The Effect of Total-Body X-Irradiation on the Thymus and the Number of Its Cells

Three criteria were used for determining the effect of whole-body x irradiation on the thymus, thymic weight, total number of thymocytes, and the number of thymocytes per milligram of tissue. It was found that two independent cell populations exist in the thymus, one radiosensitive and the other radioresistunt. The decrease in radiosensitive cells occurs more rapidly than the decrease in weight of the thyruus at low x-ray doses, and the cell density therefore also decreases. The remairing radioresistant cells seem to be larger than the radiosensitive ones. Fourteen days after irradiation, there is complete restoration of cell density after exposure to x-ray doses at which one might suppose that some radiosensitive cells remain. Restoration of thynnic weight and of total number of thymocytes, however, is not complete. When divided doses are employed 14 days apart (150 r x 2,300 r x2, and 450 r x 2), there is a smaller decrease in thymic weight and in total number of thymocytes. Cell density, however, decreases to the same extent as after the first of the split doses. It may be concluded. therefore, that repopulation during restoration originates partly from the larger radioresistant cells. (auth)