INTRACELLULAR LUMENS IN THYROID-FOLLICLE CELLS OF THYROXINE-TREATED RATS

Intracellular lumens were present in thyroid follicle cells in rats treated with thyroxine for 1-2 days, but not in normal rats. Their intracellular nature was ascertained by serial sections examined in the electron microscope and was further marked by their lower density as compared to the extracellular follicle lumen. The limiting membrane of intracellular lumens formed microvilli which were similar to microvilli at the apical cell surface. The surface area of the average intracellular lumen was calculated to be about twice that of the apical plasma membrane. Intracellular lumens were labeled a short time after injection of 125I indicating that the limiting membrane, like that of the apical plasma membrane, had iodinating capacity. Follicle cells containing intracellular lumens were characterized by a narrow apical portion and a high concentration of exocytotic vesicles in their cytoplasm. Intracellular fusion of exocytotic vesicles appears to be the most likely mechanism of formation but the possibility of formation by invagination of the apical cell surface cannot be excluded. © 1979 Academic Press, Inc.