This study was designed to investigate radiation-induced vertebral malformations in medaka embryos irradiated with β particles from tritium. Embryos of two inbred strains (HO4 and HO5) of medaka, Oryzias latipes, were exposed either to different concentrations of tritiated water (9.25-37 MBq/ml) or to137 Cs γ rays (dose rates of 0.44-1.89 Gy/day) continuously from morula to hatching. The newly hatched fry were removed from the radiation field and kept under usual conditions for 1 month. Young fish were fixed in 10% buffered formalin, cleared in 1% KOH, stained with alizarin red S, and kept in glycerine. There was almost no difference in the response to radiation between medaka strains. No marked reduction of hatching rate was observed after chronic irradiation with β particles and γ rays, but a considerable reduction in survival of fry was detected in irradiated groups within 1 month after hatching. From observation of whole-mounted skeleton specimens, the following vertebral malformations were found in irradiated groups: fusion of two or more vertebrae, incomplete formation of vertebrae, and lack of vertebral process. The incidence of vertebral malformations increased significantly in both groups irradiated with tritium β particles and137 Cs γ rays. A similarity in the incidences was also observed between β-particle- and γ-irradiated groups. The RBE of β particles relative to γ rays was estimated to be 1 based on the dose-response relationships observed.

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