Fig. 1.

A model. During steady-state homeostasis, only a subgroup of alveolar type II cells (AT2) behave as progenitor cells and are engaged in the replacement of lost alveolar type I cells (AT1) or AT2. After injury, a larger number of AT2 are activated and behave like progenitor cells. Some of the activated AT2 differentiate into specific subpopulations that are in the middle of AT2 to AT1 transition process. Many of these intermediate subpopulations eventually differentiate into AT1 and repair the injured alveolar epithelium.