Amelogenesis

Amelogenesis is the formation of enamel on teeth and begins when the crown is forming during the advanced bell stage of tooth development after dentinogenesis forms a first layer of dentin. Dentin must be present for enamel to be formed. Ameloblasts must also be present for dentinogenesis to continue. A message is sent from the newly differentiated odontoblasts to the inner enamel epithelium (IEE) that causes epithelial cells to further differentiate into active secretory ameloblasts. Dentinogenesis is in turn dependent on signals from the differentiating IEE in order for the process to continue. This prerequisite is an example of the biological concept known as reciprocal induction, in this instance between mesenchymal and epithelial cells. Amelogenesis is considered to have three stages. The first stage is known as the inductive stage, the second is the secretory stage, and the third stage is known as the maturation stage. During the inductive stage, ameloblast differentiation from IEE is initiated. Proteins and an organic matrix form a partially mineralized enamel in the secretory stage. The maturation stage completes enamel mineralization. In the inductive stage, the morphodifferentiation phase the shape of the crown is determined by the bell stage of tooth development. There is a basal lamina between the IEE and the dental papilla. At this time, the dentin is not mineralized. The IEE cuboidal or low columnar with centralized nuclei and poorly developed Golgi complexes. The differentiation phase of the Induction stage is initiated by the presence of newly formed predentin. The IEE cells then elongate and become preameloblasts. There is a shift in polarity. Each preameloblast elongates and becomes an postmitotic, polarized, secretory ameloblast. However, there are no Tomes' process yet. It is at this stage that a signal is sent from the newly differentiated ameloblasts back across the dentinoenamel junction (DEJ) to stimulate dentinogenesis. In the secretory stage, ameloblasts are polarized columnar cells.