CHANGES IN THE NUMBER OF ASTROCYTES AND MICROGLIA IN THE THALAMUS OF THE MONKEY MACACA-FASCICULARIS FOLLOWING LONG-TERM SUBCLINICAL METHYLMERCURY EXPOSURE

The effects of long-term subclinical exposure to methylmercury on the number of neurons, oligodendrocytes, astrocytes, microglia, endothelia l cells and pericytes within the thalamus from the left side of the br ain of the monkey Macaca fascicularis has been determined by use of th e Optical Volume Fractionator stereological method. The accumulated bu rden of inorganic mercury (IHg) within these same cell types has been determined by autometallographic methods. Four groups of monkeys were exposed to methylmercury (MeHg; 50 mu g Hg/kg body weight/day) by mout h for 6 months, 12 months, 18 months, or 12 months followed by 6 month s without exposure (clearance group). Neurons, oligodendrocytes, endot helia, and pericytes did not show a significant change in cell number for any exposure group. Astrocyte cell number exhibited a significant decline for both the 6 month and clearance exposure groups. The microg lia, in contrast, showed a significant increase in the 18 month and cl earance exposure groups. Results from mercury speciation and quantific ation ana lysis of contralateral matched samples from the thalamus of the right side of the brain from these same monkeys indicated that MeH g concentrations plateaued at around 72 months exposure, whereas the i norganic levels, presumably derived from demethylation of MeHg, contin ued to increase throughout all exposure durations. Autometallographic determination of the distribution of IHg by cell type indicates that b oth the astrocytes and microglia contain substantially elevated IHg de posits relative to all other cell types. The data suggest that the ino rganic mercury present in the brains, accumulating after long-term sub clinical methyl mercury exposure, may be a proximate toxic form of mer cury responsible for the changes within the astrocyte and microglial p opulations. (C) 1996 Intox Press, Inc.