Evidence for the nonmuscle nature of the 'myofibroblast' of granulation tissue and hypertropic scar: An immunofluorescence study

Contraction is an important phenomenon in wound repair and hypertrophic scarring. Studies indicate that wound contraction involves a specialized cell known as the myofibroblast, which has morphologic characteristics of both smooth muscle and fibroblastic cells. In order to better characterize the myofibroblast, the authors have examined its cytoskeleton and surrounding extracellular matrix (ECM) in human burn granulation tissue, human hypertrophic scar, and rat granulation tissue by indirect immunofluorescence. Primary antibodies used in this study were directed against 1) smooth muscle myosin and 2) nonmuscle myosin, components of the cytoskeleton in smooth muscle and nonmuscle cells, respectively, and 3) laminin and 4) fibronectin, extracellular glycoproteins mediating cell-matrix attachment in smooth muscle and non-muscle cells, respectively. Myofibroblasts can be identified by their intense staining of actin bundles with either anti-actin antibody or NBD-phallacidin. Myofibroblasts in all tissues stained for nonmuscle but not smooth muscle myosin. In addition, nonmuscle myosin was localized as intracellular fibrils, which suggests their similarity to stress fibers in cultured fibroblasts. The ECM around myofibroblasts stains intensely for fibronectin but lacks laminin, which suggests that a true basal lamina is not present. The immunocytochemical findings suggest that the myofibroblast is a specialized nonmuscle type of cell, not a smooth muscle cell.