Fine structure and its functional properties of the endostyle of Ascidians,Ciona intestinalis

Summary For the purpose used in understanding thyroid phylogenesis, the fine structure and the iodine metabolism of the endostyle of Ascidians,Ciona intestinalis, was studied by electron microscopy and electron microscopic autoradiography. There are 8 kinds of zones in the endostyle.Zone 1, 3, and 5 cells, especially zone 1 cells, are characterized by numerous long cilia. These cells which show no indications of protein-secretion but numerous small vesicles and cytoplasmic filaments might play a role in catching and transporting food, absorption of liquid and supporting the endostylar construction.Zone 2, 4, and 6 cells are large and characterized by well developed rough endoplasmic reticulum and numerous electron-dense secretory granules which are considered to be synthesized in the rough endoplasmic reticulum and transported to the Golgi apparatus to mature. They, which are somewhat similar to the pancreatic exocrine cells in fine structure, are believed to secrete the proteinous or mucoproteinous substances which might be related to the digestion of food.Zone 7 and 8 cells which might be homologous to the thyroid cell of the higher vertebrate contains poorly developed rough endoplasmic reticulum, small Golgi apparatus, a few multivesicular bodies, a few lysosomes, and numerous small vesicles. In addition zone 8 cells bear cilia on their apical surface. The cytoplasmic characteristics of these cell types, especially of zone 8 cells, are fairly similar to those of type 2C and type 3 cells of the endostyle of a larval lamprey, though the rough endoplasmic reticulum is not so well developed. By electron microscopic autoradiography numerous silver grains were observed on the apical cell membrane region of zone 7 and 8 cells, especially of zone 8 cells, 1, 4, 6, 16 and 24 hours after immersion in sea water containing¹²⁵I. This fact suggests that the iodination takes place in the apical cell membrane region of these cells. The materials in the endostylar lumen is washed away during the fixation and dehydrating processes of the tissue. Therefore, the possibility of iodination of thyroglobulin-like substances taking place within the endostylar lumen cannot be ruled out. Grains were also found in the multivesicular bodies and lysosomes after 4, 6, 16 and 24 hours, especially 16 and 24 hours. It seems that the organic iodine might be reabsorbed into the cytoplasm of these cells.This investigation was supported by research grant from Dr. Henry C. Buswell Research Fellowship.On leave from Department of Anatomy, Hiroshima University, School of Medicine, as a Visiting Research Professor. The authors wish to express their hearty thanks to Dr. Oliver P. Jones for his valuable criticism.