Fig. 3

Early COVID-19 causes lung injury characterized by mild congestion of small capillaries in the alveolar walls (asterisk) associated with inflammation and hyaline membranes (A). Organizing diffuse alveolar damage in COVID-19 features alveolar walls (asterisk) that are thickened by exuberant fibroblastic proliferations and lined by AT2 cells (B). Some cases show juxtaposition of acute injury with hyaline membranes (asterisk), AT2 cell hyperplasia, and interstitial fibroblastic proliferation (arrow) that expands the alveolar walls (C). The interstitial fibroblastic proliferation is replaced by dense collagen that stains blue with a trichrome stain (D). The alveolar architecture is lost in the late stages of COVID-19 lung disease (A–C hematoxylin and eosin stain)