Evolution of three patterns of intra‐alveolar fibrosis produced by bleomycin in rats

In pulmonary fibrosis, it is known that fibrotic changes develop in the intra-alveolar spaces and that intra-alveolar fibrosis can be classified into three patterns, namely intra-alveolar buds, mural incorporation and obliterative changes. In order to clarify the evolution of intra-alveolar fibrosis, immunohistochemical studies of extracellular matrix proteins and electron microscopic observations were made of the lungs of rats given a single intratracheal instillation of bleomycin. All three patterns of fibrosis developed in this model. Intra-alveolar buds changed into globular lesions with dense collagen deposition, the surface of which was covered by alveolar epithelium. Electron microscopy revealed that the buds often contained spiraling collagen fibrils and numerous microfibrils, but not mature elastic fibres, beneath the regenerating epithelial lining cells; the epithelial basement membranes were discontinuous. In contrast, mural incorporation and obliterative changes were associated with alveolar structural remodeling. Electron microscopically, these lesions had bundles of normal collagen fibrils, small elastic fibers, and continuous epithelial basement membranes. These results indicate that: (i) intra-alveolar buds, that become intra-alveolar collagen globules, with an unusual extracellular matrix, do not contribute to alveolar structural remodelling; and (ii) areas of mural incorporation and obliterative changes have the usual type of extracellular matrix and are essential for alveolar structural remodelling.