Methylmercury maternal transfer

It can assess in utero exposure. Any substance in the maternal circulation can be transferred across the placenta to the developing fetus unless it is first metabolized and eliminated (Ginsberg et al. 2004). Risk assessment of the fetal period typically relies on maternal dose. However, biomonitoring of cord blood relative to maternal blood may be important to document whether there are substantial maternal-fetal differences in exposure. Evidence on methylmercury suggests that it concentrates in the fetus (Stern and Smith 2003), whereas an evaluation of 29 pesticides suggests similar concentrations across the maternal-fetal unit (Whyatt et al. 2003). 

The compartments and barriers to methylmercury transport in the tissue compartments and placenta are shown in Figure 2-6. The cell membrane is assumed to be the barrier for methylmercury transport for all tissues except the brain and placenta. The barrier to methylmercury transport to the brain is the endothelial cell wall of the cerebral vascular system (the blood-brain barrier). The placenta is modeled as four compartments, with separate transfer constants for placental barrier and placental tissue transport. There is a tissue compartment for both the maternal and fetal sides of the placenta. 

Yoshida M, Watanabe C, Satoh H, et al. 1994. Milk transfer and tissue uptake of mercury in suckling offspring after exposure of lactating maternal guinea pigs to inorganic or methylmercury. Arch Toxicol 68 174-178. 

In adult fish, females often contain higher mercury concentrations than males, possibly because they consume more food than males in order to support the energy requirements of egg production. The increased feeding rate in females causes greater dietary uptake of methylmercury however, the transfer to egg mass is a small fraction of the maternal body burden. Mercury concentrations in spiny dogfish (Squalus acanthias), were influenced by dogfish sex, length, and area of collection. 

In experiments with the perfused guinea pig placenta Kelman (1979) has studied the transfer of heavy metals from the maternal to the embryonic circulation. Using this animal model he was able to demonstrate that the placental transfer for methylmercury was 10-times higher than for anorganic mercury. 

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