Subsequently, amorphous calcium carbonate (ACC) appears in regenerating adult spines and has been shown to transform into calcite 4. The first amorphous phase in calcium carbonate systems was detected in the larval spicule of the sea urchin 2, as electrodense granules inside spiculogenic cell vesicles, which are subsequently transported to the mineralization site 3. These are made basically of calcite, aragonite and, very rarely, of vaterite. The production of amorphous minerals by organisms is a long-known fact, but it was not until the pioneering work of Towe and Lowenstam 1 that evidence for the formation of biominerals by both vertebrates and invertebrates through the corresponding amorphous transient phases began to accumulate.Ĭalcium carbonate is, with few exceptions, the most common material used by invertebrates to construct their hard structures (shells, plates, spicules, etc.). We propose that the final nanogranular structure observed is produced during the transformation of ACC into aragonite. According to their outlines, the internal transformation front of the tablets took on a complex digitiform shape, with the individual fingers constituting the crystalline cores of nanogranules. Within single tablets, the crystalline cores were largely co-oriented. The changes in composition from the amorphous to the crystalline phase indicate that there was a higher content of organic molecules within the former phase. Tablets displayed a characteristic nanoglobular structure, with the nanoglobules consisting of an aragonite core surrounded by amorphous calcium carbonate together with organic macromolecules. To gain insight into this process, we have made a high-resolution study (by means of transmission electron microscopy and other associated techniques) of immature tablets of nacre of the gastropod Phorcus turbinatus, where the proportion of amorphous calcium carbonate is high. Nevertheless, little is known as to how the amorphous-to-crystalline transformation takes place.
The globular nanostructure of most biominerals is taken as evidence of this. Currently a basic tenet in biomineralization is that biominerals grow by accretion of amorphous particles, which are later transformed into the corresponding mineral phase.
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