A histological and histochemical study of skeletal muscle regeneration in polymyositis

Abstract

The histological features of regenerating muscle fibres in biopsies from cases of polymyositis have been examined, and the activities of certain enzymes in these fibres have been assessed histochemically. Regenerative changes were most prominent in segments of muscle fibres which had undergone necrosis. Mononucleated myoblasts appeared at the periphery of necrotic fibres before completion of phagocytosis and divided by mitosis. Multinucleated syncitia were found in a similar position and extended along empty sarcolemmal tubes. No evidence of nuclear division was found in these structures, suggesting that they were formed by myoblast fusion. Budding from the remaining ends of necrotic fibres was a less frequent finding. However, changes suggestive of increased ribonucleoprotein formation in the terminal ends of these fibres were commonly found and were considered to be indicative either of regenerative activity or of lysosomal enzyme synthesis. Degenerative changes such as vacuolation and nuclear pyknosis were found in some regenerating fibres. A second biopsy from the same muscle after clinical recovery in one case showed completely normal muscle architecture indicating that complete regeneration had occurred. Regenerating fibres were found to have a high content of RNA and high LDH, SDH and NADH-dehydrogenase activities. However, only small amounts of neutral fat were present and phosphorylase activity was completely absent in regenerating fibres. The present findings demonstrate the ability of the damaged muscle fibre to regenerate completely in polymyositis and contrast with the ineffective regenerative changes found in the Duchenne form of muscular dystrophy. These differences account for the contrasting natural histories of these two myopathies.