The fine structure of the thyroid gland of the hagfish, Eptatretus burgeri was observed using the electron microscope.
The thyroid follicles are diffusely distributed and embedded in the connective tissue of the ventral region of the gill pouch arteries. Each follicle is very large in size and is recognizable by the naked eye. Though follicular epithelial cells show a few microvilli on their apical surfaces, cilia which are usually seen in the lamprey thyroid, are lacking in the follicular cell of the hagfish. The cytomembranes such as rough endoplasmic reticulum and Golgi apparatus are poor in development as compared with those of higher vertebrates. The elements of these cytoorganelles are not dilated but flattened. Injected TSH does not cause so marked alterations on the fine structure of the epithelial cells. By electron microscopic autoradiography it was shown that silver grains are chiefly localized over the follicular lumen and the apical cell membrane region one hour after the injection of ¹²⁵I. The main site of iodination of thyroglobulin is the luminal colloid, the same as that in higher vertebrates. The colloid might be reabsorbed into the cell by pinocytosis. Large dense bodies, 0.5-3.0μm in diameter, which might be secondary lysosomes or residual bodies produced by the reabsorbed colloid and primary lysosomes, are often seen in the cytoplasm.
Blood capillaries are relatively scarce in distribution. Though the endothelial fenestration is absent, numerous pinocytotic vesicles are characteristic in the cytoplasm.
In respect to their function the hagfish thyroid cells are considered to exhibit a relatively low degree of activity as compared with those of elasmobranchs, teleosts, and higher vertebrates.