The role of biomarkers in reproductive and developmental toxicology.

A massive outbreak of methylmercury poisoning took place in the winter of 1971-1972 due to the consumption of homemade bread contaminated with a methylmercury fungicide. The longitudinal analysis of the mother's head hair, collected after delivery of the baby, provided a means of recapitulating exposure to methylmercury during pregnancy. Methylmercury is incorporated into newly formed hair at a concentration that is proportional to the simultaneous concentration in blood. Since hair grows at a rate of approximately 1 cm/month, longitudinal analysis of the hair strand, centimeter by centimeter, will give a month by month recapitulation of blood levels. Depending on the length of the hair strand, it is possible to recapitulate several years of exposure. Using longitudinal hair analysis, it was possible to compare the methylmercury levels in the mother during pregnancy with the severity and frequency of effects in her offspring. As in the previous incidents, high levels of prenatal exposure led to severe brain damage. However, it was also possible to identify milder effects of methylmercury as manifested by delayed development. It was possible to demonstrate a dose-effect and dose-response relationship between the maximum concentration of methylmercury in maternal hair during pregnancy and evidence of delayed development and mild neurological abnormalities in the offspring. These relationships provided quantitative evidence that the developing nervous system is more susceptible to damage than the mature brain.