Role of Organic Matrix Molecules in the Formation of Very High Magnesium Calcite

Biological mineralized tissues are sophisticated organic-inorganic composites that are assembled bottom-up and exhibit hierarchical architecture. Functional roles include mechanical support, feeding, locomotion, defense, and sensing. Highly evolved design enables features such as high bone toughness at low weight, self-sharpening teeth, and continuous adaptive remodeling/self-repair. Despite these highly attractive properties and great progress in bio-inspired material synthesis, many of the hallmarks of biological crystal growth have yet to be reproduced in vitro: polymorph control, shaping of non-faceted, curving and/or branching single crystals, and the creation of compositionally and functionally graded crystals with compositions far from equilibrium. Sea urchins make calcite in their adult test, spines, lantern, stereom, teeth, and embryonic spicules. One of the most interesting aspects of the tooth structure is the development of strengthening inter-plate columns. The arrangement of the cellular components of the tooth allows continuous growth and regeneration, in response to tooth wear and abrasion during feeding. The arrangement of the mineral phase structure is comprised of distinctly different high Magnesium containing calcite crystal elements. In the sea urchin tooth, calcite Ca1-xMgxCO3, the composition of the plates, prisms are x ~ 0.13 and that of the inter-plate columns which are responsible for the hardness of the tooth are x ~ 0.33. The tooth is the only mineralizing element in the sea urchin that makes very high magnesium calcite crystals. Since all biomineralization is matrix mediated an understanding of the nature of the proteins involved is essential in elucidating its mechanism. Previous work in the laboratory led to the hypothesis that the formation of the crystal elements is guided and regulated by the components of the organic sheaths constituted from the cell syncytial membranes and extracellular or surface-associated proteins. The resultin