To investigate the biological effectiveness of neutrons at energies below 1MeV with regard to ear toxicity, we exposed mice to 1.0 Gy of monoenergetic neutrons (1.026 MeV) or 137Cs gamma rays at 7 days of age, and observed subsequent morphological changes in the inner ear with light and scanning electron microscopes. Monoenergetic neutrons, but not gamma rays, caused acute changes in the ear. The epithelium of the greater epithelial ridge in the organ of Corti showed degeneration around 6 hours and disappeared by 72 hours post-irradiation. The apoptotic cell death of the epithelium of the greater epithelial ridge was inducible by the radiation at 3 or 4 days of age. The hair cells formed the protrusion structures of the surface by 72 hours post-irradiation. Neutron-irradiation also caused acute otitis media until 10 weeks of age.
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