Short- and long-term effects of unilateral ovariectomy in sheep: causative mechanisms

The mechanisms of ovulatory compensation following unilateral ovariectomy (ULO) are still not understood. In the present study, we investigated the short- and long-term effects of ULO in sheep using transrectal ovarian ultrasonography and hormone estimations made during the estrous cycle in which surgery was done, the estrous cycle 2 mo after surgery, and the 17-day period during the subsequent anestrus. The ULOs were done when a follicle in the first follicular wave of the cycle reached a diameter > or =5 mm, leaving at least one corpus luteum and one ovulatory-sized follicle in the remaining ovary. Ovulation rate per ewe was 50% higher in the ULO ewes compared with the control ewes at the end of the cycle during which surgery was performed, but it did not differ between groups at the end of the cycle, 2 mo later. This compensation of ovulation rate in ULO ewes was due to ovulation of follicles from the penultimate follicular wave in addition to those from the final wave of the cycle. Ovulation from multiple follicular waves appeared to be due to a prolongation of the static phase of the largest follicle of the penultimate wave of the cycle. Interestingly, the length of the static phase of waves was prolonged in ULO ewes compared with control ewes in every instance where the length of the static phase could be determined. Changes in follicular dynamics due to ULO were not associated with alterations in FSH and LH secretion. In conclusion, ovulatory compensation in ULO sheep involves ovulation from multiple follicular waves due to the lengthened static phase of ovulatory-sized follicles. These altered antral follicular dynamics do not appear to be FSH or LH dependent. Further studies are required to examine the potential role of the nervous system in the enhancement of the life span of the ovulatory-sized follicles leading to ovulatory compensation by the unpaired ovary in ULO sheep.