Bufo Toad Embryonic Development Deiodinase Gene DIO2, Study Characteristics DIO3 Expression

Thyroid gland is an important endocrine gland in vertebrate species. The hormones (T3and T4) synthesized in the thyroid control basal metabolic rate and energy metabolism in adult mammals, while in most vertebrates it regulates metabolic processes involving nitrogen balance, lipid degradation, etc. Furthermore, thyroid hormones play important roles during the development, metamorphosis and growth of animals. The functional unit of the thyroid gland is the thyroid follicle, a spherical structure filled with colloid serving as a reservoir of materials for production. Colloid changes in follicle have been widely used to assess thyroid hormone production during anuran development and metamorphosis.The activation and inactivation of thyroid hormones are catalyzed by deiodinases. The type II deiodinase (D2) catalyzes the pro-hormone T4into the receptor-active form T3via the out rings deiodination (ORD). The type III deiodinase (D3) exclusively converts, via inner rings (IRD), a process that results in the degradation of both T4and T3to inactive derivatives. The type I deiodinase (D1) carries out both ORD and IRD reactions. D2and D3activities have major roles in determining the timing of tissue changes and controlling cellular responses during metamorphosis. In this study we have investigate the onset of thyroid gland function in the tadpole, the distribution and relative expression of DIO2and DIO3mRNAs in the embryo of Bufo gargarizans. The main results as follows:1. The thyroid gland of B.gargarizans tadpole at Gosner stage38lied dorsal to the developing heart, ventral posterior to the copula II, ventral to the anterior margin of the hypobranchial plate and just between the two geniohyoideus. The thyroid gland was consisted of two ellipse lobes. Each lobe was composed of many transparent spherical follicles. The transparent follicle lumen was lined by the thyroid follicle cells.2. Thyroid gland was not found at Gosner stage25. The appearance of developing thyroid gland was observed at Gosner stage28. At this stage, the thyroid gland was small in size. The follicles were low in number with no colloid present. Follicles number was increased at Gosner stage33.The cuboidal follicular cells of thyroid gland were distinctly observed. The follicle lumina were filled with homogeneous colloid. The results reveal that the thyroid gland first appeared at late premetamorphosis and the thyroid gland could produce TH during prometamorphosis. 3. In situ hybridization reveals faint signals for DIO2mRNA was first detected in the animal hemisphere of embryos at Gosner stage6. The expresseion persisted in the animal hemisphere of embryos at Gosner stage8and stage9. DIO2was detected in the neural folds, branchial placode and the prospective cement gland region at Gosner stage14. DIO2expressed in the midbrain, optic vesicle, cement gland, somites and tailbud of embryos at Gosner stage17. The expression of DIO2in hindbrain, spinal cord, tail, somites, branchial arches, cement gland, oral apparatus, nose and pharynx was seen in embryos at Gosner stage20. At Gosner stage23the expression was observed in tail, somites, otic vesicle, branchial arches, nose, cement gland, oral apparatus, spinal cord and pharynx but absent from the brain. DIO2expression detected in otic vesicle, somites and branchial arches and expression in liver, eye and gut was also detectable at Gosner stage26.The DIO3expression was detected prior to Gosner stage6. The fiant expression was observed in the animal hemisphere of embryos at Gosner stage6,8and9. DIO3was detected in the branchial placode at Gosner stage14. DIO3expressed in the otic vesicle, branchial arches, somites, tailbud and cement gland of embryos at Gosner stage17. The expression of DIO3was present in the forebrain, spinal cord, branchial arches, oral apparatus, cement gland, somites and tail of embryos at Gosner stage20. Expression was visible in the otic vesicle, eye, branchial arches, oral apparatus, cement gland, somites and tail while absent in the brain of embryos at Gosner stage23. In the embryos at Gosner stage26, the expression of.DI03was observed in gut, liver, somites, otic vesicle, spinal cord and branchial arches.4. Both DIO2and DIO3mRNAs were detectable in unfertilized eggs (GO). The expression of DIO2and DIO3in embryos first appeared at Gosner stage1and reached the maximum at Gosner stage14(neural fold stage). The results indicate that DIO2and DIO3mRNAs could be involved in neurogenesis.The level of DIO2mRNA increased from Gosner stage23to Gosner stage42in the whole body of tadpoles. The expression of DIO2peaked at Gosner stage42(metamorphosis climax). The DIO3expression increased from Gosner stage23to Gosner stage38and reached maximum at Gosner stage38(prometamorphosis). The results suggest that DIO3may contribute to the protection during prometamorphosis and the onset of the late developmental events during metamorphosis.