Mortal nonstem germ cells in culture are differentiating. (A) Analysis of time-lapse image sequences showed that among germ cells that have divided at least once in culture, 99.4% of the cell deaths (n = 314) are synchronized among sibling groups; only 0.6% died as isolated cells. (B) Images from a time-lapse sequence of a typical mortal germ cell–derived clone (Video 1) and a plot of the cell number within the clone over time show the synchrony of cell divisions and cell death of sibling germ cells in the clone. (C) The experimental scheme for a functional assay specific for intercellular bridges. It involves photobleaching of EGFP within one cell of a group and monitoring of EGFP fluorescence intensity of each cell by time-lapse imaging. (D–F) Time-lapse image sequences before and after photobleaching of the cell indicated by an asterisk, and a time trace of EGFP fluorescence intensity in each cell of a germ cell group in culture (D and E, also see Video 2) or in testis (F). Bleached regions are delineated by yellow lines. Arrows point to visible intercellular bridges. The numbers in D–F indicate which cells correspond to the fluorescence intensity traces shown on the right. Bars, 10 µm.