Dentinogenesis is the formation of dentin, which begins before enamel formation. Dentin is formed by odontoblast cells in two phases: first the formation of an organic collagen matrix, followed by deposition of hydroxyapatite crystals. As dentinogenesis begins, odontoblasts elongate and secrete an unmineralized collagen matrix called predentin. Over time, predentin adjacent to the pulp mineralizes and forms dentin while new predentin is deposited, resulting in incremental dentin growth of approximately 4 micrometers per day. Dentinogenesis continues throughout life but slows after eruption.