Biological hydroxyapatite crystals are either small, as in bone and dentin, or large as in enamel. Enamel crystallites are unique since each is initiated and grows in length, thickness and width until the entire layer of enamel is secreted. In maturation, these extremely long crystallites grow only in thickness and width. Crystal growth in vitro follows physico-chemical principles, but lacks biological intelligence; in vivo this intelligence is contributed by protein templates. The location of the organic template in enamel is congruent with the crystallite, constituting the crystal ghost. Since crystals cannot accommodate proteins, the explanation is logically inconsistent. In sections, enamel crystallites, appear hexagonal, and this is interpreted as their cross-sectional shape. Since this hexagonal image also contains the crystal ghost, the notion that hexagons do not represent true cross-sections was explored with models of crystallite shape. Hexagonal rods were compared to rectangular or rhombohedral rods. Whereas segments of hexagonal rods in the section should project as octagons at the electron microscope imaging plane, and octagonal profiles are never found, rectangular or rhombohedral rod segments project as hexagons. Assuming the organic template covers the crystal exterior, the projected rhombohedral segment, appearing hexagonal, would seem to contain the protein, hence explaining the apparent presence of the crystal ghost.
"External Shape of Enamel Crystals,"
Scanning Microscopy: Vol. 1
, Article 39.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss4/39