Biopsy material from the quadriceps femoris muscle in a case of subacute polymyositis associated with polyarthropathy and transient cardiac involvement has been examined with the electron microscope. Observations have been made on the ultrastructural changes in necrotic muscle fibres as well as on the origin and differentiation of myoblasts participating in the regenerative processes in damaged fibres. An unexpected finding was the presence of granular cytoplasmic inclusions and nuclear abnormalities in some regenerating muscle fibres. Although the particles in the inclusions are similar to those seen in cells infected with Coxsackie virus, the possibility that the inclusions are unusually large aggregates of glycogen granules cannot be excluded. Thickening of endothelial cells and thickening and reduplication of the basement membrane of capillaries and venules was also found but is not considered to be of primary importance in the pathogenesis of the muscle fibre damage
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