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

Abstract

Three criteria were used to determine 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 ("cell density"). X-ray doses were 150, 300, 450, 600 and 900 r, and the animals were killed on the 3rd and the 14th day after irradiation. Results were expressed as percentage of normal control values. Two independent cell populations exist in the thymus, one radiosensitive and the other radioresistant. The decrease in radiosensitive cells occurs more rapidly than the decrease thymic weight at low X-ray doses, and the cell density therefore also decreases. The remaining 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 some radiosensitive cells remain. Restoration of thymic 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 x 2, 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.