Radioautographic Studies of Bone Marrow Lymphocytes in Vivo and in Diffusion Chamber Cultures

Abstract

Radioautography with tritiated thymidine has been utilized to examine the turnover rate and origin of small lymphocytes in the bone marrow of the guinea-pig. Very few marrow lymphocytes were initially labeled by a single injection of tritiated thymidine, but thereafter the number of labeled lymphocytes rapidly increased to high maximum levels at 3 days. Analysis of the labeling curves and grain counts indicates that the population of marrow lymphocytes is maintained in a dynamic steady state with an average turnover time of 3 days or less. Suspensions of bone marrow cells were isolated from the circulation within intraperitoneal diffusion chambers after short-term labeling with tritiated thymidine in vivo. Although very few small lymphocytes were labeled when introduced into the diffusion chambers, a considerable percentage became labeled during the subsequent culture period. Tritiated thymidine was also administered intravenously whilst excluded from one hind limb by the application of an occlusive compression bandage for 20 minutes. Very few labeled small lymphocytes were found after 72 hours in the tibial marrow of the initially occluded limb, whereas the normal high percentage was labeled in the control tibial marrow. These experiments do not demonstrate any large-scale influx of small lymphocytes from the blood stream into the marrow parenchyma. They suggest that newly formed small lymphocytes appear in the marrow as a result of the division of locally situated precursor cells, but the mechanism of intramedullary lymphocytopoiesis is uncertain. "Transitional" cells, intermediate in morphology between blast cells and small lymphocytes, synthesize DNA and are actively proliferative, but they do not appear to account fully for the rate of lymphocyte production. Certain large, undifferentiated labeled cells appeared in the bone marrow as a result of hematogenous migration. Some implications of these findings are discussed.