RADIATION RESPONSE IN RELATION TO THE CELL CYCLE IN VIVO

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Abstract
TdR-H3 percentage labeling indices, mitotic indices, and the frequency of anaphase-telophase chromosomal aberrations have been used to examine the influence of the cell cycle stage on the response to X radiation in vivo. A high degree of synchronization was obtained in proliferating parenchymal cells after partial hepatectomy in the regenerating liver of the young male rat, and animals were irradiated (700 rads) prior to synchronization (G0 cells), and at 14 hours (late G1 cells), 25 hours (late S cells), 30 hours (G2 cells), and 32.5 hours (M cells) after synchronization. Modification to DNA synthesis and mitotic and division delay were measured at frequent intervals during the first cell cycle after irradiation, and the frequency of chromosomal aberrations was determined during the first postirradiation cell division at 33 hours after synchronization. The data indicated that for proliferating parenchymal cells in vivo, modification to DNA synthesis and mitotic delay were greatest when cells were irradiated during the late G1 phase, less in the G2 phase, and least in the late S and G0 phases. Radiationinduced chromosomal aberrations occurred most frequently in G2 cells, less in late G1 cells, and least in late S and G0 cells. The results are discussed in terms of experimental studies on radiation effects and radiosensitivity during the generation cycle in plant cells and mammalian cells in vitro, and of age-response functions in relation to growth and cell division, biochemical effects of DNA synthesis, and chromosomal aberrations affecting the regenerative capacity of the proliferating parenchymal cell system.

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