Demonstration of the extracellular alveolar lining layer (surfactant) in human lungs

The structure of the extracellular alveolar lining layer in 3 human lungs is described. This lining was composed of 2 layers: a) a highly osmiophilic surface layer, b) a base layer. The surface layer consisted of lamellar structures with 40 Å intervals. It was fragile and broken in many areas. The base layer molded the irregularities of the alveolar surface; it frequently appeared in the form of “tubular myelin”. In the latter the intervals between lamellae were about 450 Å. The demonstration of these structures was achieved by routine intratracheal instillation of glutaraldehyde and osmium post-fixation. The lining layer is interpreted as a constituent of pulmonary surfactant.