Fig. 1

A Model: After lung injury, some AT2s are activated and behave as facultative stem cells. A fraction of the activated AT2s differentiates into intermediate cells in the middle of AT2 to AT1 transition. Many of these transient intermediate cells differentiate into AT1s and repair the alveoli subsequently. Depending on the age and type of injury, sometime AT1s can differentiate into AT2s. After some severe lung injuries, subgroups of airway cells can be mobilized and migrate into alveoli. These cells became certain intermediate cell types and might also be able to transit into AT2s or AT1s