The fine structure of the pulmonary alveolar wall reveals extracellular components that are typical expressions of connective tissues. Boundary (basement) membranes (basal laminae) are found in close association with both pulmonary epithelium and capillary endothelium. In them a feltwork of fine primary fibrils (30 to 50 Angstroms in diameter) supports a finely granular amorphous material. Where the blood-air barrier is thinnest, a single boundary membrane is located between epithelium and endothelium. Between capillaries the tissue space contains reticular fibers and elastic fibers. Reticular fibers are composed of numerous, small unit collagen fibrils which average 200 to 300 A in diameter and are arranged in roughly parallel array. Elastic fibers are small, usually being less than 0.5μ in diameter. They possess a homogeneous core which is surrounded peripherally by hollow microfibrils. Occasional fibroblasts are present.