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Labeling patterns with generally labeled tritiated deoxcytidine ([G-3H]dCyd) in lymphoid tissues were quite different from those labeled with tritiated thymidine ([methyl-3H]dThd). Thymic cortex and germinal centers of lymph node, spleen and Peyer's patch were labeled intensely with 3HdCyd but only a few lymphocytes in medulla of thymus and medullary cords of lymph node labeled with 3HdThd. The incorporation of 3HdThd into DNA was reduced significantly in the thymic cortex and germinal center of lymphoid tissues by the simultaneous administration of nonradioactive dCyd. On the other hand, the autoradiographic intensity in the labeled lymphocytes in medullary cord of lymph node did not change by administrating nonradioactive dCyd. These results support the proposition that the metabolic activities of pyrimidine utilization for DNA synthesis differ within lymphocyte populations with respect to the specific sites of origin and development within the lymphoid tissues.

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