Mapping residual organics and carbonate at grain boundaries and the amorphous interphase in mouse incisor enamel.

Lyle M Gordon,Derk Joester

doi:10.3389/fphys.2015.00057

Abstract

Dental enamel has evolved to resist the most grueling conditions of mechanical stress, fatigue, and wear. Adding insult to injury, it is exposed to the frequently corrosive environment of the oral cavity. While its hierarchical structure is unrivaled in its mechanical resilience, heterogeneity in the distribution of magnesium ions and the presence of Mg-substituted amorphous calcium phosphate (Mg-ACP) as an intergranular phase have recently been shown to increase the susceptibility of mouse enamel to acid attack. Herein we investigate the distribution of two important constituents of enamel, residual organic matter and inorganic carbonate. We find that organics, carbonate, and possibly water show distinct distribution patterns in the mouse enamel crystallites, at simple grain boundaries, and in the amorphous interphase at multiple grain boundaries. This has implications for the resistance to acid corrosion, mechanical properties, and the mechanism by which enamel crystals grow during amelogenesis.

Highlights

Enamel, the hardest tissue in vertebrates, is composed of 98 wt% hydroxylapatite (OHAp) along with 1–2 wt% organic molecules and water (Eastoe, 1960)
We previously found that at multiple boundaries, and probably at simple grain boundaries, an intergranular phase exists that we identified as Mg-substituted amorphous calcium phosphate (Mg-ACP, 0.5–6 wt% Mg2+) (Gordon et al, 2015)
We find that all impurities are elevated in the amorphous intergranular phase, Mg-ACP, that is present at multiple grain boundaries, but that the distribution within OHAp crystallites and at simple grain boundaries is specific to the impurity

Summary

Introduction

The hardest tissue in vertebrates, is composed of 98 wt% hydroxylapatite (OHAp) along with 1–2 wt% organic molecules and water (Eastoe, 1960). Ameloblasts first secrete a soft extracellular organic matrix comprised of water (∼50 wt%), mineral (∼30 wt%), and a number of proteins (∼20 wt%), including the amelogenins, ameloblastins, and enamelins, among others (Robinson et al, 1998). Mature enamel is composed of crystallites of OHAp that are highly elongated parallel to the crystallographic c-axis and have polygonal cross sections with edge lengths of 20–50 nm in the a-b-plane. There are variations of specific aspects of this architecture that depend on the location of the enamel on the crown of a particular tooth, between teeth with different functional morphologies, and between the teeth of different species. The similarities remaining are such that rodent teeth, in particular those of rats and mice, are well-established as model systems for amelogenesis and dental caries of human teeth (Bowen, 2013)

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