Myosin heavy chain IIB knockdown selectively blocks differentiation in fibroblasts on fibrotic lung. Normal human lung fibroblasts were allowed to transdifferentiate on bleomycin-treated lung tissue sections (24 h, 37 °C, ± 100 nm MHC IIB siRNA or 100 nm non-targeting (NT) control siRNA). Panel A, representative 10× photomicrographs of fibroblasts on normal and fibrotic lung stained for F-actin stress fibers (see 2× magnification inset). Panel B, effect of MHC IIB siRNA on F-actin density in HLFs on normal versus fibrotic lung; *, p < 0.001 compared with change on normal ± IIB siRNA. Panel C, representative 10× photomicrographs of HLFs on normal and fibrotic lung stained for α-SMA. Panel D, effect of MHC IIB siRNA on α-SMA density in HLFs on normal versus fibrotic lung; *, p < 0.001 compared with change on normal ± IIB siRNA.