The pattern of lymphocytes in the thymus and spleen after labeling with3H-thymidine and3H-deoxycytidine

Adult male untreated mice (NMRI) were investigated after radioactive labeling with3H-thymidine and3H-deoxycytidine to find out whether the lymphocytes in the cortex and medulla of the thymus as well as in the perifollicular and periarteriolar regions of the spleen show a labeling pattern which allows a classification into T- and B-lymphocytes. The percentages of radioactively labeled small lymphocytes and their mean grain counts were determined. The percentages of radioactively labeled small lymphocytes after3H-TdR and3H-CdR showed no significant differences in both splenic zones. The grain counts over the lymphocyte nuclei in the periarteriolar zone showed lower values after3H-TdR than after3H-CdR. The lymphocytes in the perifollicular zone were strongly labeled with3H-TdR and weakly labeled with3H-CdR. In the thymus medulla, lymphocytes were weakly labeled with3H-thymidine and strongly labeled with3H-CdR. In the cortex no significant differences were observed. 75 to 80% of the small lymphocytes in the peripheral blood were weakly and 20–25% strongly labeled after3H-TdR. Therefore there are similarities in the radioactive labeling pattern of thymic medulla lymphocytes and that of small lymphocytes of the periarteriolar zone of the spleen by both DNA precursors. The small lymphocytes in the peripheral T-dependent tissue zones, for example in the spleen, as well as in the mixed lymphocyte population of the peripheral blood can be differentiated from the B-lymphocytes through the difference in the amount of incorporation of3H-thymidine and3H-deoxycytidine.