FREEZE-FRACTURE STUDY OF ALVEOLAR LINING LAYER IN ADULT-RAT LUNGS

The freeze-fracture replication technique was used to study the structure of the alveolar lining layer in the rat lung. The surface film appeared to be a finely granular membranous film devoid of membrane-associated particles. The density of its granularity varied from place to place, particularly near zones of emergence of linear grooves on the film. Linear grooves overlay and constituted the upper wall of underlying tubular myelin. Occasionally, there were small saccular depressions in the surface film. The hypophase generally appeared thin but was abundant over niches in the alveolar walls, especially at the junctions of alveolar septa. It was generally a structureless layer, but occasionally contained spherical bodies and tubular myelin, which tended to gather in the niches. Apparently, the surface film is a phospholipid monolayer containing no bound protein. Tubular myelin, saccules and spherical bodies may be forms of surfactant molecules in reserve, prior to incorporation into the surface film.