Figure 5. Schematic model representing calcium entry in enamel cells.

Working model for Ca²⁺ uptake by enamel cells showing maturation stage ameloblasts forming a cell barrier joined by tight junctions at the apical pole. In the endoplasmic reticulum (ER) we find that enamel cells express the sarco/endoplasmic reticulum SERCA2 as the main Ca²⁺ refilling pump. Inositol 1,4,5-trisphosphate receptors (IP₃R) and ryanodine receptors (RyR) are also identified as release channels with the former likely being the active release system. STIM1 has a wide distribution throughout the ER and ORAI1 is found in the plasma membrane of enamel cells. As Ca²⁺ pools are depleted in the ER, STIM1 clusters enable Ca²⁺ entry via the ORAI1 channel.