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.