Introduction: Sporadic inclusion body myositis (sIBM) is a progressive disease that leads to extensive muscle weakness. The aim of this study was to determine whether the number and distribution of fibroblasts differ in sIBM when compared with polymyositis. Methods: Immunofluorescence double labeling was performed on 35 biopsies with antibodies directed against perlecan and CD90, procollagen I, CD34, and CD105. In addition, nonserial ultrathin sections were studied from 3 biopsies of each condition. Results: Fibroblasts expressing CD90 and CD34 accumulated in the endomysial compartment in polymyositis and sIBM. In addition, cells expressing CD90 were found beneath the basal lamina in both conditions. At the ultrastructural level in polymyositis, fibroblasts invaded the myofiber, with focal destruction of the basement membrane. In sIBM, by contrast, invasive fibroblasts were ensheathed by the intact myofiber basement membrane. Conclusions: The impact of intruding fibroblasts on satellite cells remains to be established. Muscle Nerve 49: 175–180, 2014
