Ovarian follicular populations in pony and saddle-type mares

Five pony and 5 saddle-type mares were used to compare total ovarian follicular populations between breeds. The animals were hemi-ovariectomized at the preovulatory stage during the breeding season and the ovary bearing the large preovulatory follicle was studied using histological techniques. Pony and saddle-type mares did not differ as to mean number of primordial follicles, mean number of growing follicles, the variability of these numbers and follicular distribution into various size classes. No difference was detected either in the initiation processes of follicular growth, oocyte growth, granulosa cell multiplication and follicular atresia. However, significant differences were found in the pattern of antral formation (P less than 0.01). The present data suggest that pony mares are a valuable tool for the study of follicular populations in equines. The main features of the equine ovary are the small number of follicles (35 950 primordial and 100 growing follicles) and the high individual variability of this number (5 600 to 75 000 primordial and 20 to 300 growing follicles).