Structure and function of secretory ameloblasts in enamel formation.

Secretory ameloblasts have multiple functions including the synthesis and resorption of enamel matrix proteins and calcium transport during enamel formation. We have examined these functions by means of cytochemistry and immunocytochemistry. Enamel proteins, amelogenins and enamelins are localized in the biosynthetic pathways of ameloblasts and in the forming enamel. Sulfated glycoconjugates are present in secretory ameloblasts. The distal junctional complex of ameloblasts may act as a permeability barrier to enamel proteins, thereby confining the secreted proteins to the growing enamel front. Secretory ameloblasts contain lysosomal enzymes in the Golgi lysosome endoplasmic reticulum system and also exhibit absorptive capacity, which might be associated with an early decrease in extracellularly degraded enamel proteins. Active calcium transport through the ameloblasts towards the growing enamel is indicated by the demonstration of Ca-ATPase activity along the plasma membranes. A calcium-dependent modulator protein, calmodulin, is localized in ameloblasts, suggesting that early enamel mineralization is dependent upon calmodulin-regulated Ca-ATPase in ameloblasts. These results suggest that the secretory ameloblast is a highly specialized multifunctional cell in the production, resorption and degradation of enamel matrix and in the active calcium transport essential for matrix mineralization during enamel formation.