The incorporation of fucose-3H in rat thyroid follicles was studied by radioautography in the light and electron microscopes to determine the site of fucose incorporation into the carbohydrate side chains of thyroglobulin, and to follow the migration of thyroglobulin once it had been labeled with fucose-3H. Radioautographs were examined quantitatively in vivo at several times after injection of fucose-3H into rats, and in vitro following pulse-labeling of thyroid lobes in medium containing fucose-3H. At 3–5 min following fucose-3H administration in vivo, 85% of the silver grains were localized over the Golgi apparatus of thyroid follicular cells. By 20 min, silver grains appeared over apical vesicles, and by 1 hr over the colloid. At 4 hr, nearly all of the silver grains had migrated out of the cells into the colloid. Analysis of the changes in concentration of label with time showed that radioactivity over the Golgi apparatus increased for about 20 min and then decreased, while that over apical vesicles increased to reach a maximum at 35 min. Later, the concentration of label over the apical vesicles decreased, while that over the colloid increased. Similar results were obtained in vitro. It is concluded that fucose, which is located at the end of some of the carbohydrate side chains, is incorporated into thyroglobulin within the Golgi apparatus of thyroid follicular cells, thereby indicating that some of these side chains are completed there. Furthermore, the kinetic analysis demonstrates that apical vesicles are the secretion granules which transport thyroglobulin from the Golgi apparatus to the apex of the cell and release it into the colloid.

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