Effects of cadmium exposure on thyroid gland and endochondral ossification in Rana zhenhaiensis

Discovery of elevated concentrations of cadmium in the natural environment has increased awareness because of their potential threats. Amphibians are negatively affected due to their moderate sensitivity to cadmium. Here, we conduct acute and subchronic toxicity tests to examine whether, and to what extent, cadmium exposure disturbs metamorphosis, growth, and kinetic ability of Rana zhenhaiensis. We set different concentration treatment groups for the subchronic toxicity test (0, 10, 40, 160 μg Cd L-1). Our findings demonstrate that cadmium exposure reduces growth parameters and the cumulative metamorphosis percent of R. zhenhaiensis. Decreases in follicular size and follicular epithelial cell thickness of thyroid gland are found in the treatment group. Further, subchronic exposure to cadmium decreases ossification ratio of hindlimbs in all treatment. Also, adverse effects of cadmium exposure on aquatic tadpoles can result in the reduced physical parameters and weak jumping ability in adult frogs. In this sense, our study suggests that cadmium adversely influences body condition and metamorphosis of R. zhenhaiensis, damages thyroid gland and impairs endochondral ossification. Meanwhile, we speculated that cadmium-damaged thyroid hormones inhibit skeletal development, resulting in the poor jumping ability, which probably leads to reduced survival of R. zhenhaiensis.