THE DEVELOPMENT AND HORMONE LEVELS OF THYROID GLAND DURING RANA NIGROMACULATA METAMORPHOSIS

The metamorphosis of amphibians is considered as a good model to study thyroid hormone (TH) disruption. In the present study, we used Rana nigromaculata, a domestic species, to investigate thyroid development and the changes in TH levels during metamorphosis.We first described the morphological change during the tadpole development (stage 26-46) that the hind limbs appeared and finally formed five toes from stage 26 to 40. At the climax of metamorphosis, the dramatic changes include the protruded forelimbs and start-to-resorb tail. Tail resorption completed at stage 46, indicating the completion of metamorphosis. Corresponding to the morphological changes, histological features of thyroid gland changed significantly. From stage 26 to 37, the thyroid gland was small and developed slowly. At stage 38, the thyroid gland enlarged dramatically. The climax of thyroid gland development happened at the climax of metamorphosis. The thyroid gland began to shrink at the completion of metamorphosis. The thyroid hormone (T3) levels were associated with thyroid gland development. T3 content in per lower jaw was very low before metamorphosis, while increased dramatically during metamorphosis; it reached the peak level at the climax of metamorphosis and then decreased. During Rana nigromaculata metamorphosis, thyroid hormone levels were correlated with thyroid gland histological features. The current study creates the foundation for the research of thyroid hormone disruption.