Tubular myelin structures in the lungs of amphibia. The mode of formation.

The lungs of the tree frog Hyla arborea and the toad Bufo bufo were fixed in glutaraldehyde with the addition of picric acid or tannic acid to visualize tubular myelin (TM), one of the forms of pulmonary surfactant. The lungs of both Hyla and Bufo are complex in structure and possess one type of pneumocytes containing numerous lamellar bodies (LBs). The alveolar lining layer in the lung of Hyla includes numerous LBs at different stages of transformation into a monolayer film, but the lattice is sparse, delicate and unstable. The structure of smooth membranes in the alveolar layer is not clearly visible and their thickness varies from 4.0 to 7.2 nm. In the lung of Bufo TM occurs commonly in the alveolar lining layer. Membranes of LBs are smooth particle-free bilayers of 8.1 to 12.0 nm thick. During transformation into TM, the membranes become covered by fuzzy coat and irregularly spaced rod-like particles of 8 to 13 nm in diameter. I suppose that these particles facilitate the maintenance of TM structures.