Figure 10.

Three options for stem cell division. Three possible outcomes of stem cell activation and cell division are illustrated: an “α” division represents the classic asymmetric cell division with renewal of the mother cell and generation of a daughter that enters a downstream transit population. A “2α” cell division generates two identical stem cells, increasing the number of total stem cells by one. Finally, a “μ” division generates two cells that enter a downstream transit population, depleting the number of stem cells by one. The balance between “2α” and “μ” events determines whether a given stem cell pool will increase or decrease in number.