Combined effects of low-level methylmercury and x-radiation on the developing mouse cerebellum

The effects of low-level methylmercury and x-radiation in combination on the developing mouse cerebellum were studied. Pregnant BALB/cJcl mice were divided into two groups. Dams of a group were orally given 9 micrograms Hg/g body weight of methylmercuric chloride on d 17 of pregnancy. The other dams were not treated. They were allowed to give birth, and the male pups were exposed to a single x-radiation at doses of 0.125 or 0.5 Gy on the day following birth. Sham-exposed pups were also prepared. The pups were killed at various postexposure periods, and their cerebella were removed and processed for microscopy and mercury analysis. Mercury was retained in the pup cerebellum at levels of 4-8 micrograms/g for 4 d and then lowered. Appearance and incidence of cell death in the external granular layer (EGL) were similar in groups exposed to 0.125 Gy alone or 0.125 Gy and methylmercury in combination. Fourteen percent of cells in the EGL were killed by exposure to 0.5 Gy with or without methylmercury. Restoration of the EGL from the damage was slightly retarded by methylmercury, but by 10 d of age these cerebella overtook the normal ones in development. These findings indicate that methylmercury does not modify the apoptotic cell death caused by x-radiation in the EGL but retards the tissue restoration from the damage.