Subapical Vesicles Research Articles

Morphological aspects on the site of iodination of thyroglobulin in the thyroid gland

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.

Fine structure and functional properties of the thyroid of the anura, Rana nigromaculata nigromaculata (a part of the phylogenetic studies of the thyroid gland).

As a part of the phylogenetic studies of the thyroid gland, the anuran thyroid was studied by electron microscopy, electron microscopic autoradiography with 125I and by histochemistry for acid phosphatase activity.The fine structure of the anuran thyroid cell is almost similar to that of mammalia and fish. The rough endoplasmic reticulum is well developed in the basal part of the cell. Subapical small vesicles which have been considered to be derived from the Golgi apparatus and to contain secretory materials to the follicle lumen are also seen as in mammalia and fish. Characteristic are relatively well developed tonofilaments, numerous and large intracellular colloid droplets, and the occurrence of filamentous structures in some of the droplets. These findings are similar to those in some species of teleost and chimaeroid fish. About two thirds of the large droplets, most of the small dense granules and some parts of the Golgi apparatus show an acid phosphatase activity. The small dense granules which are sometimes in contact with the large colloid droplets are considered to be primary lysosomes derived from the Golgi apparatus. The large droplets likely correspond to reabsorbed colloid to be hydrolyzed by lysosomes. This presumed pattern of cell function in the amphibian thyroid is almost the same to that in mammals.The capillary endothelium shows numerous pores as that of the higher vertebrates and some species of fish.The present findings indicate that the iodination of the thyroglobulin takes place chiefly in the follicle lumen and in the apical cell membrane region.