Ultrastructural changes during the life cycle of human odontoblasts.

In developing premolars there are four successive stages of odontoblasts related to their location within the tooth, namely pre-odontoblasts, secretory, transitional and aged odontoblasts. Pre-odontoblasts are at the growing tip of the root; they are immature and slightly polarized. They become fully differentiated to give rise to the most active, secretory stage when primary dentine is produced. After completion of primary dentine formation in the coronal region, the aged odontoblasts have a reduced number of organelles which are relocated within the infranuclear region. This final stage is regressive and results from an autophagic process that is prominent in the transitional stage.