[Gamma irradiation and delayed effects: muscular fibrosis]

This study was performed on an experimental porcine model of acute local gamma irradiation to simulate accidents which occurred among humans. It enabled us to determine the development and the physiopathological characteristics of the fibrous tissue which developed in skeletal muscle. In the first month after irradiation, the strong inflammatory reaction which initiated the radiation induced fibrosis was characterized by edema as visualized on MRI imaging and X rays computed tomography and by acute phase reactant proteins changes, associated with elevations of local and general temperatures in irradiated animals. At the margin of the irradiated tissue myofibroblasts isolated among collagen bundles or grouped in nodullary reinforcements, are seen associated with intense capillary neogenesis. Several months after irradiation normal skeletal muscle was replaced by atrophic fibrosis delimited by an inflammatory perifibrotic tissue. The muscular fibrosis was characterized by a high density of myofibroblasts and by an inflammatory distribution pattern of collagen types I, III, IV, laminin, fibronectin and fibrinogen as visualized by immunohistochemical methods. Biochemical results showed an increase in collagen content and synthesis in fibrotic tissue whereas the cells in the perifibrotic zone synthesized more non collagenous proteins as compared with the normal muscle. The contributions of granulation tissue, cellular mediators and inhibition of muscular regeneration in the persistence of the invasive character of the muscular radiation induced fibrosis are discussed.