The present study was undertaken to determine the precise stage of growth at which the ability to resume meiosis is acquired in bovine oocytes. Oocytes of various sizes were isolated from ovaries by mechanical dissection using an 18-gauge needle followed by a razor blade. This method yielded an average of 26.2 +/- 7.4 growing and fully grown oocytes from an ovary. Cumulus-enclosed oocytes were cultured in vitro in tissue culture medium 199 containing 10% fetal calf serum. Oocytes less than or equal to 90 microns in diameter did not resume meiosis. However, germinal vesicle breakdown was observed in oocytes whose diameters exceeded 91 microns. Polar body formation was observed in oocytes with diameters exceeding 101 microns. About 80% of the oocytes with diameters greater than or equal to 121 microns were able to extrude the polar body. The percentage of large oocytes (101 to 120 microns) with first polar body increased when incubated in medium containing dibutyryl cyclic adenosine 3',5'-monophosphate; however, oocytes 90 to 101 microns did not extrude the first polar body even when cultured in a medium containing dibutyryl cyclic adenosine 3',5'-monophosphate. These observations indicate that the capability to resume meiosis is acquired gradually during development of oocytes and that dibutyryl cyclic adenosine 3',5'-monophosphate can improve the meiotic competence of bovine oocytes in culture.
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