Fluctuations in surface pH of maturing rat incisor enamel are a result of cycles of H + ‐secretion by ameloblasts and variations in enamel buffer characteristics (1098.1)

The large hydroxyapatite (HA) crystals in dental enamel results from a long lasting process called enamel maturation, which follows a stage of enamel matrix secretion. Thus, after the full thickness of enamel is reached, the enamel still requires about 60‐70 % mineral to be deposited by HA‐crystals growing in width and thickness. This is facilitated by the ameloblasts, which during maturation change morphology every 6‐8th hour. This study aims to test the role of the ameloblasts in maintaining enamel pH in the maturation zone in order to elucidate the role of the cyclic changes in ameloblast morphology for HA‐crystal growth. Colorimetric pH‐indicators and ratiometric fluorometry were used to measure surface pH in maturation zone enamel of rat incisors. Alternating acidic (down to pH 6.24±0.06) and alkaline zones (up to pH 7.34±0.08) corresponding to ruffled‐ended and smooth‐ended ameloblasts, respectively, were found along the tooth thus coinciding with ameloblast morphological cycles. Underlying the cyclic pattern, a gradual decrease in pH towards the incisal edge was seen. Vinblastine or FR167356 (H+‐ATPase‐inhibitor) disturbed ameloblast acid‐secretion, especially in the early parts of acidic zones. At the pH‐values observed, PO43‐ would be protonated (pKa >12) and HA dissolved. However, by molecular dynamics simulations we estimate the pKa of HPO42‐ at an ideal HA surface to be 4.3. The acidic pH measured at the enamel surface may thus only dissolve non‐perfect domains of HA crystals in which HPO42‐ is less electrostatically shielded. As a result of the repeated alkaline/acidic cycles induced by ameloblasts, near‐perfect domains will therefore gradually replace less perfect domains and large near‐perfect HA crystals will be produced. In conclusion, fluctuations in surface pH of maturing rat incisor enamel are a result of cycles of H+‐secretion by ameloblasts and variations in enamel buffer characteristics.