Multinucleation in mouse fibroblasts cultured in methocel medium

3T3‐4E cells formed multinucleate cells with high frequency when incubated in methocel medium. The experiment with hydroxyurea and the cytological observation of mitoses showed that multinucleate cells were produced by nuclear division in the absence of cytokinesis. When transferred onto a solid substratum, most of the multinucleate cells divided within seven hours into mononucleate cells through the process of cytoplasmic division, indicating that cell spreading induced cytokinesis. Other mouse fibroblast lines examined so far showed only the low frequency of multinucleation. These findings indicate that in 3T3‐4E cells cultivated in methocel, nuclear division occurred independently of cytokinesis, and that cytokinesis was also anchorage‐dependent. This system will be available for studying cytoplasmic division of mammalian cells.