Are the molecules ocluded in the matrix and mineral precursors self-organized or is their organization “supervised”? Apparently both, as can be observed in the following from the formation of mineralized structures in coral. See the simplified diagram below representing the different models of biomineralization.
Figure 1.6: Diagram of induced and controlled models.
The “biologically induced” model or the “biologically controlled” model? According to the first model, biomineralization is a subproduct of cellular metabolism. Given the supersaturation of salt water in tissue (in relation to Ca2+ and CO32- ions), combined with the elevated pH from photosynthesis (alkalization) and the necessity to remove calcium ions from the cell, calcification would be induced through normal cellular pathways. The counter argument that even though these activities happen throughout practically all cells in coral, calcification doesn’t happen in all regions of the organism and species specific architecture of the skeleton would be even more surprising (Figure 1.6). These observations suggest that there is some degree of biological control over where calcification occurs.
In tests involving the formation of enamel in rodent teeth it was observed that mice with removed genes did not show failures in mineralization. This could occur via redundant function genes, but it could be that biomineralization is not regulated by individual proteins but by complex multi-proteinaceous structures.
Proteins with a large capacity to capture calcium can be encountered in the endoplasmic reticulum of enamel epithelium; the precipitation of calcium salts could occur, in some cases, within intracellular vacuoles probably combined with matrix proteins, and the mineral precursor formed would be exocytosed at the location where enamel is formed.
Figure 1.7: Diversity in mineralized structures in coral – illustrations of Ernst Haeckel, 1899
Bioinorganic model: coral controls when and where the saturation level is increased, making it possible for nucleation and growth of mineral phases by way of primarily inorganic processes. Removal of inhibitive compounds, ion pumps, facilitated diffusion, liberation of fluid phase reaction accelerating enzymes, use of transport molecules or transport mediated by vesicles for the purpose of altering the ionic composition of the extracellular environment.
Additive mediation model: coral controls other factors besides the saturation state; the liberation of composites that interact in specific ways with crystals (“poisoning” the growth on certain faces, promoting preferential phase nucleation or inhibiting all nucleation). Hopefully an organic additive extracted from the organic matrix in in vitro precipitation experiments responds the same way within the organism. Phospholipids extracted from coral skeletons (from the Scleractinia group) were capable of bonding with calcium, suggesting that they might act as site markers for calcium carbonate (CaCO3) nucleation.
Organic matrix mold controlled model: the organism produces a three dimensional structure that initializes, directs, and limits crystal growth. The three-dimensional arrangement of the different functional groups is critical in the determination of skeletal growth. This model is widely observed in mollusk shells as well as bones.
Figure 1.10: Organic matrix mold controlled model.
Based on discrete biomineralization models, three principal mechanisms are observed, summarized in three case studies. Case studies have shown that not all organisms use the same methods for nucleation. The following figures schematically represent the three common pathways the presumably share some of the same nucleation stages:
Mineralization of extracellular matrices
Figure 1.11 – Nucleation pathway scheme involving the extracellular matrix. Exemplified if the formation of shells in radial foraminiferas and mineral formation of bones, dentin, enamel, mollusk shells or crustacean cuticles.
Mineralization within a large vesicle (syncytium)
Figure 1.12 – Nucleation pathway scheme involving the restricted vesicle space (syncytium). e.g., sea urchin larva spicule
Mineralization within vesicles that are located in intracellular space
Figure 1.13 – Nucleation pathway scheme involving the formation of mineralized elements in a intracellular vesicles, where the final mineralized product can remain on the interior of the cell (e.g., guanine crystals in fish skin) or be transported to the cell surface (e.g., foraminiferas).
