Electron microscope immunohistochemical localization of thyroglobulin in the rat thyroid gland

In order to make a precise identification of the organelles containing thyroglobulin (TG) in the thyroid follicular cells, an electron microscope immunohistochemical localization of TG was conducted in the rat thyroid gland. In both control and TSH-treated rats, TG was detected in the follicular lumen, in large vesicles which correspond to the classical colloid droplets, in apical vesicles, and also in the apical cytoplasm. When the formation of colloid droplets was prevented by the previous administration of L-thyroxine, a positive reaction for TG was observed only in the colloid and apical vesicles, and diffusely in the cytoplasm. These results clearly establish for the first time the presence of immunoreactive TG in apical vesicles, luminal colloid and colloid droplets, and cytoplasm (possibly cell sap). They confirm previous indirect evidence suggesting that apical vesicles are involved in the transport of TG and that colloid droplets whose formation is induced by TSH, do contain TG.