FIGURE 5. Stem cells can facultatively use both symmetric and asymmetric divisions.

a, Division in the plane of the epithelium generates two morphologically similar daughter cells that are both likely to be stem cells (orange). Grey line, basement membrane. b, Division perpendicular to the plane of the epithelium generates one stem cell and one differentiated daughter (green). Such asymmetric divisions by stem cells are thought to predominate during late fetal development and adulthood in the basal layer of epithelia^{7, 61} and in the ventricular zone of the brain^{40, 53}. Although spindle orientation seems to correlate with cell fate in this manner in various systems, it is not an obligate relationship because current data on progenitor identity and daughter cell fates are incomplete, and divisions in the plane of the epithelium can sometimes yield progenitors that acquire different fates⁵⁴. c, During development, symmetric divisions expand the stem-cell pool. d, In healthy adults, divisions perpendicular to the epithelial plane typically maintain normal numbers of stem cells and differentiated cells in the basal layer of epithelia and in the subventricular zone of the brain. e, In healthy adults, cells can be lost to injury (X). Symmetric divisions are proposed to regenerate additional stem cells, and asymmetric divisions to regenerate differentiated daughters. f, We speculate that defects in regulation of the switch between symmetric and asymmetric divisions can be deleterious. Left, a defect favouring symmetric divisions results in tumorigenesis. Right, a defect favouring asymmetric divisions results in decreased capacity for tissue repair. Both tumorigenesis and poor wound healing are typical of ageing animals, raising the question of whether defects in switch mechanisms accumulate with age.