EFFECTS OF WHOLE BODY IRRADIATION WITH X-RAYS ON CELL CYCLE PROGRESSION OF THYMOCYTES IN MICE

The effects of whole body X irradiation (WBI) with various doses on cell cycle progression of thymocytes in mice were investigated using flow cytometry. The results show that DNA synthesis was stimulated at 24~72 h after 75 mGy irradiation, reaching the level of sham irradiated group on day 7. Meanwhile, after WBI with 2.0 Gy X rays DNA synthesis was suppressed with significant G 2 block beginning from 4 h and reaching its peak at 12 h, but the cell cycle progression was accelerated at 72 h and returned to the level of sham irradiatied group on day 7. After WBI with 50 mGy X rays,the progression of cell cycle of thymocytes from G 0/G 1 to S was promoted and DNA synthesis was enhanced. However, G 1 block began to occur with 100 mGy. The number of S phase thymocytes decreased in a dose dependent manner and the number of G 0/G 1 phase cells increased after WBI with 0.5~4.0 Gy. In the same dose range, the number of G 2/M phase cells increased in a dose dependent manner. These data suggest that DNA synthesis is suppressed with occurrence of both G 1 and G 2 block. The study shows that the threshold dose for G 1 block was 0.1 Gy in thymocytes after WBI and that for G 2 block was 1 Gy. (