Transplacental transfer of radionuclides. A review.
Transplacental transfer of tritium (H), cesium (Cs), iodine (I), strontium (Sr) and plutonium (Pu) has been observed in all examined species with an active transfer mechanism for iodine, strontium and plutonium. Cesium and tritiated water freely cross the placenta and are distributed approximately uniformly in maternal as well as in fetal tissue. In pregnant mice, 137Cs concentrations in maternal organs were 4-6 times higher than in the foetus. With increasing weight, Cs amounts rise in the foetus. 131I distribution in the foetus before and after the beginning of thyroid function is totally different. In an 8-week-old human foetus, there was almost no accumulation of 131I in the thyroid gland, while on week 14 of gestation more than 40% of the fetal iodine was accumulated in the thyroid gland. Radioiodine given only once results in distinctly higher fetal-maternal ratios than after long-term exposure. Experiments performed to evaluate the extent of placental barrier of Sr in comparison to Ca showed a maximum of Sr/Ca discrimination early in pregnancy while at the end of pregnancy Sr and Ca are transported to almost equal proportions across the placenta. A well-known phenomenon is the accumulation of Pu in the yolk-sack of rats such that 80% of the total activity is contained in the feto-placental unit. For the extrapolation of results from animals to man, experiments should not be confined to laboratory animals but include primates. Furthermore, computer softwares like MIRDOSE could be used to simulate transplacental distribution.