The yolk sac of zebrafish embryos as backpack for chemicals?

The zebrafish embryo (Danio rerio) has developed into one of the most important non-sentient animal models for the hazard assessments of chemicals but the processes governing its toxicokinetics (TK) are poorly understood. This study compares the uptake of seven test compounds into the embryonic body and the yolk of the zebrafish embryo using TK experiments, a dialysis approach, thermodynamic calculations, and kinetic modeling. Experimental data show that between 95% (4-iodophenol) and 67% (carbamazepine) of the total internal amount in 26 hpf embryos, and between 80% and 49% in 74 hpf embryos were found in the yolk. Thus, internal concentrations determined for the whole embryo overestimate the internal concentration in the embryonic body; for the compounds of this study, up to a factor of 5. Partition coefficients for the embryonic body and a one-compartment model with diffusive exchange were calculated for the neutral test compounds and agreed reasonably with the experimental data. For prevalently ionic test compounds at exposure pH (bromoxynil, paroxetine), however, the extent and the speed of uptake was low and could not be modeled adequately. A better understanding of the TK of ionizable test compounds is essential to allow assessing the validity of this organismic test system for ionic test compounds.