Mineral Acquisition Rates in Developing Enamel on Maxillary and Mandibular Incisors of Rats and Mice: Implications to Extracellular Acid Loading as Apatite Crystals Mature

Abstract

The formation rates of mineral in developing enamel were determined by microweighing of incisors of mice and rats. Computations indicated that a large excess of hydrogen ions would result from creating apatite at the calculated rates. Enamel organ cells (ameloblasts), therefore, likely excrete bicarbonate ions to prevent pH in fluid bathing enamel from becoming too acidic.

The anion exchanger Ae2 is required for enamel maturation in mouse teeth

“…Ruffle-ended ameloblasts comprise about 80% of the maturation stage ameloblast population in the rat incisor. These ameloblasts are thought to be responsible for controlling the movement of calcium and other ions into the enamel space and thus control enamel crystal growth as well as enamel matrix resorption (Smith et al, 2005). It is interesting to note here that within the population of wild-type maturation ameloblasts some groups of cells stained weakly for Ae2 and we assume that these cells were less active and may correspond to the smooth-ended ameloblasts.…”

“…During formation of the hydroxyapatite crystals in the enamel compartment, protons are generated that need to be neutralised to maintain a physiological pH in order to drive the mineralisation process (Smith et al, 2005). The number of protons generated is dependent upon the phosphate precursor used and can vary between four and 14 moles of hydrogen per mole of apatite formed (Simmer and Fincham, 1995).…”

“…The relatively low rate of mineralisation during the secretory stage and the presence of large amounts of amelogenins with high buffering capacity in the enamel extracellular space are thought to be the mechanism responsible for neutralising the protons formed during OCP formation (Ryu et al, 1998;Simmer and Fincham, 1995). During the maturation stage, however, the situation is different: the rate of mineralisation is very high and the generation of protons is expected to be high as well while at the same time, the amelogenins that were responsible for buffering the extracellular compartment during the earlier secretory stage are being removed from the enamel compartment (Smith et al, 2005). If maturation ameloblasts are actively involved in proton buffering, one would expect that they secrete bicarbonate into the forming enamel.…”

Amelogenins as Potential Buffers during Secretory-stage Amelogenesis

“…However, it was proposed recently that the initial long crystallite ribbons in the secretory stage consist not of crystalline but amorphous calcium phosphates (ACP) and that adsorption of amelogenins followed rather than induced the shapes of the crystals (Simmer et al 2012). A second possible function of amelogenins is that they act as a buffer to neutralize the protons that are generated during crystal formation (Smith, 1998;Smith et al 2005). In this respect, amelogenins contain 14 histidine residues, which bind protons, such that a single unprotonated amelogenin molecule can bind 11 to 15 protons per molecule in vitro (Ryu et al 1998).…”

“…This would influence the dynamics of crystal growth. It might also evoke a response of the ameloblasts to normalize pH by the secretion of bicarbonates, which normally occurs at the maturation stage (Smith et al 2005;Lacruz et al 2013). To further acidify the forming enamel, we exposed AmelX -/-mice to fluoride to induce the formation of hypermineralized lines, releasing more protons (Lyaruu et al 1989;Lyaruu et al 2014).…”