Life span of small lymphocytes in the thymolymphatic tissues of normal and thymus-deprived BALB/C mice.

Autoradiography and scintilation counting have been used after various schedules of 3H-thymidine injections to evaluate lymphocyte kinetics in normal and thymus-deprived BALB/C mice. The thymus was found to be an active production site of small lymphocytes, the majority of these cells having thymic residence times of three to five days. Peripheral lymphoid tissues--including spleen, lymph nodes and Peyer's patches--were dominated by long-lived lymphocytes and produced very few small lymphocytes. Nearly identical percentages of long-lived lymphocytes with comparable grain counts were found in the peripheral tissues. In adult thymectomized animals, and in mice thymectomized and reconstituted with bone marrow cells following total body irradiation, percentages and lifespan of long-lived lymphocytes were found to be of the same order as in normal or sham-operated controls. It is concluded that T cells in the peripheral long-lived pool can be formed outside the thymus and that the bone marrow probably produces long-lived B lymphocytes.