Secretory processes in follicular cells of the bat thyroid. II. The occurrence of organelle‐associated intercellular junctions during late hibernation

The fine structure of bat thyroid follicular cells was examined throughout the late hibernation period (January 17 to April 1), when thyroid gland function undergoes marked increase. The follicular cells during this period are squamous and possess few microvilli, an inconspicuous Golgi complex, scanty rough‐surfaced endoplasmic reticulum and a small number of mitochondria. The cells also exhibit conspicuous membrane‐limited dense bodies and atypical intercellular contacts. The atypical junctional complexes are characterized by a localized dense thickening of the apposed lateral plasma membranes which run parallel to each other for approximately 1 μ. The two dense membranes are separated by an intercellular space 75 Å wide and there is close apposition of rough‐surfaced endoplasmic reticulum and mitochondria. The occurrence of these unusual junctional complexes during late hibernation is discussed in relation to the known increase in thyroid function which occurs at this time.