Fig. 2. Microscopic findings in lung tissue in patients with ARDS.

In acute respiratory distress syndrome (ARDS), features of diffuse alveolar damage (DAD), such as in the acute ‘exudative’ phase (~7 days) (panel a), are typically followed by alveolar type II (ATII) cell hyperplasia and interstitial fibrosis in a ‘proliferative’ phase. Eosinophilic depositions termed hyaline membranes are defining features of DAD (pink structure lining the central alveolus, indicated by the arrowhead in panel b) are defining features of DAD. Leukocytes are embedded in the hyaline membranes (arrows in panel b). Electron microscopic analyses (panel c) demonstrate that alterations in endothelial and epithelial cells are critical features of acute alveolar injury in ARDS^37,46. Focal epithelial destruction of alveolar type I (ATI) cells and denudation of the alveolar basement membrane occur early in ARDS, whereas endothelial continuity is preserved with modest alterations in most cases. The pattern shown in panel c was identified in the lungs of a patient with indirect acute lung injury resulting from sepsis^37,46. A, alveolar space; BM, basement membrane; C, capillary; EC, erythrocyte; EN, endothelial cell; HM, hyaline membrane; LC, leukocyte. Reprinted with permission of the American Thoracic Society. Copyright © 2019 American Thoracic Society. Matthay, M. A. & Zimmerman, G. A. (2005) Acute lung injury and the acute respiratory distress syndrome: four decades of inquiry into pathogenesis and rational management. Am. J. Respir. Cell Mol. Biol. 33, 319–327. The American Journal of Respiratory and Critical Care Medicine is an official journal of the American Thoracic Society.