Ultrastructural autoradiography and cytochemistry of the iodine‐binding cells in the ascidian endostyle

Abstract

AbstractThyroid‐like activity was investigated in the endostyles of Molgula manhattensis, M. occidentalis and Styela plicata. Animals were immersed in sea water containing 125I for one to 24 hours. Although electron microscopic autoradiography revealed an extracellular accumulation of silver grains associated with zone 8 in all three species and with zone 9 in two of them, intracellular accumulations were found only in zone 7. Within this zone in each of the three species, grains were present over the apical cell membrane, multivesicular bodies and the apical cytoplasmic matrix after brief exposure to 125I. After longer exposure, grains were also present over large dense bodies and more basal regions of the cytoplasmic matrix.Certain features, suggestive of thyroidal functions, were observed in the cells of zone 7. These included morphological evidence for secretory and pinocytotic activities and cytochemical evidence both for peroxidase activity and for the presence of carbohydrate material which may represent a glycoprotein.It is concluded that active binding of iodine is restricted to zone 7 in the endostyles of Molgula and Styela, where it occurs at both the apical cell membrane and within multivesicular bodies. Some of the iodinated product may be immediately passed into the lumen of the pharynx. The rest is stored intracellularly within multivesicular bodies and dense bodies of zone 7. Intracellular binding and storage of iodine are considered to represent primitive conditions preceding the evolution of the follicular structure of the thyroid gland.