Secretion by parafollicular cells beginning at birth: ultrastructural evidence from developing canine thyroid.

The ultrastructure of developing canine parafollicular cells has been examined. The hypothesis that secretion is relatively inactive prior to birth but very active at and following birth was tested. Parafollicular cells develop and accumulate characteristic secretory granules prior to birth. However, there is little or no evidence of exocytosis at this time. At birth and during the neonatal period, but not in adult thyroids, signs of exocytosis of granular contents by parafollicular cells are abundant. Just prior to the expected date of birth and before evidence of exocytosis appears, parafollicular cells accumulate intracisternal granules within rough endoplasmic reticulum. These observations are consistent with the view that parafollicular cells first become actively secretory around the time of birth and are more active at this time than in early fetal or later adult life.