parative physiology. Initial, fish have adopted to aquatic environments in the course of evolution and have developed specialized anatomical characteristics, e.g., gills, swim bladders, scales and extracorporeal fertilization. While developmental similarities have already been discovered in gills and in lungs, structural organization, developmental origin and physiological function remain rather diverse [15]. Secondly, loss of genes, neo-functionalization of gene products, and gene-duplication have accrued within a teleost-specific (and salmonid-specific) whole genome duplication in the course of evolution [16,17]. These events resulted to some extent in gene expression changes, signalling pathway alterations and gene function adaptations. Consequently, particular care must be taken by direct comparison amongst fish species and higher vertebrate genomes, as evolutionary distance and several whole genome duplication events have to be regarded as and resulted in genetic diversity involving species [17,18]. Third, fish have retained the capacity of regenerating organs immediately after harm all through their lifetime. Regenerating tissues include extremities, heart and neuronal cells and employ very specialized molecular processes missing in higher vertebrates [19]. Besides these selected examples, a wider number of biological differences can be observed in organ improvement (e.g., sex differentiation), adaptive immunology, behaviour (e.g., parental care, social behaviour), and in neurology (e.g., lack of neocortex) [10]. As a result, the transition of novel findings from fish directly to other common laboratory animals and humans is seldom straight forward and nonetheless requirements validation in mammals. In accordance with these points, the suitability of a fish model to the particular scientific hypothesis and towards the planned assay has to be carefully thought of just before conducting experiments in zebrafish. Nonetheless, by carefully taking in account these variations, a increasing number of comparative interspecies research has been successfully performed plus the benefits would be the foundation for implementation of fish species in investigation of molecular processes Akt2 list typical to all vertebrates as well as their application in toxicological testing [20]. 1.2. Prerequisites for Use of Zebrafish for Toxicity Testing Fish species are extensively employed in ecotoxicology, e.g., by investigation with the impact of chemical compounds and environmental contaminants on fish populations [21,22]. Various fish species, which includes zebrafish, are integrated inside the internationally accepted OECD Suggestions to assess systemic toxicity in fish, i.e., The Testing of Chemicals with all the Fish Acute Toxicity Test (OECD 203) along with the Fish Embryo Acute Toxicity Test (OECD 236) [23,24]. At the moment the European Commission Directive 2010/63/EU, permits experimentation in fish embryos at earliest life stages without getting regulated as animal experiments (Present kind: http://data.europa.eu/eli/dir/2010/63/2019-06-26; BRPF3 Molecular Weight accessed 9 December 2021 EFSA opinion: doi.org/10.2903/j.efsa.2005.292; accessed 9 December 2021). This consists of zebrafish embryos and early larval stages till free-swimming and independent feeding, corresponding to five dpf (days post fertilization) just after raising at 28.five C. These regulationsInt. J. Mol. Sci. 2021, 22,3 ofthus permit toxicological studies in zebrafish at these early developmental stages as an alternative model to animal testing in other vertebrates, e.g., rodents, but typically limits these investigations to developmental and to acute toxic effec
