The early effects of methylmercury on the developing rat brain.

The effects of organic mercury compounds on the development of the brain are well known since the exposure of people at a large scale to methylmercury in the Minamata Bay area and in Iraq. The neuropathological examination of the brains of children prenatally exposed revealed dysplasia of the cerebral and cerebellar cortex, neuronal ectopia and several other developmental disturbances. In this experimental study we examined developmental mechanisms involved in methylmercury-induced cerebral anomalies. By examining the fetuses soon after treatment we concentrated in the initial effects of the treatment. The pregnant rats were given 10 mg/kg methylmercury chloride i.p. on day 18. Already at 2 h after administration mitochondrial degeneration occurred in the endothelium of the cerebral capillaries. Subsequently hemorrhages developed interfering with the cellular arrangement in the ventricular zone, with neuronal migration in the intermediate zone and with the development of the cortical cytoarchitecture. Macrophages and cavities appeared in the hemorrhagic areas. It is suggested that the abnormalities seen in the experiments can be considered as the initial methylmercury-induced effects which, in combination with various other toxic effects, ultimately result in the anomalies that have been observed in the brains of children prenatally exposed to methylmercury.