Endocytosis in thyroid follicle cells: III. An electron microscopic study of the cell surface and related structures

Abstract

Endocytosis in thyroid follicle cells from untreated, suppressed and TSH-stimulated glands were studied. In untreated glands, images indicating a micropinocytic process resulting in the production of bristle-coated vesicles were observed at the apical surface. In suppressed glands, signs of apical micropinocytosis were only rarely encountered. After TSH stimulation, the coated vesicles reappeared. In addition, pictures indicating endocytosis through engulfment of colloid by large cytoplasmic protrusions were observed. Similar pictures were rarely encountered in normal glands. In all the experimental groups, signs of micropinocytosis by coated vesicles were observed along the lateral cell surface as well as along narrow infoldings in the basal plasma membrane. Along parts of the basal cell surface bordering on basement membrane, smooth vesicles suggesting a different form of endocytosis were observed. Differences in structure and thickness of the plasma membrane of the different surfaces of the cells were described, as well as differences in the localization of ATPase activity. Small vesicular structures, displaying ATPase activity were observed both in the basal portions of follicle cells and in the thyroid capillary walls. Heavy deposits of ATPase reaction products were found on the plasma membrane of parafollicular cells and in the interspace between a parafollicular and a follicle cell. Only scattered deposits were found between the lateral surfaces of adjacent follicle cells. Desmosomes were found between adjacent follicle cells as well as between parafollicular cells, but were not observed along the contact between a parafollicular and a follicle cell.