We investigated delayed reproductive death and chromosome bridge formation in mitotic cells in colonies of normal human embryonic cells which survived exposure to 6 Gy of X rays. Eighteen primary clones each derived from control and irradiated cultures were isolated and secondary colonies were grown up. Six of the primary clones surviving the irradiation showed significantly lower cloning efficiencies than the control clones (P < 0.001), while the rest of the surviving clones showed a cloning efficiency similar to those of the control clones. The average cloning efficiency of control and surviving clones was 16.4 and 7.2%, respectively. Reduced cloning efficiencies were also observed in the tertiary colony formation, indicating that the persistent decrease in cloning efficiency was inherited over 40 generations after X irradiation in normal human cells. Five of nine primary clones surviving the irradiation also frequently contained multiple giant cells in the colonies, while this was a rare event in the progeny of control clones. Furthermore, we analyzed in situ chromosome segregation in anaphase cells appearing during the formation of the secondary colonies. A significantly higher frequency of chromosome bridge formation was found in four of the surviving clones which received 6 Gy (P < 0.001), and the frequency of micronuclei was also increased in the surviving clones. These results provide evidence that genetic instability is induced in normal human embryonic cells by low-LET radiation, and that misrejoining of the broken chromosome ends is increased in the progeny of cells surviving X irradiation.

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