Effects of Ionizing Radiation on Rat Lymphoid Tissuein Vivo

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Abstract
The effects of 300 rads whole-body x-irradiation on DNA-synthesis in rat lymphoid tissue in vivo have been studied by means of biochemical, autoradiographic and histological techniques. The DNA-synthetic process of the large and medium lymphocytes has been shown to be more radiosensitive in vivo than in vitro. The reasons for the differences in sensitivity under these two conditions have been discussed in the light of recent observations relating to the biphasic nature of the DNA-synthetic cycle in rat thymocytes and human bone marrow. Histological and autoradiographic techniques have allowed the separation of effects due to population changes from those due to direct biochemical alteration. The study of control rats has revealed that the average generation time for the large and medium lymphocytes of the lymph nodes is 20–30 hours. Further evidence is presented in favour of a long life-span for the small lymphocytes. The question of re-utilization of portions of the DNA molecule in the synthesis of new DNA has been examined and evidence put forward in support of this concept.

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