Human placenta as a 'dual' biomarker for monitoring fetal and maternal environment with special reference to potentially toxic trace elements. Part 3: toxic trace elements in placenta and placenta as a biomarker for these elements.

The human placenta as a body component is exposed to several harmful substances, depending upon the environmental conditions encountered. In the case of toxic metals, placental tissue can be regarded as a dual biomarker to assess maternal and fetal health. The average range of concentrations for toxic trace elements in placenta based on wet weight are found to be: cadmium 1-6 ng/g; total mercury 2-13 ng/g; methyl mercury 1-14 microg/g; and lead 5-60 ng/g. The placenta appears to be at least a partial barrier for Cadmium. Cadmium transport includes a broad variety of mechanisms. Once in circulation, it mainly interferes with Ca and Zn transportation. On the other hand, placenta appears to be a weaker harrier for Pb than for Cd. In the case of Hg, predominantly the organic form is absorbed and readily crosses the placenta. In fetal blood, the organic mercury content is equal or even greater than in maternal blood, raising questions on normal fetal development. Placenta as a biomarker could be taken as an alternative to repeated maternal blood sampling for assessing lead exposure in utero. Placenta samples are usually obtained at the time of parturition, a one-time event. Hence, each pregnancy has to be looked upon as an RTM (real time monitoring) process since the affected species is exposed to the placental source of pollutants only during the course of that particular pregnancy.