From cutting edge to guideline: A first step in harmonization of the zebrafish embryotoxicity test (ZET) by describing the most optimal test conditions and morphology scoring system

In the last couple of years, the interest in the zebrafish embryotoxicity test (ZET) for use in developmental toxicity assessment has been growing exponentially. This is also evident from the recent proposal for updating the ICHS5 guideline. The methodology of the ZET used by the different groups varies greatly. To further evaluate its successfulness and to take the ZET to the next level, harmonization of procedures is crucial. In the present study, based on literature and empirical data, the most optimal study design regarding temperature, test chamber, exposure period, presence of chorion, solvent use, exposure method, choice of concentrations, and teratogenic classification is proposed. Furthermore, our morphology scoring system is reported in detail as protocol to further enhance study design harmonization.