Amelogenesis

Overview
Amelogenesis is the formation of enamel on teeth and occurs during the crown stage of tooth development after dentinogenesis, which is the formation of dentine. Since dentine must be present for enamel to be formed, this prerequisite is an example of the biologic concept, termed reciprocal induction.

Amelogenesis is considered to have two stages. The first stage is known as the secretory phase, and the second stage is known as the maturation stage. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage. The maturation stage completes enamel mineralization.

Secretory stage
In the secretory stage, ameloblasts are polarized columnar cells. In the rough endoplasmic reticulum of these cells, enamel proteins are released into the surrounding area and contribute to what is known as the enamel matrix, which is then partially mineralized by the enzyme alkaline phosphatase. When this first layer is formed, the ameloblasts move away from the dentin, allowing for the development of Tomes’ processes at the apical pole of the cell. Enamel formation continues around the adjoining ameloblasts, resulting in a walled area, or pit, that houses a Tomes’ process, and also around the end of each Tomes’ process, resulting in a deposition of enamel matrix inside of each pit. The matrix within the pit will eventually become an enamel rod, and the walls will eventually become interrod enamel. The only distinguishing factor between the two is the orientation of the calcium crystals.

Maturation stage
In the maturation stage, the ameloblasts transport substances used in the formation of enamel. Microscopically, the most notable aspect of this phase is that these cells become striated, or have a ruffled border. These signs demonstrate that the ameloblasts have changed their function from production, as in the secretory stage, to transportation. Proteins used for the final mineralization process compose most of the transported material. The noteworthy proteins involved are amelogenins, ameloblastins, enamelins, and tuftelins. During this process, amelogenins and ameloblastins are removed after use, leaving enamelins and tuftelin in the enamel. By the end of this stage, the enamel has completed its mineralization.