SELECTIVE DEGENERATION OF DORSAL-ROOT GANGLIA AND DORSAL NERVE ROOTS IN METHYL MERCURY-INTOXICATED RATS - A STEREOLOGICAL STUDY

The components of the nervous system of rats that are most critically affected by methyl mercury are still a matter of debate. A recent ster eological study of rats with typical symptoms resulting from methyl me rcury intoxication demonstrated that the morphology of cerebellar gran ule cells and Purkinje cells were unchanged at the light microscopic l evel, even though there was pronounced degeneration of myelinated axon s in dorsal nerve root nerves. In the present study, unbiased stereolo gical methods were used to quantify morphological changes in the dorsa l root ganglion, and dorsal and ventral nerve roots of the rats used i n the previous study. The rats were treated with methyl mercury (2 mg daily/kg, per os) for a 19-day period that was followed by a 32-day pe riod without treatment. The means of the total numbers of A-cell and B -cell perikarya in the dorsal root ganglion of the intoxicated rats we re reduced by 60% and 24%, respectively. The mean volume of A-cell per ikarya in rats of the experimental group was reduced by 22%, whereas t he mean volume of B-cell perikarya was the same in the two groups. In the experimental group, the total number of myelinated axons in the do rsal nerve roots was reduced by 60%, whereas no difference was found i n the ventral nerve roots. The areas of axon and myelin sheath, dorsal and ventral nerve roots were not affected. This study demonstrates th at extensive loss of dorsal root ganglion cells and myelinated axons i n dorsal nerve roots precedes light microscopical changes in the ventr al nerve roots and the cerebellum of rats intoxicated with methyl merc ury.