Early Morphological Changes in Cortical Medullary Thymocytes of the Rat after Whole-body Irradiation

SummaryElectron-microscope observations revealed that the early morphological change which precedes pyknosis in rat thymocytes after whole-body irradiation with 1000 R was the alteration of nuclear shape by increased nuclear indentation, followed by loss of the typical chromatin pattern in the nucleus. It was confirmed that radiation caused destruction of more thymocytes in the cortex than in the medulla.In the normal medulla, the following three structures were observed: (1) narrow channels between the reticular cell and the thymocyte, or between thymocytes; (2) a structure resembling a tight junction between the thymocyte and the reticular cell, suggesting a transfer of substance between the cells; and (3) rows of small vesicles in the reticular cells and the thymocytes.The mechanisms of the radiation-induced destruction of thymocytes, and the different radiosensitivity of cortical and medullary thymocytes, are discussed in relation to cellular structure.