Quantitative Studies on the Normal Decay of Lymphocytes in the Thymolymphatic System

Studies on the number of dying and dead lymphocytes in unfixed lymphocyte suspensions from thymus, mesenterial lymph nodes and different peripheral lymph nodes from mice were done by employing the Dye Exclusion Test. The number of decaying lymphocytes demonstrates a striking variation within the various age‐groups. During the first months of life the number of dying lymphocytes in the thymus is rather low ‐ from 3 to 5%. It amounts to 15 to 20% between the ages of 3 and 12 months. In suspensions from mesenterial and peripheral lymph nodes the lymphocyte decay during the first two months of life is twice as large as the thymocyte decay. After this the cell decay in the different lymphoid organs becomes gradually similar. About the time of 5 months of age and later the number of decaying lymphocytes and thymocytes are almost identical. Corresponding studies on thymus suspensions from mouse foetuses showed that a rather striking thymocyte decay takes place during the 16th day of foetal life (8%), while the number for the 18th day of foetal life were as low as 2%. The findings have been discussed on the basis of knowledge of two lymphocyte populations, one short‐lived and one long‐lived, as well as the theory concerning thymocyte migration.