| Bufo gargarizans,endemic to eastern Asia,is common in artificial ponds,grasslands and cultivated lands around China.Local abundance,wide distribution and environmental sensitivity make this the species of choice to evaluate the toxic effects of contaminants.In the chronic toxicological experiment of cadmium(Cd),we used B.gargarizans tadpoles to evaluate the chronic toxic effects of Cd exposure on metamorphosis,body size,thyroid gland,bone formation and gene expression through various Cd concentrations of 0,5,10,50,100 and 500?g/L.In chronic toxicological experiments of mercury,the aim of the present work was to evaluate the adverse effects of sub-lethal concentration(0,6,12,18,24 ? 30 ?g/L)of mercury(Hg)exposure on growth,thyroid gland and liver condition,ultrastructural features,oxidative stress and hepatic lipid metabolism gene expression of B.gargarizans.The main findings were as follows:1.Our results showed that 100 and 500?g/L of Cd concentrations increased mortality and decelerated metamorphosis rate.In addition,significant body size reduction at Gosner stage 42 was observed at 100 and 500 ?g/L of Cd treatments.Average body length and hind-limb length were significantly decreased in the 500 ?g/L of Cd group but body mass was not significantly different at Gosner stage 46.Moreover,bone formation was delayed in high Cd concentration treatments(50,100 and 500 ?g/L)at both Gosner stage 42 and 46.Histopathological changes of the thyroid gland showed that follicular cell hyperplasia and malformation were induced by high Cd concentrations(50,100 and 500 ?g/L).Furthermore,real-time polymerase chain reaction analysis suggested that the larvae exposed to high-dose Cd exhibited a significant decrease in deiodinase(Dio2)and thyroid hormone receptor(TR?)mRNA levels at Gosner stage 42 and 46.2.Our results showed that high Hg concentration(24 and 30 ?g/L)increased mortality and decelerated metamorphosis rate.Average total length,tail length,hind-limb length and body mass were significantly decreased in the 18,24,30 ?g/L of Hg group.In addition,Histological changes of the thyroid gland showed that follicular cell hyperplasia and malformation were induced by high Hg concentrations.Histological changes characterized by an increase in the number of melanomacrophage,Sinus dilatation,damage in normal palisade arrangements of the hepatocytes,cytoplasmic vacuolization and unclear cell nucleus boundaries were observed in liver at high Hg concentrations.Moreover,marked ultrastructural alterations including nuclear deformation,less numerous of mitochondria,clear Vacuoles of mitochondria and cellular debris were also observed in high Hg concentrations.Furthermore,real-time polymerase chain reaction analysis suggested that exposure to high Hg concentrations significantly up-regulated mRNA expression of HSP,SOD,FAE,FAS and ACC,but down-regulated mRNA expression of Dio2,TR?,TR?,PHGPx,CPT,SCP,ACOX and PPARa.In conclusion,high-dose Cd caused metamorphic deceleration,body size reduction,and delayed skeletal development through disrupting the thyroid system in B.gargarizans larvae.Hg at high levels induced metamorphic deceleration,body size reduction in B.gargarizans tadpoles.Moreover,high Hg concentrations could affect the normal processes of lipid metabolism though increasing lipid synthesis and reducing the ability of fatty acid ?-oxidation,and disturb thyroid hormone pathways in liver,and induced oxidative stress.This study revealed the toxicological effects of cadmium and mercury pollution on the growth and development,tissue physiology,morphology,oxidative stress,lipid metabolism and thyroid hormone gene expression of tadpoles of toad Bufo gargarizans.The experimental results can vividly reflect the living conditions of amphibians in cadmium and mercury polluted areas,and serve as a warning for heavy metal environmental pollution.It can also provide valuable advice and data support for the management of ecological environment and the protection of amphibians. |
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