HIGH-RESOLUTION ELECTRON-MICROSCOPY - STRUCTURE AND GROWTH MECHANISMSOF HUMAN DENTIN CRYSTALS

Biological crystal formation was postulated to begin by a nucleation p rocess. Such processes have been demonstrated for human amelogenesis a nd bone mineralization. The aim of this study was to confirm if such m echanisms occur during dentin crystal formation. The structure of huma n fetal dentin crystals and the earliest stages of mineral growth were followed by High Resolution Electron Microscopy (HREM) associated wit h digitalized image analysis. Micrographs of the mineralization front were first digitalized, and selected areas were transformed in the rec iprocal space by Fast Fourier Transform. The resulting diffractograms were compared with computer-simulated diffractograms and used to deter mine the orientation of crystals. Dentin crystals, found close to the mineralization front, show a structure closely related to that of hydr oxyapatite (HA), as determined by comparison of HREM images with simul ated images. These crystals present numerous structural defects such a s dislocations and grain boundaries. These defects appear to be presen t in dentin crystals at an early stage of growth. We have also observe d nanometer-sized particles in mineralization areas. Calculated diffra ctograms of these areas show significant similarities with HA diffract ion patterns, and in one case, their structure could be correlated to HA structure through an image simulation process. These nanometer-size d particles could be related to the nucleation process, and their grow th, orientation, and formation appear to be mediated by extracellular matrix components.