic sheep, and little evidence for selection of recruited follicles and follicular dominance.