Malignant histiocytosis. Ultrastructural and immunocytochemical characterization.

The immunohistochemical and ultrastructural characteristics of involved lymph nodes from four patients with typical clinical and histopathologic features of malignant histiocytosis are reported. The neoplastic cells in all four cases had numerous irregular surface projections and pseudopodia, as well as lobulated or biolobed nuclei with macronucleoli. The cytoplasm contained many lysosomes, mitochondria, and polyribosomes, and displayed evidence of phagocytosis. Langerhans' granules were not identified. The ultrastructural features in the four cases were similar, effectively excluded an epithelial origin in each instance, and delineated morphological characteristics common to neoplastic macrophages. Intracytoplasmic muramidase (CM), an enzyme marker essentially restricted to cells of myeloid and true histiocytic origin, was identified with the unlabeled immunocytochemical technique and was strongly positive in neoplastic histiocytes at all stages of differentiation. Thus, fine structural features and the presence of CM are useful confirmatory studies in the diagnosis of malignant histiocytosis.