A study of granule formation in the bat parafollicular cell.

The fine structure of the bat thyroid parafollicular cell has been examined at monthly intervals throughout the hibernating period. During November and December parafollicular cells appear either partly or totally degranulated and intact dense secretory granules are relatively sparse. The degranulated cells exhibit an inconspicuous Golgi complex and relatively few lysosome-type bodies. Few degranulated parafollicular cells are present in thyroid glands from bats collected in January. When found they are characterized by the presence of whorls of cytoplasmic agTanular membranes which enclose a central mass of cellular debris. January bat thyroids are characterized by the presence of three different types of parafollicular cell. One type contains no secretory granules. The cytoplasmic matrix of this type is rich in granular endoplasmic reticulum and free ribosomes and its small Golgi complex consists of several slightly dilated saccules. In close proximity to the Golgi complex are numerous small to medium-sized vesicles which often appear to merge with Golgi elements. Such vesicles are considered to represent the vehicle by which secretory product is transferred from the endoplasmic reticulum to the Golgi complex. The second type of parafollicular cell differs from the first in containing large numbers of intact dense secretory granules. It is also characterized by an extensive Golgi complex which appears to be forming new secretory granules, and by a less extensive granular endoplasmic reticulum. The third type of parafollicular cell shows a structure intermediate between the first two. The cytoplasm of all three types of January parafollicular cells contains many structures belonging to the lysosomal-vacuolar system, including autophagic vacuoles, vacuolated dense bodies and multivesicular bodies. By February and March only parafollicular cells of type 2 are observed. They contain few lysosome-like structures. It is concluded that during mid-hibernation (January), parafollicular cells undergo a series of intracellular changes during which new dense secretory granules are produced. Accompanying granule formation is an augmentation of lysosome-like structures which probably serve as a means of digesting debris from previous secretory cycles.