Endocytosis in thyroid follicle cells: II. A microinjection study of the origin of colloid droplets

Abstract

A solution of equine ferritin was instilled into single rat thyroid follicles by microinjection. Simultaneously, thyrotropic hormone (TSH) was administered, either intravenously or dissolved in the microinjection fluid. After 10–20 minutes the follicles were fixed in vivo and prepared for electron microscopy. Ferritin particles were found in vacuoles located in psuedopods protruding into the follicle lumen, and in vacuoles, recognizable as colloid droplets, deeper in the cytoplasm. Ferritin was also seen in small vesicles in the apical cytoplasm, and in bristle-coated pits in the plasma membrane, indicating a micropinocytotic process. In animals pretreated with thyroxine and not stimulated with TSH, no ferritin-containing vacuoles or vesicles occurred. The findings were interpreted as evidence for the view that colloid droplets represent absorption vacuoles. The movement of cytosomes toward the cell apex during TSH stimulation was described and was suggested to be independent of the endocytosis itself.