Ion-thinning techniques for the preparation of transmission electron microscope samples of mature enamel were used to examine ultrastructural details that included crystallite size and orientation in relation to the enamel rod geometry. The three-quarter arcade of the sheath is visible as the perimeter of the keyhole. Diffraction studies indicate a preferred orientation in the c axis of the crystallites in the head and tail of the keyhole and in other directions in the head region.
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