Plasminogen activation in healing human wounds

Tissue injury is followed by formation of a provisional, fibrin-containing matrix. It is later on replaced by granulation tissue. Replacement involves extracellular proteolysis by fibrinolytic enzymes. Plasmin is a fibrinolytic proteinase and is generated from ubiquitous plasminogen by cell-derived urokinase-type (uPA) or tissue-type (tPA) plasminogen activator. To explore the cells and components involved in plasminogen activation, we have performed a combined immunohistological and zymographic study on human skin wounds produced iatrogenically by debridement. The fibrin(ogen)-specific staining indicated the progressive removal of a fibrin-containing provisional matrix. Plasmin(ogen) was present over the entire observation period. It was diffusely distributed and also displayed a conspicuous association with cells of the granulation tissue, in particular with monocytes/macrophages and fibroblasts. Also, uPA was associated with monocytes/macrophages and fibroblasts, whereas the uPA-receptor (uPA-R) was stained in monocytes/macrophages only. The uPA was potentially active as indicated by zymography. No tPA-specific staining was found. The findings point at the importance of monocytes/macrophages and fibroblasts in uPA-mediated plasminogen activation in healing human skin wounds.