The role of exposure window and dose in determining lead toxicity in developing Zebrafish

Heavy metal contamination is recognized worldwide as a serious threat to human health and wildlife, and reducing their emissions is a priority of international and EU actions. Due to its persistence, high bioaccumulation tendency, and toxicity properties, lead (Pb) is one of the heavy metals of greatest concern. Even at low concentrations, lead induces various clinical and subclinical conditions in both humans and animals, and it has been included in the priority list of hazardous substances. In the present study, we used zebrafish's early stages as a model, given their well-acknowledged predictive value in the risk assessment of chemicals. This study was designed to investigate the morphological and morphometric alterations induced by Pb during zebrafish's early development and disclose the putative effects stage- and/or dose-dependent. We examined injuries induced by two environmentally relevant and extremely low concentrations of Pb (2.5 μg/L and 5 μg/L) during two exposure windows: early (between 1 and 7 dpf) and late (between 2 and 8 dpf). We clearly demonstrated that the incidence and severity of morphological abnormalities increased with increasing Pb dose and exposure time in both early and late-exposed groups. Furthermore, we revealed that malformation severity was significantly higher in the early exposed group than in the late exposure group at all exposure times and for both tested doses, thus highlighting the high sensitivity of zebrafish during the initial stages of development. The information presented in this paper emphasizes the effectiveness of morphological biomarkers in unveiling threatening situations and supports the role of zebrafish embryos and larvae in risk assessment and environmental monitoring.