Both chromatin bridges and lagging chromatids were generated far more frequently during the multipolar mitoses than the normal bipolar mitoses. Formation of micronuclei from multipolar mitoses had been suggested by studies on fixed oral cancer cells and live Chinese hamster cells under phase contrast microscopy. After the completion of multipolar mitoses, cytokinesis did not always separate eachdaughter nucleus, thus multinucleated cells were frequently generated. We found 74% of the tripolar and all of the tetrapolar mitoses generated multinucleated cells.By CCX140 following the fate of a portion of these cells, we found that the mononuclear cells from the multipolar mitoses underwent apoptosis during subsequent interphase, whereas a portion of the multinuclear cells could reach the next mitoses. The apoptosis of mononucleated cells should not be a result of loss of a specific gene because all cells had undergone apoptosis despite these cells received about two-thirds of the genome. Rather, the total amount of genome complements, i.e. less than diploid, is expected to induce the apoptotic response. We hereafter call such hypoploid daughter nuclei ����small nuclei����, in order to discriminate them from micronuclei that originate from acentric chromatin. It was known that micronuclei were structurally heterogeneous with respect to the chromatin condensation level and the presence of nuclear lamina, however, the reason for this had not been determined. In this study we found that one factor contributing to heterogeneity was the mechanism of generation of micronuclei. The chromatin condensation level was quite different between micronuclei generated by different mechanisms. In our study, chromatin condensation was roughly estimated by the intensity of Vecuronium Bromide fluorescence from H2B-GFP or DAPI stain, because these intensities reasonably correlate with chromatin condensation, e.g. heavier label or stain at heterochromatic regions.Representative time-lapse images illustrate that chromatin in the lagging chromatid-derived micronuclei was relaxed compared to that in the main nucleus during telophase to early G1 phase.