Mechanisms by which the Enamel Organ Controls Calcium Entry into Developing Enamel

The secretory ameloblasts and the bands of ruffle-ended maturation ameloblasts function as tight epithelia that control calcium entry into developing enamel by cellular membrane transport processes. In secretory ameloblasts, this transport process appears to be a Ca-activated ATPase distributed over the entire membrane surface. Because about 80% of the plasma membrane surface is proximal to the intercellular tight junctions, the pumping action is directed away from enamel so that calcium entry into secretory enamel is very limited.

The majority of the plasma membrane surface area is distal to the intercellular tight junctions in the bands of ruffle-ended ameloblasts. Therefore, the cellular membrane transport process for calcium is directed toward the enamel. It is postulated that this process is a Na ⁺ ⋅Ca ²⁺ exchange.

Calcium entry into enamel overlaid by the bands of smooth-ended maturation ameloblasts seems less important. ⁴⁵ Ca auto-radiographic studies show only a moderate incorporation of the tracer into enamel overlaid by these bands, which account for only about 20% of the maturation enamel surface area in the rat incisor.