Fig. 1. Mechanisms of stem cell divisions. If individual stem cells in a niche are differently marked (color coded green or white) the mechanism of division can be assessed (for clarity, only stem cells but not differentiating cells are depicted). (A) If regulated at the single cell level, each stem cell division is strictly asymmetric to give rise to an new stem cell and a differentiating cell (shaded arrows, cells not shown). The distribution of genetic sectors does not change. (B) If regulated in a population mode, symmetric and asymmetric divisions occur. Owing to symmetric divisions genetic sectors can enlarge at the expense of others. See text for details.