Colloidal gold immunoultrastructural localization of rat surfactant

Using a polyclonal antiserum against the nonserum proteins in purified rat surfactant, we have localized protein antigen within the lamellar bodies of rat alveolar Type II cells perfusion-fixed with 2% cacodylate-buffered paraformaldehyde and postfixed with 0.5% osmium. A postembedment indirect immunogold ultrastructural localization was used and 20 nm gold particles were localized over the lamellae in Type II cell lamellar bodies, in tubular myelin, and in some of the secondary lysosomes of alveolar macrophages. Occasional labeling was seen in the rough endoplasmic reticulum and multivesicular bodies in some Type II cells, but the amount of this staining was not different from nonspecific background. There was, however, an invariant lack of labeling over all other lung cell types. These results demonstrate the presence of surfactant proteins within the lamellar body secretory product and support the idea that the surfactant lipoprotein complex is formed within intracellular sites prior to its secretion into the alveolar space.