In this study, we have shown that butyrolactone I (BL-I), a potent inhibitor of cyclin-dependent kinases, affects oocyte cytoplasmic morphology and nuclear function in terms of nucleolar ultrastructure and immunocytochemistry. Bovine oocytes were recovered from three classes of follicle size: 1.5-2, 2-3, and 3-6 mm. The oocytes were incubated for 40 hr with BL-I, and subsequently processed for transmission electron microscopy or immunocytochemistry. A control group of oocytes were processed immediately upon recovery (0 hr). In general, incubation with BL-I for 40 hr disrupted contact between cells of the cumulus oophorous and the oocyte, caused degeneration of the cortical granules and the peripheral migration of all cytoplasmic organelles. At the level of the nucleus, it induced convolution of the nuclear membrane and caused acceleration of nucleolar compaction in oocytes from follicles < 3 mm and fragmentation of nucleoli, particularly evidenced by immunocytochemistry, in oocytes from follicles > 3 mm. Furthermore, the effects appear to be more profound in fully-grown oocytes.
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