FIGURE 21-11 Patterns of stem-cell differentiation. Different patterns of stem-cell division produce different proportions of stem cells (red) and differentiating cells (green). Stem-cell divisions must meet three objectives: they must maintain the stem-cell population, they must sometimes increase the number of stem cells, and at the right time, they must produce cells that go on to differentiate. (a) Stem cells can undergo asymmetric divisions, producing one stem cell and one differentiating cell. This pattern does not increase the population of stem cells. (b) Some stem cells can divide symmetrically to increase their population, which may be useful in normal development or during recovery from injury, at the same time that others in the same population can be dividing asymmetrically as in (a). (c) Some stem cells may divide as in (b) while at the same time other stem cells produce two differentiating progeny. See S. J. Morrison and J. Kimble, 2006, Nature 441:1068–1074.