Stereologic characterization of bovine (Bos taurus) cumulus-oocyte complexes aspirated from small antral follicles during the diestrous phase

Bovine ovarian cumulus-oocyte complexes (COCs) are used for in vitro matura tion and fertilization after selection by size and morphology, but their de velopmental 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 aspirate d from diestrous ovaries of Holstein-Friesian cows, fixed in glutaraldehyde , randomly embedded in glycol-methacrylate, and sectioned at 20 mum. The un biased nucleator principle was used for estimating the mean volumes of comp lexes, 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 opt ical disector procedure was used for cumulus cell sampling. Volume estimati on 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 u nique reference point. Quantitative data showed that COCs appear heterogene ous for all studied parameters and that the cumulus mass contains three dif ferent cell populations.