# Morphological and Molecular Alterations Induced by Lead in Embryos and Larvae of _[PERSON]_
## 4 Discussion
### Gene-Expression  
Exposure to heavy metals triggers several adaptive responses, among which the induction of metal-binding proteins (MTs) involved in both transport and detoxification of metals. Stage-specific differences in the implementation of compensatory responses have been discussed several times in fish exposed to heavy metals [59]. [PERSON] and colleagues [67] showed no significant differences in MTs' expression after exposure to high lead concentrations in embryonic tissues of African catfish (_Clarias gariepinus_), suggesting that high sensitivity during early stages may be due to the failure of homeostatic mechanisms. On the contrary, the significant increase in _mtf_ observed here clearly indicates zebrafish embryos and larvae's ability to mount an adaptive response to lead injuries. According to [PERSON] et al. [59], the differential activation of homeostatic mechanisms, including MT induction, may be responsible for species-specific and stage-specific differences in metal sensitivity observed in fishes. Besides, several studies reported that MTs significantly contribute to protecting the organism from oxidative stress acting as scavenging against free oxygen radicals [60; 61; 62]. Heavy metals are known to induce oxidative stress by stimulating the production of reactive oxygen species (ROS), considered a common toxicity mechanism driven by different environmental stressors in fish. Through their scavenging activity, antioxidant enzymes represent a protection mechanism against elevated ROS levels and play a crucial role in preventing pathological processes [68]. In zebrafish embryos, it has been previously reported that short-term exposure to high Pb concentrations induces increased ROS production and upregulation of the antioxidant enzymes [51]. We showed that two environmentally relevant concentrations of Pb induce a significant and time-dependent upregulation of _so11_, a key antioxidant enzyme implicated in the first-line defense against oxidative stress [35; 47; 51; 53; 69; 70]. Since Pb concentrations tested here are very low, our results reinforce the role of sod as sensitive and valuable biomarkers of lead-induced oxidative stress with important consequences thereof for the protection of aquatic life and human health. Furthermore, the increase in SOD activity observed in both embryo and larvae indicates an attempt to counterbalance increased ROS generation. However, in the early stages of development, when morphological, biochemical, and physiological development is incomplete, the overall antioxidant capacity may not fully counteract ROS production [71; 72]. The imbalance between ROS formation and the antioxidant defence system can alter the normal developmental process and therefore be involved in the appearance of the morphological abnormalities observed here. The LDH has been recognized in both toxicology and clinical chemistry as an indicator of energetic changes and a biomarker of cellular, tissue, and organ damage following exposure to xenobiotics [71; 73; 74]. In juveniles and adult fishes, both an increase and decrease of LDH have been observed after metal exposure [75; 76; 77]. Literature data on the effects of lead in the early developmental stages are scarce, and the only available study reports a decrease in LDH activity in _Clarias gariepinus_ after exposure to three high concentrations of lead nitrate (100, 300, 500 \(\mu\)g/L) [74]. We do not have clear evidence regarding the mechanisms responsible for these different outcomes; nevertheless, we found in zebrafish a significant increase of _ldl_ expression at all concentrations and exposure times. Similar results have been reported in the early life stages of several species, including zebrafish, after exposure to different pollutants, and confirmed the high energy requirement originating from chemical stress is involved in the upregulation of LDH in zebrafish [71; 78; 79; 80].