Bovine ovarian cumulus-oocyte complexes (COCs) are used for in vitro maturation and fertilization after selection by size and morphology, but their developmental potential remains low. Stereology could provide more objective criteria for selecting the most competent complexes, but its application is lacking in cattle. COCs from small (1–4 mm) antral follicles were aspirated from diestrous ovaries of Holstein-Friesian cows, fixed in glutaraldehyde, randomly embedded in glycol-methacrylate, and sectioned at 20 μm. The unbiased nucleator principle was used for estimating the mean volumes of complexes, oocytes, cumulus cells, and nuclei of oocytes and cumulus cells. The thickness of the zona pellucida and the relative numerical percentages of the several morphologic types of cumulus cells were also evaluated. The optical disector procedure was used for cumulus cell sampling. Volume estimation based on a real physical unique point did not differ from those based on a particular point among many or on a virtual central point, and the mean cumulus cell volume was estimated by using the single section bearing the unique reference point. Quantitative data showed that COCs appear heterogeneous for all studied parameters and that the cumulus mass contains three different cell populations.
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