ALVEOLAR LINING LAYER IS THIN AND CONTINUOUS - LOW-TEMPERATURE SCANNING ELECTRON-MICROSCOPY OF RAT LUNG

The low-temperature electron microscope, which preserves aqueous struc tures as solid water at liquid nitrogen temperature, was used to image the alveolar lining layer, including surfactant and its aqueous subph ase, of air-filled lungs frozen in anesthetized rats at 15-cmH(2)O tra nspulmonary pressure. Lining layer thickness was measured on cross fra ctures of walls of the outermost subpleural alveoli that could be soli dified with metal mirror cryofixation at rates sufficient to limit ice crystal growth to 10 nm and prevent appreciable water movement. The t hickness of the liquid layer averaged 0.14 mu m over relatively flat p ortions of the alveolar walls, 0.89 mu m at the alveolar wall junction s, and 0.09 mu m over the protruding features (9 rats, 20 walls, 16 ju nctions, and 146 areas), for an area-weighted average thickness of 0.2 mu m. The alveolar lining layer appears continuous, submerging epithe lial cell microvilli and intercellular junctional ridges; varies from a few nanometers to several micrometers in thickness; and serves to sm ooth the alveolar air-liquid interface in lungs inflated to zone 1 or 2 conditions.