Contractile cells in rat myocardial scar tissue.

In earlier studies of rat myocardial tissue reactions to necrotizing injuries, we observed in the resulting scar tissue a large number of smooth muscle cells unassociated with blood vessels. Since these cells are not normal components of ventricular myocardium, we studied the appearance and fate of all cells with smooth muscle-like features in healing and healed lesions at intervals up to 10 weeks after ischemic or freeze-thaw injuries. Observations were made with light and electron microscopes, using conventional methods and immunostaining methods to detect alpha-smooth muscle actin. In healing and healed lesions, smooth muscle actin was detected histologically in capillary pericytes, myofibroblasts, and both vascular and nonvascular smooth muscle cells. Its association with cytoplasmic microfilaments in nonsarcomeric myocytoskeletal arrangements was confirmed ultrastructurally. The pericytes and myofibroblasts predominated during the earlier hypercellular healing period. The smooth muscle cells appeared near the end of the first week of repair; they were initially located mainly in presumptive vascular structures, identified by residual basal lamina sheaths, but subsequently located mainly in nonvascular locations. After the second week until the end of the study the number of nonvascular smooth muscle cells increased and that of myofibroblasts decreased. The nonvascular smooth muscle cells predominated in the larger mature scars, especially the transmural ones. From these observations, we have concluded that contractile cells other than cardiac myocytes have important roles in myocardial tissue repair, have suggested that their roles are related to the forces of myocardial contractions, and have discussed their possible functions and lineage interrelationships.