Iodine metabolism of the endostyle of larval lampreys, Ammocoetes of Lampetra japonica. Electron microscopic autoradiography of 125-I.

Abstract

Experiments were carried out to study the iodine metabolism of the endostyle of the larval lamprey which is considered to be homologous to the thyroid gland. Larval lampreys, ammocoetes of Lampetra japonica were intraperitoneally injected with 200 μc of Na 125I; their endostyles were removed 30 minutes, 1, 2, 4, 6, 8 and 24 hours after the treatment. Type 1 and type 4 cells (Marine) were almost inactive in binding iodine. Silver grains appeared within 30 minutes after the injection over the apical cell membrane including the surfaces of microvilli and cilia of type 2 c and type 3 cells. These grains increased in number until 2 hours. A few of apical small vesicles of the same cells were labeled 1 to 2 hours after the injection. Small dense granules large dense bodies, and multivesicular bodies in the type 2 c and type 3 cells were labeled especially at 6 to 24 hours. The ratio in number of the labeled dense granules, or bodies to the unlabeled ones tended to increase markedly with time. Large or small vacuoles, dense or light in the cytoplasm of some type 5 cells which lack indications of protein-synthesis sign in the cytoplasm were labeled 6 to 24 hours after the injection of 125I, and the number of the labeled vacuoles increased with time. From these facts, we conclude that: (1) iodination of the thyroglobulin of type 2c and type 3 cells takes place almost entirely at the apical cell membrane region, (2) the thyroglobulin-like protein contained in the apical small vesicles of type 2c and type 3 cells is slightly iodinated, (3) although it is difficult to determine whether the dense granules and bodies, which might be lysosomes, are secretory substances or reabsorbed materials, the possibility of the occurrence of reabsorption and hydrolysis of the thyroglobulin in the type 2c and type 3 cells should be considered, and (4) reabsorption of the thyroglobulin from the endostylar lumen by some type 5 cells should be also considered.