The ultrastructure of macrophages found in contact with plasma cells in the bone marrow of patients with multiple myeloma.

The ultrastructure of a macrophage found in the bone marrow of 15 patients with multiple myeloma is described. The macrophage is designated the dendritic bone marrow macrophage, due to the various branching cell forms in which it can be found. It is further distinguished by having contact with plasma cells. At the contact zones a thickening of the inner leaflet of the cytoplasmic membrane of the plasma cells is observed, whereas no morphological changes are present in the adjoining membranes of the macrophage. These macrophages show a great variation in their phagocytic capacity, and only few show morphological evidence of being actively phagocytic. A few dendritic bone marrow macrophages show a localized thickening of the inner leaflet of their cytoplasmic membrane, but this thickening is never found in the regions of contact with plasma cells.