Coexistence of Amorphous and Crystalline Calcium Carbonate in Skeletal Tissues

This paper describes a composite skeletal tissue in which amorphous and crystalline calcium carbonate coexist in well-defined domains separated by an organic sheath. Each biogenic mineral phase shows a characteristic Mg content (5.9 and 1.7 mol%, respectively) and concentration of intramineral proteins (0.05 and 0.01 wt%, respectively). Macromolecular extracts from various biogenic amorphous calcium carbonate (ACC) skeletons are typically glycoproteins rich in glutamic acid and hydroxyamino acids. The proteins from the crystalline calcitic phases are aspartate-rich. Macromolecules extracted from stable biogenic ACC inhibited crystallization of calcite and induced the formation of stabilized ACC in vitro. The yield of the synthetic ACC was 15-20%. The presence of Mg facilitated the stabilization of ACC: the protein content in synthetic ACC was 0.12 wt% in the absence of Mg and 0.07 wt% in the presence of Mg (the Mg content in the precipitate was 8.5 mol %). In contrast, the macromolecules extracted from the calcitic layer induced the formation of calcite crystals with modified morphology similar to that expressed by the original biogenic calcite. We suggest that specialized macromolecules and magnesium ions may cooperate in the stabilization of intrinsically unstable amorphous calcium carbonate and int he formation of complex ACC/calcite tissues in vivo.