revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 178
2. HEALTH EFFECTS
This finding supports the assumption by Suda et al. (1989) that ingested methylmercury is dealkylated to
inorganic mercury in the brain.
Monkeys were fed 0.05 or 0.09 mg Hg/kg/day as methylmercury, containing 5% impurity of inorganic
mercury, for 0.5–1.5 years (Lind et al. 1988). The low-dosed monkeys were found to have 10–33% of the
total mercury present in the inorganic form in brain cortices, while the high-dosed monkeys had 90% in the
inorganic form. Demethylation of methylmercury in the brain, as well as in other organs, including the
kidneys and liver, is believed to contribute substantially to the high concentration of inorganic mercury in
the brain. Following oral exposure to methylmercuric chloride, regional distribution of total mercury in
the brain of monkeys was observed; the highest levels were in the thalamus and hypothalamus (Rice
1989b).
In contrast, in the brain of 21-day-old neonatal rats that had been previously exposed to a gavage dose of
6.4 mg Hg/kg as methylmercury chloride in utero, the cerebellum had the highest mercury concentrations
and the brainstem had the lowest (Braghiroli et al. 1990). By 60 days of age, concentrations in the brain
reached normal values, with an estimated half-life of approximately 37 days (Braghiroli et al. 1990).
Therefore, age can affect regional distribution in the brain of animals.
Massie et al. (1993) reported no significant change in the total amount of mercury in the organs (lung,
heart, kidney, brain, and liver) of male C57BL/6J mice ranging in age from 133 to 904 days of age
maintained under conventional conditions with no known source of mercury exposure other than
background concentrations. The ratio of mercury in the brain to that in the liver or to that in the kidney
was significantly increased with age, indicating that older mice are less able to maintain a low brain-toliver
ratio of mercury regardless of the total body content of mercury.
In a study of organs from sledge dogs fed methylmercury-laden meat and organs from predatory marine
animals (Hansen and Danscher 1995), the highest concentration of total mercury was found in the
mesenterial lymph nodes, followed by liver and kidneys, indicating that the lymphatic system may play an
important role in the transport of mercury to target organs. The tissue concentrations of mercury observed
in this study were found to be age-related, and the results suggest that demethylation takes place in all
organs, except the skeletal muscle. Demethylation of methylmercury was found to be lower in the brain
than in other organs (Hansen and Danscher 1995).
Mercury accumulates in hair following exposure to methylmercury in humans and mice (Grandjean et al.