Polyploidization in MK results from a late reversal of cytokinesis. Primary cultured MKs from human bone marrow were transduced with a plasmid encoding H2B-GFP. Time-lapse images were obtained by confocal video microscopy. Time relative to the first image is indicated in minutes on each photograph. Bars represent 10 μm. (A) A mitosis/endomitosis transition. The cell was stained with CellTracker Orange and serial images were obtained at 3 minutes interval. Endomitosis proceeded exactly as a mitosis until the late telophase when the 2 nuclear masses are separated and the 2 daughter cells are connected by a cytoplasm bridge (images 54,66). Thereafter, the 2 daughter cells moved backward toward each other (images 75-96) and reassembled into 1 single cell with only 1 nuclear mass (image 114). (B) An endomitosis in a polyploid MK. Serial images were obtained at 5-minute intervals. Endomitosis proceeded as a mitosis but DNA was scattered at 4 poles until the end of cytokinesis (images 0-40). The 4 daughter cells remained connected (images 30,35,40). Then the incipient daughter cells moved backward and are reunified 2-by-2 (images 45-85). These time-lapse images are available in Videos S1 and S2.