Atomic-scale compositional mapping reveals Mg-rich amorphous calcium phosphate in human dental enamel

Abstract

Human dental enamel, the hardest tissue in the body, plays a vital role in protecting teeth from wear as a result of daily grinding and chewing as well as from chemical attack. It is well established that the mechanical strength and fatigue resistance of dental enamel are derived from its hierarchical structure, which consists of periodically arranged bundles of hydroxyapatite (HAP) nanowires. However, we do not yet have a full understanding of the in vivo HAP crystallization process that leads to this structure. Mg(2+) ions, which are present in many biological systems, regulate HAP crystallization by stabilizing its precursor, amorphous calcium phosphate (ACP), but their atomic-scale distribution within HAP is unknown. We use atom probe tomography to provide the first direct observations of an intergranular Mg-rich ACP phase between the HAP nanowires in mature human dental enamel. We also observe Mg-rich elongated precipitates and pockets of organic material among the HAP nanowires. These observations support the postclassical theory of amelogenesis (that is, enamel formation) and suggest that decay occurs via dissolution of the intergranular phase. This information is also useful for the development of more accurate models to describe the mechanical behavior of teeth.

Keywords: Human dental enamel; Mg-rich amorphous phase; atom probe tomography; tooth decay.

Fig. 1. Human dental enamel from millimeter to nanometer.

(A) Reflected optical bright-field image of the mature human tooth used in this study. Enamel is the outer layer of this cross section. (B) Transmitted optical dark-field image of human tooth enamel showing the junction between dentin and enamel as well as the inner and outer enamel. (C) Transmitted optical dark-field image showing outer enamel rods, each composed of thousands of HAP nanowires surrounded by a less dense interprismatic layer. (D) Higher-magnification TEM bright-field image of aligned HAP nanowires viewed edge on.

Fig. 2. APT reconstructed volumes of human dental enamel HAP nanowires showing intergranular Mg-rich ACP.

(A to C) Mg atoms (A), Na atom distribution (B), and 0.7 atomic % (at %) Mg isosurface (C) revealing the Mg-rich ACP between HAP nanowires. (D) Cross-sectional view of 0.7 at % Mg isosurface that highlights the ribbon-like shape of the HAP nanowires. (E) Proximity histogram (proxigram) from Mg-rich ACP based on 0.7 at % Mg isosurface [based on the sum of interfaces shown in (D)].

Fig. 3. APT 3D reconstructed volumes of human dental enamel HAP nanowires containing a Mg-rich precipitate and organic matter.

(A) ²⁴Mg²⁺ (overlapping with ¹²C⁺) atom distribution. (B) Na isosurface (0.5 at %) showing Mg-ACP intergranular phase. (C) Isosurface ¹²C²⁺ (0.18 at %) (precipitate A), 1 at % ²⁴Mg²⁺ isosurface (precipitate B), and 0.7 at % ²⁴Mg²⁺ (Mg-ACP intergranular phase). (D) Proxigrams of precipitate A, precipitate B, and Mg-ACP (based on the sum of ACP interfaces).

Fig. 4. TEM image of human dental enamel HAP nanowires.

TEM central bright-field image. Arrows show an intergranular layer (light contrast) between HAP nanowires and a triple point (circled) thought to be amorphous. The electron diffraction pattern is indexed as [2-1-13].

Fig. 5. APT 3D reconstructed volumes from sample 2 with isosurfaces from the main species that arise from organic regions, carbonates, or Mg-rich phases.

(A) Isosurfaces of ¹²C²⁺, ¹⁴N²⁺, ⁴²CNO⁺, and H (decomposed) highlighting the organic matter. (B) Isosurfaces of ⁴⁴CO₂⁺ highlighting the carbonates. (C) Isosurfaces of ²⁵Mg²⁺ and ²⁶Mg²⁺ highlighting the Mg-rich precipitate and ACP. (D) Isosurfaces of ²⁸CO⁺ and ²⁹COH⁺ highlighting both organic matter and carbonates.