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Ch. Hellmich, F.-J Ulm:
"Hydroxyapatite is Uniformly Concentrated in the Extracollagenous Ultrastructure of Mineralized Tissues";
in: "Proceedings of the Fifth International Symposium on Computer Methods in Biomechanics and Biomedical Engineering", J. Middleton, N.G. Shrive, M.L. Jones (Hrg.); University of Wales College of Medicine, 2001.

At the ultrastructural observation scale of mineralized tissues (l=5-10 micrometer), transmission electron micrographs (TEM) reveal that hydroxyapatite (HA) is situated both interfibrillarly and extrafibrillarly. 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, whether intrafibrillar or extrafibrillar, of mineralized tissue. 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 matter, which takes effect in a liquid environment, regulates the chemical boundary conditions for mineralization of hydroxyapatite in the extra-collagenous space.

Keywords: mineralized tissues, hypdroxyapatite, collagen, extracollagenous space, uniform mineral concentration, non-collagenous proteins