Ultrastructural finding of vascular degeneration in myositis ossificans circumscripta (fibrodysplasia ossificans).

Abstract

A case of myositis ossificans circumscripta (fibrodysplasia ossificans) of the masseter muscle has been studied ultrastructurally. It showed atrophy of muscle fibres with deranged myofilaments, adjacent to dense fibrous tissue, osteoid and bone. A large number of the cells found in the dense fibrous tissue showed vacuolation and separation of membrane bound vesicles similar to matrix vesicles released by chondroblasts and osteoblasts during calcification. It is possible that, as in normal calcification of bone, these vesicles are responsible for initiating calcification in the ectopic bone found in the present condition. It was also shown that most of the blood vessel found in the blocks examined exhibited various degrees of degenerative changes. The finding of arterial occlusion together with capillary degeneration suggests that the main changes in myositis ossificans circumscripta may be those affecting blood vessels leading to ischaemic muscle atrophy. The fibrosis and ossification that follow are probably secondary changes.