On the Mechanism of Radiation Effect on DNA Synthesis

Abstract

Bone marrow culture and autoradiography. The technique for culturing cell suspensions of human bone marrow in vitro and for a high-resolution autoradiography on smear preparations from such cultures has been described previously (2, 3). Irradiations. The radiation employed was in the range of 140 to 300 kv, with dose rates to the cells of 250 to 400 rads/min. The nonuniformity of the thickness of the walls of the culture bottles made radiations of 250-300 kv preferable, but none of the doses delivered to the cells is expected to be in error by more than ? 10 %. All radiations were performed at room temperature within I to 3 hours after the cultures were set up. Preliminary experiments did not indicate any difference whether the radiation was delivered at room temperature or at 37? C, immediately after setting up of the cultures or after 3 to 4 hours of standing at room temperature. Grain counting. Grain counts were performed on stained autoradiographs (Fig. 1). Unless otherwise specified, 50 myelocytes and promyelocytes were counted, and the results are expressed as average grain counts per cell. The background usually was less than 1 grain per cell, but for ease of counting and to avoid errors induced by random grouping of background grains no cell was counted positive which showed a grain count below 10 grains. Where grain counts were performed on nucleated red cells (basophilic and early polychromatic normoblasts, or megaloblasts) this is specified. Grouping of nucleated red cells and myelocytes together in counts was avoided because of the difference in the cycle time of these two series of