Generalizing, it can be said that the pathologic cells seen in smears of the bone marrow in multiple myeloma resemble the plasma cell and vary from the very anaplastic and immature cell to the well-differentiated and almost characteristic plasma cell.
The feature which the "myeloma" cell shares with the plasma cell is the abundant, granular, basophilic cytoplasm which tends to be fragile and undergo the same degenerative changes in each; namely, the formation of Russell bodies and vacuolization. Fairly frequently a perinuclear clear area or Hof is present and the nucleus tends to be eccentrically placed. Cytoplasmic extensions or pseudopodia may also be seen in either case, but they occur more often and more dramatically in instances of multiple myeloma. Multinucleated cells are commonly seen.
In addition, myeloma-plasma cells will often have a large clear nucleolus and a leptochromatic nucleus and will exhibit a tendency to the formation of isolated areas of condensed chromatin. Cytoplasmic extrusions, free cytoplasmic bodies, occasionally complete with Russell bodies and vacuoles are almost universally present.
All cases were of the plasma cell type; there was no exception. In these cases, the myeloma-plasma cell constituted from 2.5 to 96 per cent of the leukocytic elements present. The opinion was expressed that all so-called types of multiple myeloma are merely variations in differentiation of this same cell.
It was noted that anaplasia, hypernucleation and lack of plasma cell predominance in certain cases were diagnostic pitfalls. Additional evidence was adduced to confirm the reticulo-endothelial origin of the myeloma-plasma cell. It was further observed that certain prognostically valuable information could be gleaned from a careful review of the cytologic characteristics in these cases.