Chemical and histochemical studies of human alveolar collagen fibers.

Light and electron microscopic studies have established that the normal human alveolar argyrophilic (reticulum) fiber is collagen fiber. The silver impregnation method is highly sensitive and specific for histologic demonstration of the elaborate collagen fiber network of alveolar septa. The argyrophilic alveolar collagen fiber does not stain with the periodic acid-Schiff (PAS) or periodic acid-thiocarbohydrazide-osmium tetroxide (PTO) reaction. The materials positive for the PAS and PTO reactions in alveolar septa are epithelial and endothelial basal laminas, which are nonargyrophilic. Chemically, lung collagen fibers are composed of Type I and Type III collagens, which differ in amino acid composition, chain composition, and carbohydrate content. The chemical heterogeneity of lung collagen may have important biologic implications in the maintenance of normal structure and in the repair of lung injury.