Alveolar type I and type II cells.

The alveolar epithelium comprises two main cell types: the alveolar type I and alveolar type II cell. The type I cell is a complex branched cell with multiple cytoplasmic plates that are greatly attenuated and relatively devoid of organelles; these plates represent the gas exchange surface in the alveolus. On the other hand, the type II cell acts as the "caretaker" of the alveolar compartment. It responds to damage of the vulnerable type I cell by dividing and acting as a progenitor cell for both type I and type II cells. In addition, it synthesises, stores and releases pulmonary surfactant into the alveolar hypophase, where it acts to optimise conditions for gas exchange.