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The site of iodination of thyroglobulin in the thyroid gland revealed through morphological studies was critically reviewed in relationship to biochemical points of view. The phylogenetic and evolutional aspects of this problem were also reviewed and discussed. To conclude, the present author wishes to emphasize the following:1. In adult vertebrates whose thyroid follicular structure has been well completed, the main site of the iodination of thyroglobulin is generally the follicular lumen and the apical plasma membrane region.2. The iodination may also take place partly in the cytoplasm. Autoradiographic data from the chick embryo and from the dissociated thyroid cell of the rat and sheep, and the electron microscopic histochemical data for peroxidase reaction indicate the possibility that the iodination could occur also in the cytoplasm, such as in the rough endoplasmic reticulum, Golgi apparatus and subapical vesicles.3. In usual adult animals, however, thyroglobulin in the follicular lumen is far larger in quantity than that in the cell cytoplasm and numerous molecules of thyroglobulin in the follicular lumen might have not yet completely iodinated and therefore injected iodine is considered to be combined with luminal colloid preferentially. This seems to be the reason why the iodination takes place almost entirely in the follicular lumen (and apical plasma membrane region) in adult animals having usual follicular structures.4. Based on the data of larval lampreys and ascidians, iodine metabolism in the thyroidhomologous cells in cyclostomes and protochordates seems to show the similar pattern to that of higher vertebrates. The main site of iodination of protein in these animals is the apical plasma membrane region of certain types of endostylar cells, and the possibility of iodination taking place also in the endostylar lumen is not ruled out. Some of iodinated protein must be reabsorbed into the certain types of endostylar cells in these animals.

Morphological Aspects on the Site of Iodination of Thyroglobulin in the Thyroid Gland