Needle-like crystals in plasma cells in a patient with a plasma cell proliferative disorder

A patient with a plasma cell proliferative disorder and a monoclonal IgG-kappa protein in the serum was observed over a period of four years. Two unusual features were seen in this patient: (1) An apparently benign course in spite of relatively large and slowly increasing amounts of Bence Jones protein (monoclonal kappa light chains) in the urine, and (2) crystalline structures located outside the cisternae of the endoplasmic reticulum in a large number of bone marrow plasma cells. The crystals, which ultrastructurally appeared to be composed of protein material, were not stained by immunofluorescent antisera to immunoglobulins. They were, however, observed only in cells staining for gamma heavy and kappa light chains. This suggest an association between the crystalline structures and the synthesis of monoclonal immunoglobulin proteins.