Figure 2. Schematic illustration of ion transport through the enamel organ layer and associated extracellular events taking place in the forming enamel. Ions transported through the cell layer are incorporated into the lattice positions. The precipitation rate is determined by the degree of supersaturation, fluid volume, crystal surface areas, protein coating, and the presence of inhibitors (e.g., carbonate and Mg²⁺) and promoter, i.e., fluoride. As a result, the Ca²⁺ concentration in the fluid ([Ca]_s) surrounding the forming crystals is determined in principle by the balance between the rates of Ca²⁺ supply (R_D) and consumption for crystal precipitation (R_C). Proteolytic processing of the amelogenins and other proteins, possibly in a form of nanosphere aggregates, may depend on the [Ca]_s levels.