Secretory processes in follicular cells of the bat thyroid. III. The occurrence of extracellular vesicles and colloid droplets during arousal from hibernation

Abstract Ultrastructural changes that occur in follicular cells of the bat thyroid gland just prior to, and immediately after arousal from hibernation are discussed in relation to the known changes which occur in thyroid function during arousal from hibernation. The most distinctive ultrastructural change that takes place just before emergence from hibernation is the occurrence, extracellularly, of concentrations of small vesicles lying in the colloid near the cell's apical plasma membrane. Similar accumulations of vesicles are absent in the apical cytoplasm of the follicular cell. Other principal changes from the early hibernating state found at this time are an increase in the number of apical vacuoles, dense granules and multivesicular bodies. These changes are followed at arousal itself by the appearance of large numbers of intracytoplasmic colloid droplets, often intimately associated with dense granules. An unusual feature of these follicular cells is that although they are rich in colloid droplets, apical pseudopods cannot be found.