Seasonal Variations of Sex Steroid Hormones in Plasma and Ovarian Follicular Morphologies of Mares in Southern Kyushu

Abstract

Seasonal variations of sex steroid hormones in plasma and the ovarian follicular morphology were examined in order to characterize the breeding season in mares in Southern Kyushu, Japan. A total of 280 fattening mares slaughtered at abattoir were examined all year round. Mean plasma progesterone levels varied from 3.0ng/ml in February to 8.3ng/ml in July, while mean plasma estradiol-17β levels varied from 3.3pg/ml in November to 13.1pg/ml in June. Plasma estradiol-17β levels were relatively higher during the breeding season, March to July. A total of 2,202 follicles≥10mm in diameter were subjected for morphological examination. Follicles≥30mm in diameter existed in most ovaries all year round, and reduced its number during the non-breeding season, October to December. However, the percentage of non-atretic viable follicles in May was significantly higher than that in January (70% vs, 32% ; P<0.05). Viable follicles in non-breeding season, November to February, showed relatively thinner granulosa cell layer, higher rate of shrinkage and nuclear condensation in theca cells compared to those found in breeding season, March to October. In conclusion, mares fattened in Southern Kyushu showed measurable level of reproductive profiles in non-breeding season, but non-atretic viable follicles≥30mm showed thinner granulosa cell layer and lack in lutenized theca cells compared to those found in breeding season.