Late diffuse alveolar damage

High power photomicrograph shows changes typical of the proliferative or late stage of diffuse alveolar damage. Although hyaline membranes are still identifiable, the histologic picture is now dominated by thickening and reorganization of interstitial structures due mainly to marked proliferation of mesenchymal spindle cells, including both fibroblasts and myofibroblasts.

High-power photomicrograph shows alveoli containing capillaries within a narrow interstitium. The alveoli are lined with thin, elongated type I pneumocytes (arrow) and smaller numbers of cuboidal type II pneumocytes (dashed arrow).