MORPHOLOGY AND RESPONSIVENESS OF THE TWO LARGEST OVARIAN FOLLICLES IN ANESTROUS EWES

The morphology and the responsiveness of ovarian follicles to an ovulating stimulus imposed in the presence or absence of previously induced corpora lutea (CL) were studied in anestrous DLS ewes (Dorset × Leicester × Suffolk). Induction of CL formation was done with 500 IU of PMSG given intravenously. In CL-bearing ewes, the diameter of the largest follicle was significantly larger 12 and 16 days after PMSG injection, while that of the second largest follicle was increased only on day 12. When ovulation was induced with 500 IU PMSG on day 16, the presence of a previously induced CL did not modify the ovulation rate. The two largest follicles identified with India ink markings 4 days prior to PMSG injection did not respond by ovulation and most of these follicles had become completely atretic by day 21 of the experiment. When the two largest follicles, however, were identified and marked on the day of PMSG injection, 65% of them responded to the stimulus. This indicated that turnover of the large follicles was occurring during the anestrous season as is the case during the normal breeding season.