Since many years it is well-known that calcium phosphates are the most important parts of biomineralization around the organic bone parts which are built by collagen triple helices. On the other hand, up to now it is not completely understood how the triclinic octacalcium phosphate (OCP) and hexagonal hydroxy apatite (HAP) cells are arranged on the atomic scale.
We discuss one model of an arrangement which considers the coupling of OCP cells on collagen fibres using bridges of water or O-H-groups, respectively. On the other side of a half or one and a half OCP cells HAP cells are attached in multiple layers.
This model has been verified by transmission electron microscopy including electron diffraction and high resolution imaging as well as analytical methods being a powerful method to analyze just such cell arrangements. Ultrathin slices of bone parts prepared by an ultramicrotom as well as by focused ion beam techniques were used for the investigation. The calcium phosphate arrangement is stable for more than five minutes at irradiation with 300 keV electrons.
The electron diffraction patterns show that HAP is the dominant phase within the biomineralized areas. This can be confirmed by comparison of measured and simulated electron microscopic high resolution images. In this way it is also possible to find HAP (300) planes at the edges of mineralized collagen fibres. In the HRTEM images there are indications that the HAP cells are really coupled by small bridges of OCP. The goal of our further work is to confirm this finding.