In a mouse model of lung fibrosis using lineage-tracing studies, PMCs lost their polarity and cell-cell junctional complexes, migrated into lung parenchyma, and underwent phenotypic transition into myofibroblasts in response to the pro-fibrotic mediator, transforming growth factor-β1 (TGF-β1). Here, TGFB1 is linked to pulmonary fibrosis.