Malignant histiocytosis: a histological and ultrastructural study of lymph nodes in six cases.

A morphological, histological, and ultrastructural investigation was carried out on a series of six cases of malignant histiocytosis observed between 1973 and 1977. All lymph nodes were obtained prior to treatment. The histological findings revealed that the proliferating cell type was in all cases represented by histiocytes of varying degrees of atypia and with consistent phagocytic activity. In four cases, the histiocytic cellular proliferation also showed some degree of cohesiveness and, in two cases, there was capsular invasion. In three cases blood vessel invasion by malignant cells could be seen within the lymph node and in the surrounding tissue. On electron microscopy the tumour cells of malignant histiocytosis appeared to be pleomorphic with three types of cells: undifferentiated cells, histiocytes with variable degrees of differentiation, and cells with intermediate features. In particular, nonphagocytic and actively phagocytic histiocytes could be identified. The histological and ultrastructural data further support the idea that malignant histiocytosis is a disease that is related to the neoplastic proliferation of moderately differentiated histiocytes and their precursors. The latter may undergo various transformations with either apparently benign or definitely atypical morphological features.