Platelets, fibroblasts, and inflammation: tissue reactions to platelets injected subcutaneously

Evidence from the literature indicates that platelets contain (besides mediators of acute inflammation) factors capable of stimulating fibroblastic growth: namely, serotonin and "platelet factors" demonstrable by their effect on tissue cultures of fibroblasts. The purpose of this study was to find out whether an inflammatory and a fibroblast-stimulating effect could be demonstrated in vivo, in the rat, using a single subcutaneous injection of concentrated platelets (platelet pellet). For comparison, rat brain, heart, and kidney tissue were homogenized under sterile conditions, spun down, and injected subcutaneously. Platelet pellets caused intense edema and neutrophil infiltration; after 8 days they had assumed a spherical shape and were surrounded by typical myofibroblasts. The 30 pellets of tissue homogenates induced a much milder acute inflammatory reaction; only one (a heart pellet) induced a recognizable myofibroblast reaction. The delayed appearance of myofibroblasts around platelet pellets was probably mediated by the neutrophils, which accumulated in large amounts around platelet pellets. Using this in vivo model, a direct fibroblast-stimulating effect of platelets was not demonstrable. It is pointed out that there are analogies between cellular reactions induced by injected platelet pellets and by intravascular platelet thrombi.