Bone tissue incorporates in vitro gallium with a local structure similar to gallium-doped brushite

Abstract

During mineral growth in rat bone-marrow stromal cell cultures, gallium follows calcium pathways. The dominant phase of the cell culture mineral constitutes the poorly crystalline hydroxyapatite (HAP). This model system mimics bone mineralization in vivo. The structural characterization of the Ga environment was performed by X-ray absorption spectroscopy at the Ga K-edge. These data were compared with Ga-doped synthetic compounds (poorly crystalline hydroxyapatite, amorphous calcium phosphate and brushite) and with strontium-treated bone tissue, obtained from the same culture model. It was found that Sr²⁺ substitutes for Ca²⁺ in the HAP crystal lattice. In contrast, the replacement by Ga³⁺ yielded a much more disordered local environment of the probe atom in all investigated cell culture samples. The coordination of Ga ions in the cell culture minerals was similar to that of Ga³⁺, substituted for Ca²⁺, in the Ga-doped synthetic brushite (Ga-DCPD). The Ga atoms in the Ga-DCPD were coordinated by four oxygen atoms (1.90 Å) of the four phosphate groups and two oxygen atoms at 2.02 Å. Interestingly, the local environment of Ga in the cell culture minerals was not dependent on the onset of Ga treatment, the Ga concentration in the medium or the age of the mineral. Thus, it was concluded that Ga ions were incorporated into the precursor phase to the HAP mineral. Substitution for Ca²⁺ with Ga³⁺ distorted locally this brushite-like environment, which prevented the transformation of the initially deposited phase into the poorly crystalline HAP.