The subcutaneous fascial analogue of myositis proliferans: electron microscopic examination of two cases and comparison with myositis ossificans localisata.

Two cases of the so-called fascial analogue of myositis proliferans were investigated by histological and electron microscopic methods. It was found that the structure of the fascial variant corresponds almost completely to the true myositis proliferans localized within the musculature. The electron microscopic observations show a preponderantly histiocytic differentiation of the cells and strongly activated proliferating capillaries, and exclude a myogenic origin of the characteristic ganglion-like giant cells. Ultrastructurally a traumatic genesis appears possibly, the cells of the lesion could derive from multipotent cells of the microvasculature. The relations to myositis ossificans and fascitis nodularis are discussed.