Meiotic competence in horse oocytes: Interactions among chromatin configuration, follicle size, cumulus morphology, and season

Horse oocytes were collected from an abattoir over a 15-mo period. After classification of follicle size and cumulus morphology, oocytes were either fixed immediately (0 h) or matured in vitro (24 h). There was no effect of season on the number of antral follicles present on the ovaries, or on oocyte maturation rate for any class of oocyte, The proportion of oocytes having condensed chromatin at 0 h increased with increasing follicle size. The oocyte maturation rate also increased with follicle size, and for follicles less than or equal to 20-mm diameter, was higher for oocytes initially having expanded cumuli than for those having compact cumuli, The maturation rate was strongly correlated (r(2) = 0.92) with the proportion of oocytes having condensed chromatin at 0 h, Oocytes with diffuse chromatin were found essentially only in follicles less than or equal to 20-mm diameter that yielded compact granulosa, indicating follicle viability. Presence of diffuse chromatin was inversely related to maturation rate. We conclude that the major signal for chromatin condensation, and thus acquisition of meiotic competence, occurs in viable follicles after 20-mm diameter in the horse. Condensation of chromatin in oocytes in smaller apparently viable follicles, while associated with acquisition of meiotic competence, may represent a pre-atretic change.