Alveolar fibrosis and capillary alteration in experimental pulmonary silicosis in rats.

To analyze the evolution of fibrotic and vascular changes in pulmonary silicosis, ultrastructural and immunohistochemical studies were made of the lungs of rats given a single intratracheal injection of silica particles. Early lesions were characterized by accumulations of macrophages and neutrophils in alveolar lumina and interstitium and by damage to alveolar capillaries and epithelial cells. The intraluminal masses of inflammatory cells developed into granulomas and became associated with myofibroblasts that migrated from the interstitium through the damaged epithelial lining. Type II epithelial cells and bronchiolar cuboidal cells proliferated rapidly to line the intraluminal granulomas, incorporating them into the interstitium. This process mediated the transition from intraalveolar fibrosis to interstitial fibrosis. Vascular damage was repaired by proliferation and migration of endothelial cells. Some endothelial cells in alveolar capillaries expressed Factor VIII-related antigen at 2 wk after silica infusion. In normal animals, this feature was present in peribronchiolar but not in alveolar capillaries. Two patterns of endothelial cell migration were shown by staining for proliferating-cell nuclear antigen. The first pattern was characterized by endothelial cells that extended their cytoplasm over preexisting, denuded basement membranes and replaced necrotic cells in alveolar capillaries. At 4 mo after injury, some of these cells had developed fenestrations. The second pattern consisted of budlike sproutings that developed only in peribronchiolar connective tissue. These observations indicate that peribronchiolar vessels are sources for renewal of alveolar capillary endothelium as well as for neovascularization.