Methylmercury egg injections: part 1--Tissue distribution of mercury in the avian embryo and hatchling

Abstract

Methylmercury (MeHg) is transferred by female birds into their eggs thus leaving developing embryos exposed to MeHg from the time of fertilization through hatching. Although Hg is a developmental toxicant, little is known about how it distributes among embryonic tissues and subsequently affects neurodevelopment in birds. The main objective of the present study (Part 1 of 2) was to evaluate the distribution of Hg in tissues during different developmental stages in order to better understand potential targets of Hg in the embryo and hatchling. Eight independent, yet related, egg injection studies were conducted. In five studies, white leghorn chicken embryos were air cell injected with methylmercury chloride (MeHgCl; range injected: 0.17-6.4μg/g egg) and Hg concentrations were assessed in seven tissues. We found that soft tissue distribution in embryos and hatchlings was similar to that seen in older birds, with higher total Hg concentrations in liver and kidney than in heart, muscle, and brain (e.g., 5.1, 3.8, 1.9, 2.3, and 1.9μg/g wet weight, respectively, in day 19 embryos after injection with 6.4μg/g MeHgCl). Concentrations were highest in feathers and unabsorbed yolk (e.g., 24.1 and 13.0μg/g in day 19 embryos after injection with 6.4μg/g MeHgCl). Tissue concentrations rose through embryonic days 11, 14, 16, and 19 but generally leveled off at days 1 and 7 post-hatch. We also report on pilot studies that demonstrated that tissue Hg accumulation after MeHgCl injection is similar in chicken and Japanese quail embryos, and that tissue Hg accumulation in chicken embryos after methylmercury cysteine, but not mercury (2) chloride, injection is similar to accumulation after MeHgCl injection. These findings suggest that embryos may accumulate kidney and brain Hg concentrations known to cause renal and neurotoxicity seen in older birds, but that sequestration of Hg into liver and excretion into rapidly growing feathers may offer some protection. This work also demonstrates that air cell injection studies are potentially a useful tool for studies of Hg toxicity in the laboratory.