Secretory processes in follicular cells of the bat thyroid. I. Ultrastructural changes during the preearly and mid‐hibernation periods with some comments on the origin of autophagic vacuoles

Ultrastructural changes which occur in bat thyroid follicular cells between late August and early January are discussed. During the first half of this period the principle changes are proliferation of the granular endoplasmic reticulum, an increase in size of the Golgi apparatus and an increase in the number of apical vesicles. These changes are followed by a gradual reduction in the amount of endoplasmic reticulum and in the size of the Golgi complex. During the expansion phase of the granular endoplasmic reticulum, large numbers of dense granules, and “light” and “dark” multivesicular bodies are present throughout the cytoplasm of the follicular cells. An unusual finding is that in the bat, the thyroid follicular cells also contain many autophagic vacuoles at this time. The role of the dense granules in the formation of autophagic vacuoles is discussed and evidence for the transformation of multivesicular bodies into dense granules is presented. Autophagic vacuoles were not found during the phase when the granular endoplasmic reticulum underwent a marked regression. At this time, follicular cells contained prominent complex dense bodies which resemble the “phagolysosomes” of other mammalian follicular cells. These observations are discussed in relation to the changes which occur in thyroid function of hibernators during the period studied.