The effects of3 H-thymidine on cell progression through the mitotic cycle and on cell population kinetics have been investigated. During continuous treatment there is a gradual inhibition of entry of cells from G2 into mitosis, while other stages of the cell cycle appear to be less radiosensitive. Hence there is an accumulation of cells in G2 while the proportions of cells in G1, S, and M decrease. Cells of the quiescent center are induced to synthesize DNA after inhibition of mitosis in the surrounding meristem. They do not serve to repopulate the meristem, however, since they then incorporate3 H-thymidine and are themselves arrested. It is suggested that the other slowly cycling cells of the meristem, in addition to those of the quiescent center, may also be induced to cycle more rapidly after delay in the fast-cycling population. These cells will also be arrested after a brief compensatory proliferative reaction. Thus the meristem is unable to recover from radiation damage during continuous treatment with3 H-thymidine.

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