In 12 normal subjects, DNA and RNA synthesis by individual marrow cells was studied in vitro by autoradiography after short-term incubation with tritiated nucleosides. The DNA labeling index and RNA synthesis rate were highest in normal immature cells. The DNA index was zero in all mature cells, including small lymphocytes and plasma cells. In the most actively proliferating normal immature cells, the RNA synthesis rate was about 5 times higher than in normal plasma cells. Since both normal immature cells and plasma cells are characterized by a high content of RNA and active protein synthesis, the results of the present study suggest that a high RNA turnover may not be necessary for protein synthesis to proceed in nonproliferating, protein-producing marrow cells.

In 10 patients with myeloma and in two patients with primary macroglobulinemia, the abnormal plasma cells were found to be labeled with H3thymidine in each instance. The RNA synthesis rate was about 3 times greater in plasmocytoid myeloma cells than in mature plasma cells of normal marrows, and also about 3 times greater in lymphocytoid myeloma cells than in mature lymphocytes of normal marrows. The abnormal cells in the lymphoplasmocytic disorders apparently differ from their counterparts in normal marrows not only by a higher proliferative potential, but also by a much greater rate of RNA synthesis.

In actively dividing normal bone marrow cells, high values for both DNA and RNA synthesis were found. By contrast, in the neoplastic lymphoplasma cells studied, a discrepancy was noted between relatively low values of DNA synthesis and high values of RNA synthesis. Production of abnormal proteins within long-lived neoplastic plasmocytic cells may account for this finding.

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