Type I pneumocytes (simple squamous alveolar cells) are simple squamous epithelial cells responsible for gas exchange in the alveoli and cover a majority of the alveolar surface area (>97%). While type I pneumocytes account for most of the surface area, they are approximately half as numerous as type II cells, due to differences in shape. Type II pneumocytes are large round cells, while type I pneumocytes are smaller cells stretched across a large surface area.
Type I pneumocytes are unable to replicate and are susceptible to toxic insults. In the event of damage, Type II cells can proliferate and/or differentiate into type I cells to compensate.