Parts of the protochordate endostyle are considered homologous to the vertebrate thyroid (1–3). In the adult amphioxus, the endostyle forms a groove in the floor of the pharynx lined by paired, longitudinal zones of epithelial cells. In the larval amphioxus, the endostyle is an unpaired structure in the right buccal wall (4). In the adult (5–7), as well as larval endostyle (8,9), iodination takes place in the lumen at the apical surface of a well-defined cell zone in the lateral portion of the endostyle. Iodination is catalyzed by a peroxidase, present in the apical plasma membrane (9). A thyroglobulin-like protein and iodothyronines have been found in the amphioxus (10,11). Thus, the mechanism of iodination shares many features with that in thyroid follicle cells (12).
Apical Surface Lateral Portion Apical Plasma Membrane Cell Zone Electron Microscopic Autoradiography
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