Developmental disturbances of the fetal brain in guinea-pigs caused by methylmercury

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Abstract
Pregnant guinea-pigs of Hartley strain were orally administered methylmercuric chloride once at a dose of 7.5 mg Hg/animal (weighing 500–800 g) on one of days 21, 28, 35, 42 or 49 (3–7 weeks) of gestation. They were killed on day 63 (9 weeks) and their fetuses were removed. Both maternal and fetal blood, brain, liver and kidney, and fetal hair, urine, gastric content and amniotic fluid as well, were sampled for mercury analysis. The fetal brains were also examined pathologically. The maternal kidney contained mercury at a high concentration but the fetal kidney did not. The mercury concentration was strikingly high in the fetal hair, but fairly low in the urine, gastric contents and amniotic fluid. Mercury distributed unevenly in various brain regions of both dams and fetuses after treatment at 6 and 7 weeks of pregnancy (3 and 2 weeks before sampling). The concentration was high in the neopallium and archipallium, followed by the paleopallium, diencephalon and mesencephalon, but low in the rhombencephalon, including cerebellum. Mercury contents were relatively low and distributed almost evenly in various brain regions of both the dams and fetuses following treatment at 3, 4, and 5 weeks of pregnancy. Morphologically, the fetal brains were disturbed in the development following treatment at 3, 4 and 5 weeks of pregnancy. The cerebral cortex was thinned, the nucleus caudatus putamen and the hippocampal formation were reduced in size, and the lateral ventricles were dilated. However, the histological architecture of the cerebral cortex was not strikingly maldeveloped; only a slight disarrangement of the cellular alignment was noted. Following treatment at 6 and 7 weeks of pregnancy, focal degeneration of the neuronal cells was observed in the fetal neocortex; the severe cases showed spongy degeneration and dysgenetic hydrocephalus.