Organization of crystals in enamel.

It has been variously suggested that the organic matrix associated with the mineral phase of enamel is present as either calcified fibrils, central dark lines, peripheral sheaths around hexagonal crystals, or organic ghosts apparently contained within crystal profiles. The most consistent findings confirm the crystal ghost conception. Grid decalcification of nearly mature sectioned enamel and staining revealed hollow, noncrystalline structures whose external measurements were statistically identical to those of the dissolved crystallites, but with internal measurements too small to accommodate the crystallites. To explain these apparent ghosts in view of the incompatibility of ghosts with crystal structure, it has been proposed that the crystallites are not hexagonal in cross-sections and the hexagonal appearance is due to projections of parallelepiped-shaped crystallite segments with cut surfaces that are rhomboidal in shape. Material on the surface of such profiles would project as if it were contained within the profile. Hexagonal forms could not be demonstrated in isolated crystallites examined by transmission electron microscopy, high-resolution scanning electron microscopy, and replicas made of the isolated crystallite preparations examined by transmission electron microscopy. Existing evidence does not rule out the possibility that the noncrystalline profiles represent stain drawn into the holes left by the dissolved crystallites as a result of high capillarity forces.