Chemical gradients in human enamel crystallites.

Abstract

Dental enamel is a principal component of teeth.[1] It has evolved to bear large masticatory forces, resist mechanical fatigue, and withstand wear over decades of use.[2] Functional impairment or loss, as a consequence of developmental defects or tooth decay (caries), has a dramatic impact on health and quality of life, with significant costs to society.[3] While the last decade has seen great progress in our understanding of enamel formation (amelogenesis) and the functional properties of mature enamel, attempts to repair enamel lesions or synthesize enamel in vitro have had limited success.[4–6] This is partly due to the highly hierarchical structure of enamel and the additional complexities arising from chemical gradients that we are only beginning to understand.[7–9] Herein, we show, using atomic-scale quantitative imaging and correlative spectroscopies, that the nanoscale crystallites of hydroxylapatite (OHAp; Ca5(PO4)3(OH)) that are the fundamental building blocks of enamel are comprised of two nanometric layers enriched in magnesium flanking a core rich in sodium, fluoride, and carbonate ions; this sandwich core is surrounded by a shell with lower concentration of substitutional defects. A mechanical model based on DFT calculations and X-ray diffraction data predicts that significant residual stresses arise as a consequence of the chemical gradients, in agreement with preferential dissolution of the crystallite core. Further, stresses may impact the mechanical resilience of enamel. Finally, the two additional layers of hierarchy suggest a new model for biological control over crystal growth during amelogenesis and have implications for the preservation of biomarkers during tooth development.