Heavy metals and placental fetal-maternal barrier: a mini-review on the major concerns

Heavy metals (HMs) are environmental contaminants with toxic properties for wildlife and humans. The placenta is a privileged organ that, along with the fetal membranes and amniotic fluid, enables growth and development of the fetus during the physiological pregnancy. It also acts as a filter reducing the passage of harmful substances, protecting the embryo and then the fetus from exposure to pollutants. The placental barrier is not completely impermeable to the passage of harmful substances; indeed, HMs were detected not only in placental tissues, but also in amniotic fluid and umbilical cord blood. The amniotic fluid can be considered as a valuable marker of prenatal exposure to exogenous factors, and as an indicator of the integrity of placental barrier. The effect of an intrauterine exposure to heavy metals has been amply evaluated during the last decades. Several studies investigated the exposure to HMs in order to evaluate the mechanism of placental transfer and the impact on fetuses and later children's health. In particular,  the early exposure to Pb, Hg, and Cd was correlated to infant health effects, such as neurological, developmental, and endocrine disorders. The aim of this mini-review is to summarise the current state of knowledge about the interaction between HMs and placental barrier, considering possible implications on fetal health.