Turnover of dominant follicles in cattle of different reproductive states

Abstract

Primordial follicles are present in cohorts of non-growing follicles in the ovaries of the heifer at birth. Through unknown mechanisms, a cohort of follicles is recruited to grow and a group of 2–6 mm follicles develops. A single follicle is selected and becomes a dominant follicle, suppressing the growth of other medium follicles. During the estrous cycle, there are recurrent periods of turnover of dominant follicles culminating in the final maturation and ovulation of the dominant follicle that is present at the time of luteolysis. Studies using daily ultrasound scanning of the ovaries have shown that there are either 2 or 3 dominant follicles present during an estrous cycle and longer cycles generally have 3 dominant follicles. The first dominant follicle can be retrospectively identified on the day of ovulation as one of a group of 3–4 follicles of 5–8 mm in diameter. The first dominant follicle increases in size daily until maximum diameter is obtained on days 6–8. It remains stable in size for 2–4 days and then gradually declines. The second dominant follicle is the ovulatory follicle in heifers with 2 dominant follicles, and it undergoes atresia, generally on days 16–17, in heifers where the third dominant follicle ovulates. It appears that final maturation and ovulation of a dominant follicle require an increase in LH pulse frequency, so that androgen and estrogen synthesis are stimulated sufficiently to give rise to a gonadotrophin surge. A similar pattern of turnover of dominant follicles occurs in heifers prior to onset of puberty, in early pregnancy, and in heifers with persistent corpora lutea. In the postpartum dairy cow, growth of 6–8 mm follicles begins within 7–10 days of calving and from this group of follicles, a single dominant follicle emerges and ovulates in 75–80% of cows between days 10 and 30 after calving. Cows ovulating after, but generally not before day 20 post partum are more likely to have a short cycle (8–13 days duration) where the first dominant follicle ovulates. In beef suckler cows the pattern of resumption of follicular development was similar to the dairy cow, but the first dominant follicle only ovulated in 2 18 cows; it took a mean of 3.2±0.2 dominant follicles to emerge before first ovulation occurred. Thus, basal levels of gonadotrophins and the expression of suppressive effects of the dominant follicle play key roles in regulating follicular growth.