Mineralization and structure of enamel (a study using scanning electron microscopy and X-ray micro-analysis of the lower incisor of the albino rat).

In order to know the morphostructural changes that occurs in the enamel during the mineralization and to study the rod tridimensional orientation, the author has examined the literature concerning these subjects comparing it to the results of his researches realized by S.E.M. and Rx-microprobe-analysis. It has been demonstrated that three phases can be distinguished during the mineralization: "fibrillation phase", "rod density phase", "interrod density phase". The fibrillation phase begins in the middle of the ameloblastic secretion phase, ends 8 mm from the cervical ansa and is characterized by the unmasking of the matrix subunits and chemically by the disappearance of the sulphur. The rod density phase occurs in the area included between 8 and 14 mm from the cervical ansa, that is in the ameloblastic modulation phase, and is characterized by the aggregation of the intraprismatic subunits and by the calcium increase. The interrod density phase that occurs during the ameloblastic pigmentation phase, is characterized by the aggregation of the interprismatic subunits and by the presence of the iron in the most superficial enamel layers. The rod orientation in the space has been studied during the second phase of the mineralization suice the intraprismatic subunits join while the interprismatic ones remain in the fibrillation phase; besides the author has compared the results of these studies to the morphology of the external opening of the honeycomb pits. In such a way it has been demonstrated that the rod originates from the dentine-enamel junction forming a 60 degree angle that opens in the cuspidalmesial or cuspidal-lateral direction. In the outer enamel layer the rod loses its mesial or lateral inclination and bends further towards the cusp delimiting a 30 degree angle; finally it bends outward and as a sharpend that is ortogonal to the enamel external surface. It is eventually discussed how this model of rod reconstruction is fit to convert the tangential forces that develop during the mastication into compression forces.