The Role of Bone Marrow of X‐irradiated Mice in Thymic Recovery

The influence of the bone marrow on the repopulation of the thymus in X‐irradiated mice has been investigated. It was observed that the thymus and a certain population of bone marrow lymphocytic cells were repopulated in parallel in a cyclic fashion. This occurred either after a single exposure of mice to 400 R or after serial weekly X‐ray treatments with 170 R. Lethally irradiated recipients which were grafted with bone marrow cells obtained 12‐24 days after four weekly irradiations of donor mice with 170 R also exhibited a cyclic repopulation of both the thymus and the bone marrow lymphocytic population. In contrast, mice which were transplanted with bone marrow cells from unirradiated donors, containing an equal number of stem cells (CFU), exhibited a continuous rather than a cyclic recovery of both cell populations. the bone marrow stem cells of mice recovering from X‐irradiation were found to have a decreased proliferative activity, since they produced significantly smaller spleen colonies in lethally irradiated recipients than marrow cells from unirradiated mice. The results were interpreted as indicating that the bone marrow lymphocytic cells may act as thymic precursor cells and that thymic lymphopoiesis is dependent on the presence of such cells. Evidently, the production of lymphocytic cells will decrease when the stimulus for granulocyte production increases due to the limited proliferative activity of the surviving bone marrow stem cells after irradiation. This may result in a cyclic variation of the production of bone marrow lymphocytic cells and it follows that thymic lymphopoiesis will run parallel.