Fig. 4.

Abnormal pneumocytes and cellular syncytia in COVID-19 lungs. A. Cytological features of SARS-CoV-2-infected lungs. A consistent and typical feature in COVID-19 lungs was the presence of major cytological abnormalities, including very large cells with dysmorphic phenotypes (a, x63; b, x40) often showing bi-o multinucleation (c,d, x40). B. Squamous metaplasia (pseudosyncytia). A common finding was the metaplasia of the alveolar epithelium, with a marked change in morphology of pneumocytes and their aggregation to form pseudosyncytia (a, b, x20; a’, b’, x63). C. Syncytia. In addition to squamous metaplasia, true syncytial elements were observed in numerous lungs, showing large cytoplasm and nuclear aggregation (a, b, x20; a’, b’, x63). D. Origin of syncytial elements. The giant and multinucleated cells forming either pseudo-syncytia or real syncytia scored positive for the pneumocyte markers Surfactant-A (a, x20), TTF1 (b, x20) and Napsin (c, x20), indicative of their epithelial origin. The COVID-19 lungs also showed the more occasional presence of CD163-positive syncytia of histiocytic origin (d, x20). In A-C, H&E: hematoxylin and eosin.