Freeze‐etch observations of rat lung

The freeze‐etch technique was used to study the fine structure of the terminal respiratory membranes of rat lung. Special emphasis was given to the ultrastructure of the type II alveolar cell and its role in formation of the surface active material. Our morphological evidence supports the theory that membrane‐bound multivesicular bodies in the cytoplasm of type II cells accumulate phospholipid material to form lamellar structures which are extruded onto the alveolar surface as one of the components of the surface active material. Subsequent loss of the lamellar configuration in the surface active layer is discussed. Electron micrographs demonstrate numerous microindentations that extend into the base of alveolar cells and help establish a close relation with the basal lamella.