Quantitative electron microscopic studies on exocytosis and endocytosis in the thyroid follicle cell

The effect of TSH (0.5 U), given iv to T4-treated rats, on the distribution of membranes in the apical part of the thyroid follicle cell was investigated by electron microscopic morphometry. In unstimulated cells, the membrane surface area of exocytotic vesicles (Ev) was slightly larger than that of the apical plasma membrane; the latter had an estimated surface area of 180-190 micron2/follicle cell. Three phases in the action of TSH could be discerned. A first phase (up to 5 min) was dominated by exocytosis; the membrane surface area of Ev decreased by about 40% and the apical plasma membrane showed a corresponding increase; pseudopods were rare. A second phase (5-20 min) was characterized by an increase in the membrane surface area of pseudopods while the area of Ev and apical plasma membrane decreased. A third phase (20-30 min) was characterized by a decrease in the membrane surface area of the pseudopods and an equivalent increase in the membrane surface area of colloid droplets. The total membrane surface area measured was about the same in all groups of rats. The membrane surface area of endocytotic structures (pseudopods and colloid droplets) at 20 and 30 min was about the same as that of the apical plasma membrane in unstimulated cells. The present results are consistent with the hypothesis that after stimulation of the thyroid follicle cell, membrane is first transferred from Ev to the apical plasma membrane. From the enlarged apical plasma membrane, pseudopods are formed. Membrane from pseudopods is then internalized in the form of colloid droplets.