The role of the placenta in fetal exposure to heavy metals.

The heavy metals mercury, lead, and cadmium are toxicants, which are well-known to cross the placenta and to accumulate in fetal tissues. Prenatal exposure to mercury and lead poses a health threat particularly to the developing brain. Fetal exposures to lead and cadmium correlate with reduced birth weight and birth size. The placental passage of cadmium is limited suggesting a partial barrier for this metal. It is very likely that metallothionein is responsible for placental storage of the metals especially of cadmium. It is unclear, however, which proteins are involved in placental uptake and efflux of the metals and where the transporters are located at the placental barrier. Hence, only certain aspects of placental metal toxicokinetics are known so far. The metals have also been shown to adversely affect placental functions. Both metal-specific placental transfer and impairment of placental function can explain the relationships between prenatal metal exposures and adverse effects on intrauterine growth and (neuro)development.