Differential Effects of Cytochalasin B on Cytokinesis in Parthenogenetically Activated Mouse Oocytes

Parthenogenetically activated mouse oocytes, induced by 7% ethanol for 5 min at room temperature then treated with a low (0.5 μg/ml) or a high (5 μg/ml) concentration of cytochalasin B (CCB), developed into different types of eggs in vitro. Pronuclei formations were observed 6 h after the commencement of ethanol treatment, and 81% of the eggs showed an immediate cleavage with a pronucleus in each blastomere at 0.5 μg/ml CCB, while at a higher concentration of 5 μg/ml CCB, 90% of the eggs showed two-pronuclei in a single cell. Without CCB treatment, single-pronuclei with extruded second polar bodies were mainly seen in the majority (63%) of the eggs. Observations of both actin filaments and nuclei by confocal laser scanning microscopy showed that no spindle rotation prior to extrusion of the second polar body resulted in immediate cleavage in the presence of a low concentration of CCB. Therefore the mechanism of spindle rotation strongly depends on actin filaments.