Fig. 6.2.

Proliferative stress requirements of stem cells at steady state and following moderate and severe injury. (a) At steady state, the majority of mitotic stem cells undergo asymmetric cell division (green cells), which is capable of compensating for cell loss during a low rate of turnover without depleting the stem cell population. (b) Following injury, basal cells may undergo symmetric differentiation (yellow cells) in order to more rapidly compensate for the loss of many cells during a higher rate of turnover. Stem cells that undergo symmetric differentiation are removed from the stem cell pool, but other stem cells may compensate for this loss through symmetric self-renewal (blue cells). (c) If the proliferative stress is sustained long enough or the injury is severe enough, the capacity of the stem cell population to self-renew is insufficient to compensate for the loss of stem cells through differentiation. In this case, reserve stem cells from neighboring regenerative stem cell pools may attempt to compensate for stem cell loss. However, in the case of chronic injury, stem cells are ultimately depleted leading to fibrosis and disease