Models of stem cell maintenance. Illustrated are two models for stem cell maintenance. In both models, the starting population is assumed to contain both stem cells (S) and more differentiated progenitor cells (D). Model A illustrates a self-renewal process where the stem cell divides asymmetrically to give rise to a committed and a pluripotent daughter cell, thus maintaining stem cell numbers at each generation. Model B illustrates a quiescence process where stem cells are maintained in a nonproliferative state and the committed progenitors divide to give rise to mature lineages.