The authors were interested in the pathological features in lung tissues of patients who have died of COVID-19. They systematically analysed lung tissue samples from 38 patients who died from COVID-19 in two hospitals in northern Italy. The most representative areas identified by macroscopic examination were selected and tissue blocks (median seven) were taken from each lung and fixed in 10% buffered formalin for at least 48 hours. Tissues were examined using haematoxylin and eosin staining, immunohistochemical staining for inflammatory infiltrate and cellular components and electron microscopy. All of the cases had features of both the exudative and proliferative phases of diffuse alveolar damage, including capillary congestion (all cases), necrosis of pneumocytes (all cases), hyaline membranes (33 cases), interstitial and intra-alveolar oedema (37 cases), type 2 pneumocyte hyperplasia (all cases), squamous metaplasia with atypia (21 cases) and platelet-fibrin thrombi (33 cases). The inflammatory infiltrate, seen in all cases, was mostly macrophages in the alveolar lumina (24 cases) and lymphocytes in the interstitium (31 cases). Electron microscopy found that viral particles were predominantly located in the pneumocytes. Therefore the predominant pattern of lung lesions in COVID-19 patients is diffuse alveolar damage, similar to that seen in SARS and MERS. Hyaline membrane formation and pneumocyte atypical hyperplasia were frequently seen. The presence of platelet-fibrin thrombi in small arterial vessels is consistent with coagulopathy, which could be a potential target for therapy.
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