The connective tissue of the rat lung: electron immunohistochemical studies.

The ultrastructural distribution of specific connective-tissue components in the normal rat lung was studied by electron immunohistochemistry. Three of these components were localized: type I collagen, fibronectin and laminin. Type I collagen was present not only in major airways and vascular structures, but also in alveolar septa. Laminin was found in all basement membranes, and only in basement membranes, demonstrating once more that this glycoprotein is an intrinsic component of the basement membrane. Fibronectin was found free in the interstitium and on the surfaces of collagen fibers. The basement membranes of bronchial, glandular and endothelial cells of large vessels lacked fibronectin; however, capillary endothelial and occasionally epithelial alveolar basement membranes contained some fibronectin in an irregular, spotty distribution. This localization suggests that in the lung, as in other tissues, fibronectin is not an intrinsic component of the basement membrane, but rather a stromal and plasma protein. Only basement membranes in the alveolar parenchyma contained "trapped" plasma fibronectin.