High-resolution electron microscope and computed images of human tooth enamel crystals

The structure of human enamel crystallites has been studied at a near atomic level by high-resolution electron microscopy. Electron micrographs have been obtained from crystallites present in human enamel with a structure resolution of 0.2 nm in the [0001], [1̄21̄0], [12̄13], [1̄100] and [45̄10] zone axes directions. In most cases it was possible to match the experimental images with images calculated using the atomic positions of mineral hydroxyapatite. However, in some cases a discrepancy between calculated and experimental image detail was observed in the c direction of the [1̄21̄0] and the [1̄100] images. This shows: (i) a structural heterogeneity of the crystals, and (ii) a loss of hexagonal symmetry of the structure. The resolution required to distinguish individual atomic sites in the different zones has been determined, and this will provide a useful basis for future work. As the determination of the “real structure” of biological crystals is of prime importance for the study of calcification mechanisms (crystal growth), biological properties and destructive phenomena of calcified tissues (i.e., dental caries and bone resorption).