The Cd in cord tissue showed no significant correlations with Cd in maternal and cord RBCs. However,

the Cd in placenta showed a significant (P < 0.001) but moderate correlation with that in maternal RBCs (rs = 0.41). In this study, the roles of the placenta in the transfer of some toxic elements from mother to fetus during gestation were well demonstrated by comparing the profiles of the elements between RG7420 chorionic tissue of the placenta and cord tissue. All of the element levels, except for MeHg, were significantly higher in placenta than those in cord tissue. The placental barrier worked most strongly against Cd, followed by I-Hg. Our study also indicated that the MeHg or T-Hg concentrations in both placenta and cord tissue were useful biomarkers for prenatal MeHg exposure in newborns. In turn, the Cd concentration in placenta can be useful for predicting maternal Cd exposure during mid-to-late gestation. Among the elements examined, only MeHg level was

significantly higher (1.6 times) in cord tissue than in placenta. The T-Hg level in cord RBCs was also significantly higher (1.5 times) than that in maternal RBCs, in agreement with previous studies (Sakamoto et al., 2004 and Stern and Smith, 2003). These phenomena may be explained by active MeHg transfer from mother to fetus across the placenta via neutral amino acid carriers mTOR inhibitor (Aschner and Clarkson, 1988 and Kajiwara et al., 1996). It is known that the developing brain during the prenatal stage is highly susceptible to MeHg toxicity. In addition, the higher MeHg accumulation in fetuses than in mothers at late gestation is an important public health issue, especially for the Japanese and other populations whose diets largely consist of fish and seafood. The I-Hg level in placenta was significantly higher (2.4 times) than that in cord tissue. The MeHg level in placenta was significantly lower (64%) than that in cord tissue as mentioned earlier. Consequently, the percentage of I-Hg vs. T-Hg in placenta was significantly and 3.3 times higher than that in cord tissue. These results indicated that, different from MeHg, the inorganic divalent Hg in maternal

blood was efficiently trapped within the placenta. The Pb concentration in placenta was significantly higher (1.4 times) than that in cord tissue, and the Pb level Phosphoglycerate kinase in cord RBCs was about 50% of that in maternal RBCs. These results indicated that the placental barrier protected the fetus from Pb exposure to a limited degree. Among the toxic elements, the Cd level in placenta was significantly and extraordinarily higher (59 times) than that in cord tissue, which implies that maternal blood Cd was most strongly trapped within the placenta. Consequently, the Cd concentration in cord RBCs was approximately 10% of that in maternal RBCs. Lower Cd concentrations in cord blood/RBCs compared with those in maternal blood/RBCs have previously been reported by a number of investigators (Baranowska, 1995, Breen et al., 1994 and Sakamoto et al.