Based on these studies, several new models to illustrate the formation mechanism
related to lamellar nacre in bivalves, and columnar nacre in gastropods are
introduced. All these explorations on the mechanism of nacre biomineralization
may greatly influence the strategies not only to improve the quality of culture pearls
and promote their production, but also to design some specific biomaterial mimics.
12.2 The Structure of the Nacreous Layer
The nacreous layer of mollusks, which is composed of 95% calcium carbonate and
less than 5% organic matrix in weight, is the best-known aragonitic structure and is
the usual model for biomineralization. SEM images show that the nacreous layer of
shells is made of polygonal aragonitic tablets (Kobayashi and Samata 2006), which
are interspaced by thin interlamellar organic matrix sheets and finely accumulated
lamellae parallel to the inner shell surface. The thickness of platelets is about
0.25 mm, and organic interlamellar matrix sheets between the tablets are
10~50 nm in thickness (Sarikaya and Aksay 1992), depending on the site of the
shell from which the sample is extracted. Recent investigation by intermittentcontact AFM revealed that each crystal within which an intracrystalline organic
matrix forms the foam-like structure, is composed of coherent flat nanograins
(45 nm mean size) which share the same crystallographic orientation (Rousseau
et al. 2005b). Similar observations were obtained using FESEM and FETEM (Oaki
and Imai 2005). They proposed a three-level hierarchical architecture model
(Fig. 12.1). The nacreous layer (tier 1) is composed of oriented aragonite plates
(tier 2), each of which is an assembly of nanobuilding blocks (tier 3). This model
will facilitate further understanding of the overall architecture in the nacreous layer
from a nanoscopic to a macroscopic scale, and to further direct the generation of
specific mimetic biominerals.
Two types of the nacre structure (Fig. 12.2) are discriminated according to their
different formation methods. The first is called the sheet nacre structure. In most
bivalves, the shell is built of tablets which, during development, form one or a few
layers at a time, usually arranged like a “brick-wall” when seen in vertical cross
section, or a “stair-step” pattern in horizontal view (Fig. 12.2a, b). The second is
called the columnar nacre structure. The crystal tablets of nacre in gastropods are
“stacked up” vertically on each other, and each stack resembles the shape of a
pyramid. However, these pyramidal structures develop only at the surface layers,
and the major portion of the layer is similar to that of bivalves (Fig. 12.2c), giving a
brick-wall appearance (Watabe 1981).
The interlamellar organic matrix, mainly composed of biomacromolecules
between calcium carbonate tablets, has a double function. On the one hand, some
Asp-rich proteins adsorbed to the matrix surface provide nucleation sites for the
next crystal layer. On the other hand, some proteins on the matrix inhibit and
terminate crystal growth to ensure uniform thickness of the nacreous layers (Addadi
and Weiner 1985; Mann 2001). Because the interlamellar organic matrix plays such
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
333
related to lamellar nacre in bivalves, and columnar nacre in gastropods are
introduced. All these explorations on the mechanism of nacre biomineralization
may greatly influence the strategies not only to improve the quality of culture pearls
and promote their production, but also to design some specific biomaterial mimics.
12.2 The Structure of the Nacreous Layer
The nacreous layer of mollusks, which is composed of 95% calcium carbonate and
less than 5% organic matrix in weight, is the best-known aragonitic structure and is
the usual model for biomineralization. SEM images show that the nacreous layer of
shells is made of polygonal aragonitic tablets (Kobayashi and Samata 2006), which
are interspaced by thin interlamellar organic matrix sheets and finely accumulated
lamellae parallel to the inner shell surface. The thickness of platelets is about
0.25 mm, and organic interlamellar matrix sheets between the tablets are
10~50 nm in thickness (Sarikaya and Aksay 1992), depending on the site of the
shell from which the sample is extracted. Recent investigation by intermittentcontact AFM revealed that each crystal within which an intracrystalline organic
matrix forms the foam-like structure, is composed of coherent flat nanograins
(45 nm mean size) which share the same crystallographic orientation (Rousseau
et al. 2005b). Similar observations were obtained using FESEM and FETEM (Oaki
and Imai 2005). They proposed a three-level hierarchical architecture model
(Fig. 12.1). The nacreous layer (tier 1) is composed of oriented aragonite plates
(tier 2), each of which is an assembly of nanobuilding blocks (tier 3). This model
will facilitate further understanding of the overall architecture in the nacreous layer
from a nanoscopic to a macroscopic scale, and to further direct the generation of
specific mimetic biominerals.
Two types of the nacre structure (Fig. 12.2) are discriminated according to their
different formation methods. The first is called the sheet nacre structure. In most
bivalves, the shell is built of tablets which, during development, form one or a few
layers at a time, usually arranged like a “brick-wall” when seen in vertical cross
section, or a “stair-step” pattern in horizontal view (Fig. 12.2a, b). The second is
called the columnar nacre structure. The crystal tablets of nacre in gastropods are
“stacked up” vertically on each other, and each stack resembles the shape of a
pyramid. However, these pyramidal structures develop only at the surface layers,
and the major portion of the layer is similar to that of bivalves (Fig. 12.2c), giving a
brick-wall appearance (Watabe 1981).
The interlamellar organic matrix, mainly composed of biomacromolecules
between calcium carbonate tablets, has a double function. On the one hand, some
Asp-rich proteins adsorbed to the matrix surface provide nucleation sites for the
next crystal layer. On the other hand, some proteins on the matrix inhibit and
terminate crystal growth to ensure uniform thickness of the nacreous layers (Addadi
and Weiner 1985; Mann 2001). Because the interlamellar organic matrix plays such
12 Molecular Approaches to Understand Biomineralization of Shell Nacreous Layer
333