[Chronological aspects of the nuclear maturation of bovine oocytes cultivated in vitro].

In this study, the chronological changes in the meiotic progress of in vitro maturation of bovine oocytes was analysed. Oocytes (n = 1044) were obtained from a local abattoir and were classified into two groups according to the presence or absence of cumulus cells. They were incubated in microdrops (5 per drop) in TCM-199 (control), TCM-199 + 10% fetal calf serum (treatment 2) and TCM-199 + 10% estrous cow serum (treatment 3). Oocytes were fixed and stained at the end of 0, 6, 12, 18, 24 and 30 h of the beginning of in vitro culture, were evaluated according to the nuclear stage of maturation. These results demonstrated that the immature oocytes at the time of collection (0 h) were in the germinal vesicle stage (GV), and that the highest maturation rate was at 24 h of culture in all treatments. Serum treatments enhanced the maturation rates obtained (52.1 and 55.7%) compared to control (serum-free) medium (42.7%; P < 0.05) in cumulus-cell-enclosed oocytes. In denuded oocytes, the maturation rates were lower compared to cumulus-cells-enclosed oocytes in all treatments. In conclusion, meiotic progression of bovine oocytes can be influenced by the inclusion of sera in the maturation media and by the presence of the cumulus cells.