Structural remodelling of bat thyroid parafollicular (C) cells during development

The development of bat parafollicular (C) cells was evaluated by electron microscopy from the time of early thyroid formation, through the neonatal period, to young adulthood. Parafollicular cells were relatively few in number (3%) at the earliest developmental stage examined, but increased with subsequent fetal stages, reaching a relatively stable level of 28% of parenchymal cell volume just prior to birth. Fetal parafollicular cells were packed with calcitonin granules. Following birth, however, parafollicular cells were totally or almost totally depleted of calcitonin granules. Moreover, another cell type that resembled the parafollicular cell was also found. This cell contained tiny granules which often contained two dense cores. Parafollicular cells of maturing, juvenile bats were characterized by the massive accumulation of secretory materal within the cisternae of the rough surfaced endoplasmic reticulum and a great deal of autophagy. Remarkably, none of these changes were found in young, adult bat parafollicular cells. In conclusion, this study provides morphological evidence to support the view that parafollicular cells play a role in developing mammals.