Distinguishing these cells is significant in the pathophysioloy of Acute Respiratory Distress Syndrome.
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. Type I cells line more than 95% of the alveolar surface.
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. Type II cells in the alveolar wall contain secretory granular organelles known as lamellar bodies that fuse with the cell membranes and secrete pulmonary surfactant.
In addition, it synthesises, stores and releases pulmonary surfactant into the alveolar hypophase, where it acts to optimise conditions for gas exchange. Type II cells are the most numerous cells in the alveoli, despite covering less surface area as the squamous type I cells.
“Histology, A Text and Atlas, Sixth Edition,” 2011, by Ross, Michael H, and Pawlina, Wojciech
Thompson et. al 2017