Methylmercury placental barrier

Methylmercury crosses the placental barrier. Pregnant women who have not displayed any signs of mercury toxicity have given birth to infants with birth defects. Some infants were mentally retarded some had palsy. 

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. 

MeHg" " is distributed throughout the body, and easily penetrates the blood-brain and blood-placental barriers (Clarkson 1993, Hansen et al. 1989, Suzuki et al. 1984). The transport of MeHg" " into tissues is mediated by the formation of a MeHg-cysteine complex (Aschner and Aschner 1990, Tanaka et al. 1991, Kerper et al. 1992). Soon after application, McHg" " is found in the blood, predominantly in the red cells. In humans, the ratio of MeHg in red blood cells to serum is approximately 20 1 (Kershaw et al. 1980). Short-chain alkyl-mercury compounds such as methylmercury or ethylmercury are rather stable in the body, whereas long-chain alkylmercury or arylmercury compounds such as phenylmercury may be metabolized relatively quickly to Hg " " ions (Roberts et al. 1979) and, therefore, show similar behavior to the Hg " " ion (Pfab et al. 

As methylmercury readily crosses the placental barrier, marked developmental toxicity has been observed in both humans and animals after gestational exposure. Infants born to mothers during the Minamata breakout appeared normal at birth. 

Methylmercury is rapidly and nearly completely absorbed from the gastrointestinal tract 90-100% absorption is estimated. Methylmercury is somewhat lipophilic, allowing it to pass through lipid membranes of cells and facilitating its distribution to all tissues, and it binds readily to proteins. Methylmercury binds to amino acids in fish muscle tissue. The highest methylmercury levels in humans generally are found in the kidneys. Methylmercury in the body is considered to be relatively stable and is only slowly transformed to other forms of mercury. Methylmercury readily crosses the placental and blood/brain barriers. Its estimated half-life in the human body ranges from 44 to 80 days. Excretion of methylmercury is via the feces, urine, and breast milk. Methylmercury is also distributed to human hair and to the fur and feathers of wildlife measurement of mercury in hair and these other tissues has served as a useful biomonitor of contamination levels. 

Tissue distribution of phenylmercury is initially similar to methylmercury. One week after administration, the distribution pattern resembles that seen after administration of inorganic compounds (Nordberg 1976). Once in the blood, phenylmercury distributes to a greater extent into the red blood cells than the plasma. Phenylmercury also predominantly distributes to the liver (Berlin 1963). It is less permeable to the placental and blood-brain barriers than methylmercury (Yamaguchi and Nunotani 1974). Phenylmercury also accumulates in the fur of rats but to a lesser extent than detected with methylmercury exposure (Gage 1964). 

The most insidious mercury toxin is methylmercury, which accumulates in various organs and is able to cross both the placental and blood-brain barriers to harm the fetus (Craig, 1986 Fergusson, 1990). 

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