Light and electron microscopic examination of skeletal muscles in the reversed passive Arthus reaction

The reversed passive Arthus reaction (RPAR) was performed in the dorsal area of the skin and skeletal muscles of the pelvic girdle of male guinea pigs. Biopsies were taken 3, 24 and 48 hours after antibody administration. In the dermal type of RPAR attention was paid to the structure of striated muscles of the subcutaneous panniculus carnosus. Histological and electron microscopic studies were performed. Changes in the dermal RPAR were more rapid and more intensive than those directly in the muscles. In both types of RPAR the affected muscle fibres demonstrated vacuolar and hyaline degeneration, segmental necrosis, internal nuclei, cellular infiltration, phagocytosis and regeneration in the late phase of reaction. Neutrophils predominated in the infiltrations after 3 hrs but after 24 and 48 hrs the main cells were macrophages and lymphocytes. Ultrastructural studies showed various stages of damage to the mitochondria, dispersion and disruption of the myofibrils and formation of tubular structures. A similar morphological appearance is characteristic of polymyositis (pm) and dermatomyositis (dm) suggesting a similar i.e. immune complex mechanism of muscle fibre damage in RPAR and in pm and dm.