In this study we investigated the presence and localization of thyroxine in Ciona intestinalis larvae and its involvement in metamorphosis. To date, the mechanisms regulating the metamorphosis of ascidians remain largely unknown. In vivo treatment of swimming larvae with exogenous L-thyroxine and thiourea, and in vitro experiments utilizing high performance liquid chromatography, radioimmunoassay, and immunoperoxidase staining demonstrate the presence of thyroxine at the larval stage. This suggests that this hormone may participate in the control of metamorphosis and thus play a different role from that observed in adults.
The effects of tributyltin(IV)chloride (TBT) on the metamorphosis of ascidian larvae of Ciona intestinalis, 2 h after hatching, were investigated. Ascidians are protochordates that lack thyroid follicles and possess thyroid hormones (THs) and their precursors, 3-monoiodo-tyrosine (MIT) and 3,5-diiodo-tyrosine (DIT), in their endostyle. According to recent findings, these hormones are also present at larval stages, localize in mesenchymal cells and their function seems to be mainly related to larval transformations. Here, we investigate the effects of TBT on thyroxine (T 4 ) content and localization by exposing larvae of C. intestinalis for different times to TBT concentrations known to block metamorphosis. The result is a blocking of the retraction of the tail, and larval immobility. As detected by immmunohistochemistry, in normal larvae, T 4 is found in all mesenchymal cells spread out in the body cavity, under the adhesive papillae and around the intestine. By contrast, in larvae exposed to 10 À5 M and 10 À7 M TBT, T 4 is detected only in 5% and 25% of total mesenchymal cells respectively. Radioimmunoassay shows a 70% decrease of T 4 content in TBTexposed larvae, with respect to unexposed larvae. In addition, neosynthesis of THs is inhibited. Even if parallelism does not exist between the endocrine systems of invertebrates and vertebrates, however, a close similarity exists for functions such as reproduction and metabolism. Our results indicate that TBT could behave as an endocrine disrupter (ED) in ascidians and could impair T 4 metabolism. These findings suggest that the ED activity of TBT could be conserved from invertebrates to vertebrates.
The ascidian endostyle is a mucus-secreting pharyngeal organ, it has iodine-concentrating activity and the biosynthesis of thyroid hormones has been well documented. According to our recent findings, ascidians possess thyroid hormones, which are localized in mesenchymal cells. We have studied the presence and localization of L: -thyroxine (T(4)) in Ascidia malaca (Traustedt), Ascidiella aspersa (Müller), Phallusia mamillata (Cuvier) and Ciona intestinalis (Linnaeus) larvae and its involvement in metamorphosis. In vivo treatment of swimming larvae with exogenous T(4) and thiourea (a thyroid hormone synthesis inhibitor), demonstrate the presence of T4 during larval development. These results were confirmed by in vitro experiments utilizing dot blotting, radioimmunoassay and immunoperoxidase staining. The hormone was localized in mesenchymal cells of all four ascidians, spread out in the body cavity, under the adhesive papillae and around the intestine. The presence of TH in mesenchymal cells could be related to blood cells, musculature and heart tissue differentiation. The results suggest that this hormone could be involved in the control of metamorphosis.
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