Crystallization Pathways in Bone

Many biomineralization processes involve the sequestering of ions by cells and their translocation through the cells to the final deposition site. In many invertebrate crystallization pathways the cells deposit an initial highly disordered mineral phase with intracellular vesicles, and this mineral is subsequently transported into the final deposition site outside the cell. As this initial mineral phase is metastable, it can easily dissolve or crystallize during sample preparation and examination. A cryogenic electron microscopy study of the forming fin bone of a zebra fish strain with continuously growing fins shows that the cells responsible for bone tissue formation do have mineral-bearing intracellular vesicles and that the mineral phase is a highly disordered calcium phosphate. We also show that globules of disordered calcium phosphate are present in the extracellular collageneous matrix and that they are not membrane bound. Close to the mineralization front these globules appear to penetrate into the collagen fibrils where they crystallize to form mature bone. This crystallization pathway is similar to pathways observed in invertebrates, and it differs from the matrix vesicle pathway documented for a variety of vertebrate mineralizing tissues as the extracellular mineral globules are not membrane bound.