Effects of methylmercury on the health and development of cultured preimplantation mouse embryos

Abstract

Human exposures to mercury compounds are currently of great concern to Federal agencies and the public. For most persons in the U.S., exposures are mainly through the consumption of methylmercury (MeHg)-contaminated fish. Sensitivity of humans to MeHg was proven in Japan during a period of heavy environmental contamination, where neurologic deficits were observed in adults and in children born at this time. Epidemiologic studies have provided conflicting results as to the lowest effective dose, but slightly elevated maternal cord blood Hg levels have been positively correlated with neurological development in children. MeHg has been shown to affect all stages of development, and the unborn fetus is more sensitive to MeHg than adults. The mechanism(s) of MeHg's toxicity remains unknown. Cultured preimplantation stage mouse embryos, which are very amenable to explantation and toxicity studies, were exposed to MeHg in vitro for up to four days and examined for alterations in development, morphology, and cell numbers in our studies. A dose-response was observed for all of these endpoints with MeHg concentrations between 50 nM and 1 muM. Microtubules, a proposed target for MeHg, were completely depolymerized within the cytoskeleton after 36 hours exposure to 1 muM MeHg. Mitotic progression was altered in embryos exposed to 100 nM for 3 days, suggesting an inhibition of spindle microtubule function. Significant increases in nuclei with altered appearances occurred at 250 nM MeHg. Reduced glutathione levels, another potential target of MeHg, were unaffected in treated embryos. Direct observation of multiple cell cycles in embryos exposed to 350 nM MeHg for 36 hours did not reveal any obvious block at a particular stage, with the possible exception of an accumulation of cells in G2 in some embryos. Overall development was the most sensitive endpoint examined, with delayed formation of expanded blastocysts occurring at 50 nM MeHg. Results are consistent with multiple hypotheses, including inhibition of microtubule function or interference with regulatory proteins of the cell cycle. These studies demonstrated the sensitivity of cultured murine preimplantation embryos, and suggest that more detailed mechanistic experiments in this system would be useful for helping to understand the toxicity of MeHg.