A comparative stereological analysis was performed in cumulus-oocyte complexes from Holstein-Friesian cows, aspirated from small (with diameter of 1-4mm) and medium (with diameter of 4-8mm) antral follicles during metestrous, diestrous and proestrous. The mean volumes of COCs, oocytes (with and without zona pellucida), nucleus of the oocyte, as well as the volumes of the follicular cells and their nucleus were estimated. The mean thickness of the zona pellucida and the relative percentage of the three follicular cell types in the cumulus mass were also obtained. Small antral follicles had an accelerate growth without synchrony between the volume of the oocyte and the respective nucleus, while in medium antral follicles an harmonious nucleus-cytoplasmic expansion was observed. These follicular cell populations were classified in their types, C1, C2 and C3, which the distribution in the cumulus mass appeared to be independent to their position in relation to the oocyte. During the estrous cycle, cell type C1 progressively developed to C2, and this one to C3.
bovine; follicular cell; estrous cycle; COC; stereology
