DIFFERENT LABELLING PATTERNS IN MOUSE LYMPHOID TISSUES WITH [3H]DEOXYCYTIDINE AND [3H]THYMIDINE

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Abstract
The percentages of labeled lymphocytes in smear preparations of mouse thymus were higher than those in similar preparations of mesenteric lymph nodes with either generally labeled tritiated deoxycytidine, [3H]CdR or tritiated thymidine, [3H]TdR. Lymphocytes in the thymus cortex and in germinal centers of mesenteric lymph nodes were intensely labeled with [3H]CdR, whereas with [3H]TdR lymphocytes in the peripheral region of thymus and medullary cords of mesenteric lymph nodes were heavily labeled. The majority of lymphocytes in thymic cortex and germinal centers of mesenteric lymph nodes were labeled weakly with [3H]TdR. Labeling patterns with [3H]CdR differed from those with [3H]TdR in lymphoid tissues of the mouse. Mouse lymphocytes can utilize [3H]CdR as a precursor molecule for cytosine and thymine in DNA. The ratio of radioactivity of thymine to that of cytosine was measured biochemically in DNA extracted from lymphocytes labeled with [3H]CdR. This radioactivity ratio in thymus was higher than that in mesenteric lymph nodes. The metabolic activities of utilizing CdR for DNA synthesis apparently differ within lymphocyte populations in various lymphoid tissues in the mouse.