Bone marrow studies of multiple myeloma revealed, in some cases, a conspicuous proliferation of "lymphoid" cells, virtually indistinguishable by light microscopy from those seen in lympho-proliferative disorders.
Electron microscopy demonstrated a variety of cells ranging from typical lymphocytes to cells with plasmocytoid features. Between these two types of elements there were cells with intermediate characteristics.
In addition, in several cases of myeloma the presence of fixed reticuloendothelial cells and "reticular" plasma cells (or reticulum cells with plasmocytic features) was frequently noted.
The presence of reticulum cells and lymphocytes and of cells apparently "intermediate" between these cellular elements and plasma cells, as judged from electron microscopic observations, is suggestive morphologic evidence of a phenomenon of cell transformation and evidence of a mixed cell proliferation in certain cases of multiple myeloma.