Lattice fringe continuity in the absence of crystal continuity in enamel

Since high-resolution transmission electron microscopy (HRTEM) provides information on a nearly atomic level, the confidence level with this method is very high. Thus, when lattice fringe continuity is found between two enamel crystals in proximity, such continuity has been taken as evidence of crystal fusion (Daculsi and Kerebel, 1977). Similarly, selected-area dark-field (SADF) electron microscopic imaging has been used to study the axial and spatial orientation of crystals. These studies have shown that there is apparent continuity between enamel and dentin crystals (Arsenault and Robinson, 1989). This observation supported the hypothesis that enamel crystallites are initiated by crystallites in dentin. We have used both HRTEM and SADF methods to identify instances of spatial relationship between crystallites in sections of rat incisor enamel and shark enameloid. In each instance of apparent continuity, goniometric tilting was used to examine the continuous interface. All instances where two crystallites seemed to come into contact, and where HRTEM imaging showed the lattice fringes to be directly continuous, were separated into individual crystallites when the specimen was tilted a few degrees. Thus, adjacent crystallites can show lattice fringe continuity in the absence of real crystallite contact. When instances of overlapping crystallites were examined by SADF imaging, the overlapping crystallites gave a single bright image. Goniometric tilting revealed separate crystallites. Thus, neither lattice fringe continuity nor image continuity under SADF can be used as evidence of crystal continuity unless goniometric rotation and tilting are applied when spatial relationships are suspected.