Histopathology.
Lung tissues were fixed in neutral-buffered formalin and embedded in
paraffin. Lung sections were cut at 4 μm, dewaxed, rehydrated, and
stained with hematoxylin and eosin. Various lung pathologic features
including acute or organizing diffuse alveolar damage (DAD), alveolar
hemorrhage, thrombosis, and fibrotic remodeling described earlier in
COVID-19 patients38-40 were evaluated by an
experienced pathologist (AK) in a blinded manner. Pathological lesions
of acute DAD were characterized by disrupted alveolar epithelium,
epithelial sloughing, formation of hyaline membranes, necrotizing
bronchiolitis, interstitial inflammation, oedema, fibrin deposition,
vasculitis, pulmonary hemorrhage, and microvascular thrombosis. The
organizing form of DAD was defined as loosely organized fibrosis,
interstitial fibrotic changes, and abnormal remodeling of epithelium
with hyperplasic type II pneumocytes and proliferative airway
epithelium. Fibrotic changes were defined as extensive and dense
interstitial fibrosis replacing lung parenchyma and disintegrating
alveolar architecture. The histopathological grading was 0-absent,
1-mild, 2-moderate, and 3-severe. Lung sections were stained with
Verhoff van Gieson (VVG) and orcein for detecting elastin fibers and
Masson’s trichrome for identification of collagen depositions.