TEM studies of calcium phosphates for the understanding of biomineralization

Examinations on calcium phosphate (Ca-P) formation and bone minerals have been the focus of research for several decades because the examination results of the Ca-P layer formed on biomaterials in physiological environment and the structure of bone minerals could provide valuable insights into the understanding of bioactivity and bone formation. The objectives of this study include: 1) Comparative study of Ca-P formation on bioceramics; 2) In situ transmission electron microscopy (TEM) examinations of octacalcium phosphate (OCP) to hydroxyapatite (HA) transformation; 3) Ultrastructure study of HA precipitation on bioceramics in vivo; 4) Study of crystal structures in bone minerals. Formation of Ca-Ps on various bioceramic surfaces in simulated body fluid (SBF) and in rabbit muscle sites was investigated. The examined bioceramics included sintered bioglass® (BG), A-W glass-ceramics (A-W), HA, α-tricalcium phosphate (α-TCP) and β-tricalcium phosphate (β-TCP). The ability of inducing Ca-P formation was compared among the bioceramics. The Ca-P crystal structures were identified by selected area electron diffraction (SAED) in TEM. The examination results showed that the ability of inducing Ca-P formation in SBF was similar among bioceramics, but considerably varied among bioceramics in vivo. Sintered β-TCP exhibited poor ability of inducing Ca-P formation in vitro and in vivo. OCP formed on the surfaces of BG, A-W, HA and α-TCP in vitro and in vivo. Apatite formation in physiological environment could not be confirmed as a common feature of bioceramics. The transformation of OCP to HA was induced by thermal effect of electron beam irradiation in TEM. The transformation products and crystal structure changes were examined via bright field images, SAED, high-resolution TEM (HRTEM), fast Fourier transform of HRTEM images and image simulation. Chemical analyses and TEM examinations suggested a solid-state transformation mechanism occurring in the OCP crystals. Image simulations and crystal structure analyses indicated the crystallographic orientations of OCP (010) // HA (01̅0) and OCP (001) // HA (001̅) , which differed from the previously proposed orientations of HA growth on an OCP layer. Porous α-TCP ceramics were implanted in dog muscle sites to induce in vivo Ca-P formation. Scanning electron microscopy (SEM) examinations indicated that the Ca-P precipitates have flattened-hexagonal rod shape. Electron diffraction in TEM revealed that the rod-like Ca-Ps have HA structure with its longitudinal direction parallel to the c-axis. Chemical analysis revealed the variations of calcium and phosphate contents in the HA rods. The chemical variations were ascribed to the embedding of a small amount of OCP crystals in the rods, as indicated by HRTEM examinations. This implied that the in vivo HA was formed via an OCP precursor phase. Based on the HRTEM examinations, the orientation relations of OCP/HA in the precipitated rods were determined to be consistent with that of a solid-state OCP/HA transformation observed in synthetic samples. Mineral crystals were extracted from human and rat bones using 10% neutral ethylenediamine tetraacetic acid (EDTA) solutions. TEM examinations showed that the dominance of bone minerals was plate-like and a few were needle-like. The length of most plate-like minerals ranged from 50 to 150 nm but could be up to 200 nm. Chemical analysis revealed that the bone minerals contained mainly calcium and phosphorus. To the author’s knowledge, OCP structure was for the first time, identified in a number of plate-like bone minerals by SAED and HRTEM. HA structure was also determined from thick plate-like and needle-like minerals. The needle-like minerals exhibited a dark contrast line in proper under-focus conditions similar to that observed in tooth enamel crystals.