On the Formation and Functions of High and Very High Magnesium Calcites in the Continuously Growing Teeth of the Echinoderm Lytechinus variegatus: Development of Crystallinity and Protein Involvement

Sea urchin teeth grow continuously and develop a complex mineralized structure consisting of spatially separate but crystallographically aligned first stage calcitic elements of high Mg content (5–15 mol% mineral). These become cemented together by epitaxially oriented second stage very high Mg calcite (30–40 mol% mineral). In the tooth plumula, ingressing preodontoblasts create layered cellular syncytia. Mineral deposits develop within membrane-bound compartments between cellular syncytial layers. We seek to understand how this complex tooth architecture is developed, how individual crystalline calcitic elements become crystallographically aligned, and how their Mg composition is regulated. Synchrotron microbeam X-ray scattering was performed on live, freshly dissected teeth. We observed that the initial diffracting crystals lie within independent syncytial spaces in the plumula. These diffraction patterns match those of mature tooth calcite. Thus, the spatially separate crystallites grow with the same crystallographic orientation seen in the mature tooth. Mineral-related proteins from regions with differing Mg contents were isolated, sequenced, and characterized. A tooth cDNA library was constructed, and selected matrix-related proteins were cloned. Antibodies were prepared and used for immunolocaliztion. Matrix-related proteins are acidic, phosphorylated, and associated with the syncytial membranes. Time-of-flight secondary ion mass spectroscopy of various crystal elements shows unique amino acid, Mg, and Ca ion distributions. High and very high Mg calcites differ in Asp content. Matrix-related proteins are phosphorylated. Very high Mg calcite is associated with Asp-rich protein, and it is restricted to the second stage mineral. Thus, the composition at each part of the tooth is related to architecture and function.