Systemic and intraovarian effects of dominant follicles on ovine follicular growth.

The objective was to study the endocrine activity in sheep with large ovarian follicles and the effects of dominant follicles on other follicles, looking for possible intraovarian differences. Induction of dominant follicles was achieved using controlled exogenous LH pulses every 90 min over 14 days in eight Scottish Blackface ewes. During this period, follicular development was assessed by daily transrectal ultrasonography and jugular venous blood samples were collected every 12 h for FSH, LH inhibin and oestradiol assay. The exogenous LH pulses caused the appearance of large follicles in all the ewes, which reached a maximum mean diameter of 7.2 +/- 0.5 mm on Day 5.5 +/- 2.6 after first detection. In the presence of a dominant follicle, no other follicle grew to a diameter larger than 4 mm and there was a decrease in the number of new growing follicles (P < 0.05) and in the number of smaller follicles (P < 0.01). This effect of dominance was mediated by changes in FSH concentration, since FSH level decreased (P < 0.05) as dominant follicles grew and the decrease in FSH levels was related to a decline in the number of remaining follicles (P < 0.05). However, the greatest decrease in the number of small follicles growing to larger sizes was observed in the ovary ipsilateral to the dominant follicle (P < 0.05). These data confirm that the presence of a large follicle depresses the recruitment and growth of other follicles by systemic factors and provide some evidence of local inhibitors blocking the final development of other putative large follicles.