Effects of γ rays on different stages of primordial germ cells were quantitatively studied in Oryzias latipes embryos. Stage 9, which is before the germ cell determination, exhibited the highest radiosensitivity to the elimination of primordial germ cells in embryos and hatching fry. Irradiation of embryos at this stage with 500-2000-rad γ-ray doses showed an exponential dose-effect relationship. A time-course study was also carried out on embryos and fry irradiated with 2000 rad at stage 9. A possibility was indicated that the onset of sex differentiation of germ cells may be independent of the population size of primordial germ cells at the genital ridges. Primordial germ cells became highly radioresistant from the stage 3 days after fertilization. In the time-course study of the embryos irradiated at 4 days after fertilization with 2000 rad, germ cells with enlarged nuclei were observed after 48 hr. Under the present experimental conditions, no "immediate cell death" was encountered.

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