Type I pneumocyte cells (also called type I alveolar cells or squamous alveolar cells) are extremely attenuated cells that line the alveolar surfaces of the lungs. They cover 97% of the alveolar surface, with type II pneumocytes covering the remainder. These cells are so thin (sometimes only 25 nm) that the electron microscope was needed to prove that all alveoli are covered with an epithelial lining. It is important that these cells are thin so that gas exchange between the alveoli and blood can occur easily. Their main role is to provide a barrier of minimal thickness that is readily permeable to gases such as oxygen and carbon dioxide.
Organelles of Type 1 pneumocyte cells such as the ER, Golgi apparatus, and mitochondria are clustered around the nucleus. This leaves large areas of cytoplasm virtually free of organelles and reduces the thickness of the cell, thus reducing the thickness of the blood-air barrier. The cytoplasm in the thin portion contains pinocytotic vesicles, which may play a role in the turnover of surfactant and the removal of small particulate contaminants from the outer surface. (However, type 2 pneumocytes are primarily responsible for secretion of surfactant). In addition to desmosomes, all type I pneumocyte cells have occluding junctions that prevent the leakage of tissue fluid into the alveolar air space.
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.