Ovarian follicular population changes with the advance of the breeding season in intact and unilaterally ovariectomized ewes

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Abstract
In ewes at the 1st, 2nd or 4th estrous cycle after unilateral ovariectomy, the ovulation rate remained constant at 1.5 in the control (sham-operated) ewes, but increased from 1.3 to 2.0 in unilaterally ovariectomized ewes. In control ewes, the proportion of preantral follicles declined significantly (P < 0.05) with each estrous cycle while the antral follicles increased as the breeding season progressed (P < 0.05). In contrast, after unilateral ovariectomy, the proportion of preantral and antral follicles remained constant throughout the cycles studied. The rate of atresia of antral follicles, especially those from small size classes, decreased significantly after 1 cycle of unilateral ovariectomy (P < 0.05). Larger antral follicles had a different rate of atresia as the breeding season advanced. Unilateral ovariectomy apparently acutely decreased the rate of atresia and maintained the within-ovary equilibrium between preantral and antral follicles which otherwise would have decreased due to the depletion of preantral follicles with the advance of the breeding season.