The ultrastructure of the bone-hydroxyapatite interface in vitro

Rat bone marrow cells were cultured on plasma-sprayed hydroxyapatite (HA). The cells formed a mineralized extracellular matrix (ECM) that exhibited several characteristics of bone tissue. The interface between this mineralized ECM and the HA was studied at the ultrastructural level with scanning and transmission electron microscopy and x-ray microanalysis. Initially, the deposition of a globular, afibrillar matrix was observed on HA. This was followed by the integration of collagen fibers in this matrix and their subsequent mineralization. At the bone-HA interface two distinctly different interfacial structures were observed. An electron-dense layer with a thickness of 20-60 nm was regularly present, which contained both organic and inorganic material and was rich in glycosaminoglycans. The interfaces differed however, in the presence or absence of an amorphous zone which was free of collagen fibers and had an average thickness of 0.7-0.8 microns. It was frequently seen interposed between the electron-dense layer and the hydroxyapatite. Similar interfacial structures have also been described in the in vivo environment, where they were referred to as lamina limitans-like or cement linelike. From the results of this study, it can be concluded that the described in vitro system is a suitable model to study bone-biomaterial interactions.