Ch. Hellmich, F.-J Ulm:
"Average Hydroxyapatite Concentration is Uniform in the Extracollagenous Ultrastructure of Mineralized Tissue";
Biomechanics and Modeling in Mechanobiology,
At the ultrastructural observation scale of mineralized tissues (l=1-10 micrometer), transmission electron micrographs (TEM) reveal that hydroxyapatite (HA) is situated both within the fibrils and extrafibrillarly. By means of mathematical modeling, this paper shows that the actual distribution of HA (the majority of the mineral lies outside the fibrils) is due to a uniform mineral concentration in the non-collagenous space of mineralized tissues, whether inside or outside the fibrils. Two independent sets of experimental observations covering a large range of tissue densities establish the relevance of our proposition: (i) mass density measurements and diffraction spacing measurements, re-analyzed through a dimensionally consistent packing model; (ii) optical density measurements of TEMs. This uniform HA-concentration in the extra-collagenous space of the ultrastructure suggests that precipitation of hydroxyapatite is not affected by the space in which it occurs, i.e. by the presence of collagen. Rather the non-collagenous organic matter, which takes effect in a liquid environment, regulates the chemical boundary conditions for mineralization of hydroxyapatite in the extra-collagenous space. The probable existence of a uniform extracollagenous HA concentration has far-reaching consequences for the mechanical behavior of mineralized tissues.
Keywords: mineralized tissues, hypdroxyapatite, collagen, extracollagenous space, uniform mineral concentration, non-collagenous proteins
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