Placental transfer of thallium was evidenced in rats treated with a single intraperitoneal dose of 2 μg 201Tl-labelled thallium/rat on the 13th day of pregnancy. Both maternai and fetal organs showed remarkable thallium retention, approx. 10% of the dose being unexcreted 8 days after injection. The highest thallium accumulation was found in maternal muscle and brain tissues. Fetal brain exhibited higher thallium uptake and faster decay rate of thallium levels than maternal brain. It is suggested that the reduced activity of the mechanisms regulating ion movements and composition of nervous tissue and the immaturity of the blood-brain barrier play a role in the peculiar pattern of thallium kinetics in the developing rat brain.
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