Effects of chronic cadmium exposure on metamorphosis, skeletal development, and thyroid endocrine disruption in Chinese toad Bufo gargarizans tadpoles

Abstract

The present study examined the effects of chronic cadmium (Cd) exposure on metamorphosis, body size, thyroid gland, and skeletal development of Chinese toad (Bufo gargarizans) tadpoles. Tadpoles were exposed to Cd concentrations at 0, 5, 10, 50, 100, and 500 μg/L from Gosner stages 26 to 46 of completion of metamorphosis. Our results showed that 100 and 500 μg/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 μg/L of Cd treatments (p < 0.01). Average body length and hind-limb length were significantly decreased in the 500 μg/L of Cd group (p < 0.05) 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 μg/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 μg/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 (TRβ) mRNA levels at Gosner stage 42 and 46. Our investigation indicated that high-dose Cd caused metamorphic deceleration, body size reduction, and delayed skeletal development through disrupting the thyroid system in B. gargarizans larvae. Environ Toxicol Chem 2018;37:213-223. © 2017 SETAC.

Sublethal toxicity of cadmium chloride (CdCl2) on the growth parameters of Xenopus laevis tadpoles (Daudin, 1802)

“…It was found that the average total body length and related morphometric parameters of length were significantly decreased (p < 0.05) in the treatment groups of 1.00 mg/L and 1.50 mg/L Cd. This observation follows the findings of [32] Sun et al, wherein they reported that chronic exposure to 0.70 mg/L of Cd caused significant reduction in average body length and hind limb length of Bufo gargarizans tadpoles. Their investigations indicated that a high dosage of Cd negatively affected the ossification in bones of the tadpoles as the metal disrupted the functioning of thyroid hormone responsible for regulating endochondral ossification and bone development.…”

Overview of Cadmium Thyroid Disrupting Effects and Mechanisms

“…Investigations in lower vertebrates also confirmed Cd direct effects on the thyroid gland. Namely, chronic Cd exposure with higher Cd concentrations (50, 100, and 500 µg/L) in Chinese amphibian toad ( B. gargarizans ) tadpoles caused histopathological changes of the thyroid gland reflected by follicular cell hyperplasia and malformation [ 64 ]. Amphibians are regarded as a good indicator for aquatic environmental changes because of their gill and skin permeability and vulnerability, and their metamorphosis is considered to be regulated by hormones produced by the thyroid gland.…”

“…In addition to changes in 5′-monodeiodinase activity, Cd exposure has been shown to induce the activity of UGT in the liver resulting in increased metabolism of T3 and T4 (lowering effective plasma concentrations) [ 43 , 48 ]. Extra-thyroidal disruption of thyroid function can also happen through inhibition of the thyroid hormone receptor (TRβ) gene expression, which was shown in Chinese toad tadpoles larvae exposed to different Cd concentrations [ 64 ]. Changes in circulating free T3 and T4 may be due to alterations in the function of Tg, the thyroid hormone binding globulin.…”

Contaminant and Environmental Influences on Thyroid Hormone Action in Amphibian Metamorphosis

“…Sun et al observed significant decreases in dio2, thra , and thrb transcripts following Cd exposures in B. gargarizans at concentrations an order of magnitude lower than previously reported to decrease metamorphic rate (83). At the lowest Cd concentration, an increase in thra expression was observed, but this may be due to using actb as a single normalizer, which can be TH-responsive (87).…”