Size does matter - Determination of the critical molecular size for the uptake of chemicals across the chorion of zebrafish (Danio rerio) embryos

FETs revealed clear-cut size-effects: the higher the actual molecular weight (= size) of the PEG, the more effects (both acutely toxic and sublethal) were found. No effects were seen with PEGs of 2000 and 3000 Da. In contrast, PEG 8000 and PEG 12,000 were found to be most toxic with LC50 values of 16.05 and 16.40 g/L, respectively. Likewise, the extent of chorion shrinkage due to increased osmotic pressure strictly depended on PEG molecular weight and duration of exposure. A reflux of water and PEG molecules into the chorion and a resulting re-shaping of the chorion could only be observed for eggs exposed to PEGs ≤4000 Da. Results clearly indicate a barrier function of the zebrafish chorion for molecules larger than 3000 to 4,000 Da.