RELATIONSHIPS AMONG OOCYTE-CUMULUS MORPHOLOGY, FOLLICULAR ATRESIA, INITIAL CHROMATIN CONFIGURATION, AND OOCYTE MEIOTIC COMPETENCE IN THE HORSE

Horse oocytes with expanded (EX) cumuli appear to have greater meiotic competence than do horse oocytes with compact (CP) cumuli but are tho ught to come from atretic follicles. We evaluated the relationships am ong cumulus expansion, follicle viability, initial chromatin configura tion, and meiotic competence of horse oocytes. Follicle walls were sec tioned for histological examination, and the follicles were scraped to obtain the oocytes. Half of the oocytes were evaluated immediately an d half were matured for 24 h in vitro. Cumulus expansion was significa ntly associated with follicle atresia. initially significantly more EX than CP oocytes had chromatin condensed into one mass within the germ inal vesicle (CC configuration; 61% vs. 32%). After culture, significa ntly more EX than CP oocytes had matured (74% vs. 30%). The proportion of oocytes with the CC configuration was lowest in viable follicles a nd increased in follicles with slight to moderate atresia. The maturat ion rate of oocytes from viable follicles was significantly lower than for oocytes from follicles with slight or moderate atresia. The CC ch romatin configuration appears to be associated with meiotic competence in horse oocytes. The association of follicle atresia with increased meiotic competence suggests that acquisition of meiotic competence is related to a loss of suppressive activity by the degenerating follicle .