The ultrastructural characteristics of alveolar (ABM) and capillary (CBM) basement membranes in the adult rat lung have been defined using tannic acid fixation, ruthenium red staining, or incubation in guanidine HCl. ABM is dense and amorphous, has 3- to 5-nm filaments in the lamina rara externa (facing the alveolus) that run between the lamina densa and the basal cell surface of the epithelium, has an orderly array of ruthenium red-positive anionic sites that appear predominantly (79%) on the lamina rara externa, and has discontinuities beneath alveolar type II cells but not type I cells that allow penetration of type II cytoplasmic processes into the interstitium of the alveolar wall. The CBM is fibrillar and less compact than ABM, has no lamina rara filaments, and has one fifth the number of ruthenium red-positive anionic sites of ABM that appear predominantly (64%) overlying the lamina densa. Incubation of lung tissue with Flavobacterium heparinum enzyme or with chondroitinase has shown that ABM anionic sites represent heparan sulfate proteoglycans, whereas CBM anionic sites contain this and other sulfated proteoglycans. The CBM fuses in a local fashion with ABM, compartmentalizing the alveolar wall into a thick and thin side and establishing a thin, single, basement-membrane gas-exchange surface between alveolar air, and capillary blood. The potential implications of ABM and CBM ultrastructure for permeability, cell differentiation, and repair and morphogenesis of the lung are discussed.

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