Average hydroxyapatite concentration is uniform in the extracollagenous ultrastructure of mineralized tissues: evidence at the 1–10-μm scale

Abstract

At the ultrastructural observation scale of fully mineralized tissues (ℓ=1−10 μm), transmission electron micrographs (TEM) reveal that hydroxyapatite (HA) is situated both within the fibrils and extrafibrillarly, and that the majority of HA lies outside the fibrils. The extrafibrillar amount of HA varies from tissue to tissue. By means of mathematical modeling, we here provide strong indications that there exists a physical quantity that is the same inside and outside the fibrils, for all different fully mineralized tissues. This quantity is the average mineral concentration in the non-collagenous space. This space is the sum of the extrafibrillar volume and of the volume of the fibrils that is not occupied by collagen molecules. Two independent sets of experimental observations covering a large range of tissue mass 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. The aforementioned average uniform HA-concentration in the extracollagenous space of the ultrastructure may emphasize the putative role played by a number of non-collagenous organic molecules in providing the chemical boundary conditions for mineralization of HA in the extracollagenous space. The probable existence of an average uniform extracollagenous HA concentration has far-reaching consequences for the mechanical behavior of mineralized tissues.