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Enamel is an epithelial secretory product. The greatest part of its water and protein content are lost during a prolonged maturation mineralisation. It hardly exists as matrix without mineral, since the crystals in the surface of the existing layer simply grow into new matrix within minutes: the maturation process consists of increasing the girth of existing crystals and may take a long time (several years in human permanent teeth).
In most enamel, an almost featureless arrangement of crystals parallel to each other and perpendicular to the surface is broken by the pitted morphology of the interface between ‘ameloblasts’ cells and their matrix. The tall columnar cells release their matrix at two sites. Crystals forming in that matrix liberated between the cells are oriented perpendicular to the general plane of that surface - which is the surface of the developing enamel in that place and at that time. Crystals in the phase which fills in pit floors are perpendicular to that surface is and roughly parallel to the prism direction, which is the path the ameloblast pursues as it secretes the tissue. In man, the floors are usually continuous with the walls on one side so that there is a continuous grade of gradual change of crystal orientation. Where floor meets wall, little or no matrix is added in the walls of the pits. Within the tissue, the wall-floor boundary is a discontinuity at which the crystal orientation changes. When the tissue fails (in use, in laboratory sample preparation, via iatrogenic interference), the boundary discontinuities join and create separate prisms.
There are other distinct Patterns of enamel prism in different taxonomic groupings of mammals. Differences between species are important when researchers try to substitute bovine, ovine or equine teeth for studies aimed at obtaining data relevant to man: there could hardly be worse choices than these. Their enamel has longitudinal rows of prisms separated by inter-row sheets of developmentally inter-pit phase (interprismatic) enamel and the difference in angle between the preferred crystal orientations in the two can be close to perpendicular. The teeth of herbivores with this enamel type are subject to severe attrition.
In most enamels, we find zones in which prismatic groups have common orientations contrasting with those in adjacent groups due to movements within the sheet of ameloblasts. In rodent incisor inner enamel single rows of ameloblasts may move against each other in opposite directions to give rise to rectilinearly crossing sheets of prisms. This give rises to a self-sharpening mechanism in which the angle of the cutting edge is controlled by the inclination of the averaged direction of the decussating prism sheets.
Layering also occurs parallel to the sequence of positions of the forming enamel surface over time. Scaling tooth surfaces to remove calcified bacterial dental plaque faults the tissue in these planes. Modern orthodontics uses ‘brackets’ bonded to a tooth surface. Unfortunately, the ‘debonding’ process occurs within enamel, and results in significant loss of natural tooth substance. Enamel was never designed to resist pulling forces.
Key words: Correlative 3D, TEM, SEM, LM Histology, Polarised Light Microscopy
