Fig. 4. Representative light micrographs of stained lung tissue sections from hydrochloric acid (HCl)– and saline (NaCl)–treated mice at 12 h, 24 h, and 2 weeks after intrabronchial instillation. (A ) HCl, 12 h, right lung. Almost all bronchioalveolar spaces are filled by fibrinoproteinaceous material with hyaline membrane formation. Alveolar septa are congested and accompanied by severe interstitial hemorrhages. Notice the medium-sized blood vessels surrounded by a prominent empty halo consistent with severe perivascular edema. Hematoxylin and eosin; scale bar , 100 μm. (B ) NaCl, 12 h, right lung. Instilled lung showed a minimal alveolar septa expansion with sparse neutrophilic infiltrates. Hematoxylin and eosin; scale bar , 100 μm. (C ) HCl, 24 h, right lung. Pulmonary parenchyma is completely effaced by marked necrotic and fibrinohemorrhagic changes associated with severe medium-sized blood vessel thrombosis and vasculitis. The latter finding is characterized by transmural disruption of vascular wall by marked neutrophilic infiltration (inset ). Hematoxylin and eosin; scale bar , 100 μm. (D ) NaCl, 24 h, right lung. Alveolar septa are moderately expanded with occasional findings of type II pneumocyte reactive hyperplasia. Hematoxylin and eosin; scale bar , 50 μm. (E ) HCl, 2 weeks, right lung. Alveolar spaces are almost completely obliterated by marked alveolar septa fibroplasia accompanied by diffuse type II pneumocyte hyperplasia and mild histioneutrophilic infiltration. Large intraalveolar groups of foamy macrophages are also multifocally evident. Hematoxylin and eosin; scale bar , 100 μm. (F ) HCl, 2 weeks, right lung. Area of scarring showing complete loss of the alveolar architecture due to abundant deposition of collagen fibers with interspersed fibroblast, newly formed blood capillaries, hemosiderin-laden macrophages, and fewer lymphocytes. Hematoxylin and eosin; scale bar , 50 μm.