RESUMEN
Iodine uptake and production of iodine-aminoacids are evolutionarily very old phenomena. Seaweed are the first eukaryotic organism presenting these functions; they are rich in iodine and are at basis of the food chain. The monoiodotirosines, precursors of thyroxine, have been identified in a wide variety of invertebrates, such as Gorgonians and Tunicates. The structure of the thyroid appears for the first time in Cyclostomes adults (Lamprey), while in Tunicates (Ciona intestinalis) and Amphioxus is present a similar structure, the endostyle, which is an invagination of the ventral wall of the pharynx containing glandular cells that are able to concentrate iodine. This is not a true endocrine gland because the secretion is poured into the alimentary canal. In the larva of Lamprey (Ammocoetes) during the metamorphosis some of the epithelial cells persist and transform in the follicles. The cyclostomes, therefore, represent a link between the endostyle of protochordates and the thyroid gland of higher chordates. This hypothesis is confirmed by molecular genetic studies which have demonstrated the expression of Thyroid Transcription Factor (TTF-1) in the endostyle of Ciona, Lamprey and Amphioxus. The TTF-1 is an ancestral transcription factor which controls the survival of thyroid follicular cells at the beginning of organogenesis and regulates the expression of thyroid-specific genes in adult life.