Abstract
Seasonal variations of sex steroid hormones in follicular fluid, and their follicular morphologies in mares were examined in order to characterize the breeding season in south-western Japan. A total of 194 follicles (>10 mm in diameter) harvested from 28 fattening mares at abattoir in 5 occassions were used for endocrinological and morphological survey. Large follicles >40 mm in diameter and corpora lutea determined with ultrasonography were observed in ovaries collected in March, May and August, whilst they were absent in non-breeding season; January. Ovaries in October showed in-between state; there was a corpus luteum but not large follciles. Follicular development were classified into 4 stages depend on the morphological findings; non-atretic, mild atretic, atretic and heavy-atretic. Most follicles were classified as atretic stages, while non-atretic large follicles >40 mm in diameter were observed in breeding seasons. During the non-breeding season, there were only middle sized non-atretic follicles, 30-40 mm in diameter. Estradiol-17β and progesterone concentrations in follicular fluid decreased as the follicle became atretic. It was concluded that the breeding season was already started by March and was undergone until August, while non-breeding season occurred in October and was utmost in January in south-western Japan. According to morphological survey, although there was neither ovulation nor corpus luteum formation in the ovary, folliculogenesis of middle sized non-atretic follicles indicated the existence of the ovarian follicular developmental activity during the non-breeding season.
