SEASONAL VARIATION IN THE MORPHOLOGY AND NUCLEAR CONFIGURATION OF PREOVULATORY AND IN VITRO MATURED MARE OOCYTES

Meiotic maturation of mammalian oocytes is a complex process and in the horse many factors hinder the results of in vitro maturation and in vitro fertilization. The aim of the present study was to examine the morphology and the chromatin configuration of the mare oocytes to asses the meiotic competence for in vitro maturation in two different seasons: winter and spring. Oocytes harvested by scraping were morphologically categorized in compact, partial, expanded, corona radiate and cumulus free. In experiment 1, all recovered oocytes were denuded immediately after collection and evaluated for initial chromatin configuration by Hoechst 33342. In experiment 2, cumulus-oocyte complexes (COCs) were matured by culture for 29 h and evaluated for cumulus expansion and for the stage of meiosis. Our data suggest that the number of oocytes with a compact cumulus at the beginning of the in vitro culture correlates strongly with the presence of the diffusely fluorescent nucleus configuration, inversely to other studies where the chromatin condensed configuration represents the meiotically competent oocyte. Finally, the maturation rate was the same in the two different seasons so, potentially, in vitro maturation can be feasible at any time regardless the reproductive season in the equine.