ABSTRACT
The placenta and its associated extraembryonic membranes have simply been considered the conduit for the transfer of molecules from mother to fetus and vice versa. This transport activity is certainly an important factor, especially when considering the transfer of toxicants, which have direct action upon the embryo/fetus, e.g., lead and methylmercury. However, the placenta may also be both a modulator of such transfer as well as a site for toxic action. A number of contributions on the reproductive and developmental toxicity of metals have already been provided in this volume as well as in other reviews (Clarkson et al., 1983; Sager et al., 1986; Domingo, 1994). Therefore, the focus of this review is the placental toxicity of metals and how placental toxicity may contribute to the known fetal effects of excess metal exposure. The evaluation of the conduit between mother and embryo/fetus, the controller of maternal physiology, and anchor for the embryo, all known as the chorioallanotic placenta and its extraembryonic membranes (including yolk sac), is limited to color and weight in developmental toxicity screening and is often only superficial in the hospital. The ability to document toxic and/or pharmacologic influences on the placenta and its associated extraembryonic membranes is further limited. In addition to this issue of toxicity, the transplacental transfer, placental localization, and metabolism of metals are also examined in the human placenta to provide insight into whether the placenta serves as a barrier for some metals. The principal metals discussed are arsenic, cadmium, lead, mercury, and selenium.
