Chronic effects of lead on metamorphosis, development of thyroid gland, and skeletal ossification in Bufo gargarizans.

We examined the Pb2+ exposure on tadpoles of Bufo gargarizans from Gosner stage 26-42. Mortality, growth and development, time to metamorphosis, size, and skeletal ossification at metamorphic climax of Bufo gargarizans were examined. Also, histological characteristics of thyroid glands in tadpoles at Gosner stage 33, 38, and 42 as well as transcript levels of thyroid hormone-related genes in the hind-limb, tail, and liver of tadpoles at metamorphic climax were examined. Pb2+ exposure induced mortality in a concentration-dependent manner in Bufo gargarizans larvae. The significant increase in growth and development, percent metamorphosis, size at metamorphic climax, and skeletal ossification were observed at 50 µg Pb2+ L-1; however, exposure to 1000 µg Pb2+ L-1 resulted in the opposite effects in tadpoles. In addition, histological alterations of thyroid gland, such as follicular cell hyperplasia and colloid depletion could be found in 50-1000 µg Pb2+ L-1 treatments. Furthermore, Pb2+ exposure at 1000 µg L-1 resulted in significantly decreased transcript levels of Dio2, TRα and TRß, and increased transcript levels of Dio3. In contrast, 50 µg Pb2+ L-1 significantly upregulated the mRNA levels of Dio2, TRα, and TRß, but it reduced the Dio3 expression. These results suggested that Pb2+ might disrupt TH homeostasis in tadpoles by histological alterations of thyroid gland and disturb the transcript levels of Dio2, Dio3, TRα, and TRß, leading to altered growth and development, as well as percent metamorphosis and skeletal ossification. Further studies are needed to elucidate the underlying mechanisms of low-dose stimulation and high-dose inhibition effects.