High resolution electron microscopy: structure and growth mechanisms of human dentin crystals

Biological crystal formation was postulated to begin by a nucleation process. Such processes have been demonstrated for human amelogenesis and bone mineralization. The aim of this study was to confirm if such mechanisms occur during dentin crystal formation. The structure of human fetal dentin crystals and the earliest stages of mineral growth were followed by High Resolution Electron Microscopy (HREM) associated with digitalized image analysis. Micrographs of the mineralization front were first digitalized, and selected areas were transformed in the reciprocal space by Fast Fourier Transform. The resulting diffractograms were compared with computer-simulated diffractograms and used to determine the orientation of crystals. Dentin crystals, found close to the mineralization front, show a structure closely related to that of hydroxyapatite (HA), as determined by comparison of HREM images with simulated images. These crystals present numerous structural defects such as dislocations and grain boundaries. These defects appear to be present in dentin crystals at an early stage of growth. We have also observed nanometer-sized particles in mineralization areas. Calculated diffractograms of these areas show significant similarities with HA diffraction patterns, and in one case, their structure could be correlated to HA structure through an image simulation process. These nanometer-sized particles could be related to the nucleation process, and their growth, orientation, and formation appear to be mediated by extracellular matrix components.