Summary
Previous study of the velocity of sound in hard tissues led to the hypothesis that collagen is stiffened because the intermolecular links between collagen molecules are embedded in mineral by the mineralizing process.
Recently published available high-resolution electron microscope studies of biological apatitic crystals frequently show periodic lattice images of the crystal basal plane. Some of these images from bone and dentin display a sequence of etch pits strung linearly along the midline indicating a chain of screw dislocations. These are interpreted to be the loci of the embedded connecting links required by the collagen stiffening hypothesis. The particular crystallites seen in the micrographs probably are from the interfibril region. Most important, the periodic lattice images show the axes of the links must be parallel to the c-axis of the crystallite. The location of the crystallites and their orientation in situ indicate that the c-axis of the crystallites must be perpendicular to the collagen axis.
X-ray diffraction studies available in the literature for a long time indicate the c-axis of HAP crystallites are parallel to the collagen molecular axes. A model of the crystallite distribution is presented which reconciles the two types of crystallite orientation.
It is assumed there is only one physical mineralization process for all apatitic crystallites. Additional data from several sources, particularly the distribution of mineral between intra- and interfibril spaces deduced by Katz and Li, are used to develop a first-order model of mineral in bone. Hole filling crystallites between ends of colinear collagen molecules have their c-axes parallel to the molecular axis. The other crystallites must have their c-axes perpendicular to the collagen molecular axis.
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Lees, S. A model for the distribution of HAP crystallites in bone—an hypothesis. Calcif Tissue Int 27, 53–56 (1979). https://doi.org/10.1007/BF02441161
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DOI: https://doi.org/10.1007/BF02441161