Myofibroblasts in subepicardium after local cold injury.

Rat hearts were exposed and injured locally by direct cryoapplication. The hearts were examined 2, 4, 20 and 48 hours, 7 and 12 days later. In the first 48 hours mast cell and polymorphonuclear (mainly eosinophilic) granulocyte infiltration was followed by proliferation of mononuclear phagocytic and fibroblastic cells and new capillaries. The underlying muscle showed freeze-thaw-rigor damage, segmental intramyofiber necrosis, phagocytosis and focal regeneration. After 7 days numerous myofibroblasts appeared and they were intimately related to myofibers undergoing degeneration and repair. After 12 days there was increased fibrosis while mesothelial covering cells of the epicardium and myofibers returned to normal. It is suggested that amines released from mast cells are involved in the formation of myofibroblasts.