In summary, both the organic matrix component and the mineral phase play an
active role in organizing the final microstructure (Checa and Rodriguez-Navarro
2005).
Although the matrix proteins from nacre have the potential to regulate calcium
carbonate deposition and crystallization, even to determine the crystal polymorph
specificity, this point of detail requires further clarification. How matrix proteins
recognize and interact with calcium carbonate will be crucial for illustrating the
molecular mechanism. Control of crystal growth seems to depend on the control of
matrix protein secretion or activation processes in the mantle cells (Jolly et al.
2004); however, there is very little research on this. How the genes encoding matrix
proteins are regulated to express in an appropriate time and space during the
development of nacreous layers is still far from understood. A better understanding
of the secretory mechanism that results in the progressive formation of the true
nacreous layer may contribute to improving the quality of culture pearls.
12.6 Conclusion
Nacre, consisting of calcium carbonate aragonite crystals and matrix, is produced
by biomineralization, having a specific structure and property. Although the organic
matrix (including chitin and matrix proteins) constitutes less than 5% of the shell
weight, it plays a crucial role in nacre biomineralization. Beside participating in
construction of the organic framework, matrix proteins not only govern the nucleation and growth of aragonitic crystals in nacre, but also control their crystallization
morphology in an appropriate microenvironment. The polymorph specificity of
calcium carbonate is presumably linked to the amino acid sequence, the conformation of specific protein(s) in the mollusk shell, and the microenvironment in which
crystal nucleation and growth takes place. Moreover, expression level of genes
encoding shell matrix proteins in the pearl sac, especially the genes encoding shell
prismatic matrix proteins, is related to the quality of culture pearls.
There are two principal types of nacre structure, lamellar and columnar. The
former exists in the shell of bivalves, whereas the latter is present in that of
gastropods. The different nacre microstructures arise from their different formation
mechanisms. Formation of nacre in bivalve shell starts by constructing the organic
sheets, consisting of beta-chitin, as a template for nucleation. The orientation of
crystal growth is a result of competitive selection between adjacent crystals and
control of the organic matrix. Gastropod nacre crystal growth may be compatible
with the spiral growth model and consequently successive nacreous lamellae are
formed through a mineral bridge. Not only the organic matrix, but also the calcium
carbonate mineral phase play important roles in the process of nacre biomineralization. The more detailed molecular mechanisms involved need to be clarified in
future.
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
347
active role in organizing the final microstructure (Checa and Rodriguez-Navarro
2005).
Although the matrix proteins from nacre have the potential to regulate calcium
carbonate deposition and crystallization, even to determine the crystal polymorph
specificity, this point of detail requires further clarification. How matrix proteins
recognize and interact with calcium carbonate will be crucial for illustrating the
molecular mechanism. Control of crystal growth seems to depend on the control of
matrix protein secretion or activation processes in the mantle cells (Jolly et al.
2004); however, there is very little research on this. How the genes encoding matrix
proteins are regulated to express in an appropriate time and space during the
development of nacreous layers is still far from understood. A better understanding
of the secretory mechanism that results in the progressive formation of the true
nacreous layer may contribute to improving the quality of culture pearls.
12.6 Conclusion
Nacre, consisting of calcium carbonate aragonite crystals and matrix, is produced
by biomineralization, having a specific structure and property. Although the organic
matrix (including chitin and matrix proteins) constitutes less than 5% of the shell
weight, it plays a crucial role in nacre biomineralization. Beside participating in
construction of the organic framework, matrix proteins not only govern the nucleation and growth of aragonitic crystals in nacre, but also control their crystallization
morphology in an appropriate microenvironment. The polymorph specificity of
calcium carbonate is presumably linked to the amino acid sequence, the conformation of specific protein(s) in the mollusk shell, and the microenvironment in which
crystal nucleation and growth takes place. Moreover, expression level of genes
encoding shell matrix proteins in the pearl sac, especially the genes encoding shell
prismatic matrix proteins, is related to the quality of culture pearls.
There are two principal types of nacre structure, lamellar and columnar. The
former exists in the shell of bivalves, whereas the latter is present in that of
gastropods. The different nacre microstructures arise from their different formation
mechanisms. Formation of nacre in bivalve shell starts by constructing the organic
sheets, consisting of beta-chitin, as a template for nucleation. The orientation of
crystal growth is a result of competitive selection between adjacent crystals and
control of the organic matrix. Gastropod nacre crystal growth may be compatible
with the spiral growth model and consequently successive nacreous lamellae are
formed through a mineral bridge. Not only the organic matrix, but also the calcium
carbonate mineral phase play important roles in the process of nacre biomineralization. The more detailed molecular mechanisms involved need to be clarified in
future.
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
347