In biologically controlled mineralization, cellular activities direct all the stages
for the precipitation of the mineral: from the determination of the initial deposition
site, to nucleation, growth, and formation of the final crystalline morphology.
Resulting biominerals acquire reproducible, species-specific, genetically determined structures and properties. Controlled biomineralization requires an isolated
environment which serves as the mineralization site. This environment can be
extracellular, intercellular or intracellular, with respect to the cells which control
the process. In general, biologically induced mineralization is found in bacteria and
lichens, whereas biologically controlled mineralization is found in foraminifera,
cephalopod statoliths, mollusks shells, bryozoan exoskeletons, scleractinian corals,
echinoderms, human bones, and teeth.
In nature, almost 50% of biominerals are calcium-bearing minerals (Lowenstam
and Weiner 1989). Calcium content in living organisms is highly regulated, as it has
key roles in metabolic processes at concentrations varying from 0.01 mM to 10 mM.
The most abundant calcium-bearing biominerals precipitate acquiring one of the
eight known calcium carbonate polymorph forms. These include seven crystalline
forms: calcite, Mg-calcite, aragonite, vaterite, monohydrocalcite, protodolomite,
hydrocerussite, and one amorphous calcium carbonate (ACC) form (Addadi et al.
2003). Most models of biomineralization invoke the involvement of membrane ion
transporters (channels and pumps) in the delivery of Ca
2+ and other ions to the
calcification site (Simkiss and Wilbur 1989).
An important parameter for the formation of a biomineral is the regulation of the
“isotopic composition” (Weber and Raup 1966) which establishes a physicochemical equilibrium with the microenvironment. The medium from which the mineral
forms is a saturated solution, which is required at the site of mineralization only.
Supersaturation can be achieved by the presence of additives which prevent the
deposition of crystalline phases, such as magnesium in concentrations similar to
those of the seawater (Raz et al. 2000). Another inhibitory factor of crystallization
is the presence of proteins which, serving as substrates, influence the solubility of
the mineral phase and stabilize different polymorphs, either ACC (Aizenberg et al.
1996) or crystalline calcium carbonate forms. In conclusion, crystal shape depends
on inorganic and organic factors: pI, temperature, microenvironment growth, solubility of the mineral phase, concentration of occluded macromolecules and ions. As
a result, calcium carbonate growth undergoes a series of phases and morphologies
to form the final calcite structure. On the contrary, the effect of whole extracts of
proteinaceous matrixes on the in vitro precipitation of calcium carbonate was
shown to selectively induce the precipitation of particular polymorphs. Some
authors have pointed out that the soluble or insoluble matrix, extracted from
nacre, controls calcium carbonate crystal polymorphs toward aragonite or calcite
formation (Falini et al. 1996). Others used chitin as a substrate and Mg
2+ as an
additive to induce aragonite double-layered composite film formation (Kato 2000).
Studies such as those mentioned above indicate that template molecules can act as
nucleators for the precipitation of the inorganic material. The surface chemistry of
each template molecule and the arrangement of the amino acid groups guide the
oriented nucleation of the cognate crystal face.
8 Echinoderms as Blueprints for Biocalcification
227
for the precipitation of the mineral: from the determination of the initial deposition
site, to nucleation, growth, and formation of the final crystalline morphology.
Resulting biominerals acquire reproducible, species-specific, genetically determined structures and properties. Controlled biomineralization requires an isolated
environment which serves as the mineralization site. This environment can be
extracellular, intercellular or intracellular, with respect to the cells which control
the process. In general, biologically induced mineralization is found in bacteria and
lichens, whereas biologically controlled mineralization is found in foraminifera,
cephalopod statoliths, mollusks shells, bryozoan exoskeletons, scleractinian corals,
echinoderms, human bones, and teeth.
In nature, almost 50% of biominerals are calcium-bearing minerals (Lowenstam
and Weiner 1989). Calcium content in living organisms is highly regulated, as it has
key roles in metabolic processes at concentrations varying from 0.01 mM to 10 mM.
The most abundant calcium-bearing biominerals precipitate acquiring one of the
eight known calcium carbonate polymorph forms. These include seven crystalline
forms: calcite, Mg-calcite, aragonite, vaterite, monohydrocalcite, protodolomite,
hydrocerussite, and one amorphous calcium carbonate (ACC) form (Addadi et al.
2003). Most models of biomineralization invoke the involvement of membrane ion
transporters (channels and pumps) in the delivery of Ca
2+ and other ions to the
calcification site (Simkiss and Wilbur 1989).
An important parameter for the formation of a biomineral is the regulation of the
“isotopic composition” (Weber and Raup 1966) which establishes a physicochemical equilibrium with the microenvironment. The medium from which the mineral
forms is a saturated solution, which is required at the site of mineralization only.
Supersaturation can be achieved by the presence of additives which prevent the
deposition of crystalline phases, such as magnesium in concentrations similar to
those of the seawater (Raz et al. 2000). Another inhibitory factor of crystallization
is the presence of proteins which, serving as substrates, influence the solubility of
the mineral phase and stabilize different polymorphs, either ACC (Aizenberg et al.
1996) or crystalline calcium carbonate forms. In conclusion, crystal shape depends
on inorganic and organic factors: pI, temperature, microenvironment growth, solubility of the mineral phase, concentration of occluded macromolecules and ions. As
a result, calcium carbonate growth undergoes a series of phases and morphologies
to form the final calcite structure. On the contrary, the effect of whole extracts of
proteinaceous matrixes on the in vitro precipitation of calcium carbonate was
shown to selectively induce the precipitation of particular polymorphs. Some
authors have pointed out that the soluble or insoluble matrix, extracted from
nacre, controls calcium carbonate crystal polymorphs toward aragonite or calcite
formation (Falini et al. 1996). Others used chitin as a substrate and Mg
2+ as an
additive to induce aragonite double-layered composite film formation (Kato 2000).
Studies such as those mentioned above indicate that template molecules can act as
nucleators for the precipitation of the inorganic material. The surface chemistry of
each template molecule and the arrangement of the amino acid groups guide the
oriented nucleation of the cognate crystal face.
8 Echinoderms as Blueprints for Biocalcification
227