Figure 1. A schematic view of stem cell differentiation.

Differentiated cells that constitute organs and tissues are generated during development. Their characteristics and identity are faithfully maintained to ensure proper organ function. Throughout life many of these cells are replenished by the differentiation of stem cells. Therefore, adult stem cells have to be maintained stably. Yet, the differentiation of stem cells requires the cell identity to be changed from a stem cell to a differentiated cell with a specialized function. This transition passes through a point when cell identity is not stable (red). Notably, during embryogenesis adult stem cells, themselves, are generated from progenitors (green arrow).