Abstract In molluscs, the shell secretion process is controlled by a set of extracellular macromolecules collectively called the shell matrix. The shell matrix, which is
produced by the mantle epithelial cells during mineralization, is predominantly
composed of proteins, glycoproteins, acidic polysaccharides, and chitin that precisely regulate the deposition of calcium carbonate outside the mantle cells. In the
present paper, we focus on the shell of Pinna nobilis, the giant Mediterranean fan
mussel, usually considered as a model for studying molluscan biomineralization
processes. P. nobilis exhibits indeed a nacro-prismatic shell, the outer layer of
which is constituted of the so-called “regular simple calcitic prisms,” according to
Carter and Clark (1985). We review here the microstructural characteristics of the
prisms and nacre and the biochemical properties of their associated matrices.
In particular, the calcitic prisms of P. nobilis are characterized by a cortege of
unusually acidic intraprismatic proteins, while the ones of the nacreous layer seem
less acidic. A brief description of the molecular characterization of three acidic
proteins, caspartin, calprismin and mucoperlin, is given. In particular, we show that
extremely acidic intracrystalline proteins such as caspartin interact with calcium
carbonate at different scales, from micrometric to crystal lattice levels.
13.1 Biomineralization of the Molluscan Shell, a Brief
Overview of the Mechanism
In the metazoan world, molluscs are often considered as the master controllers of
biomineralization processes, and their shell represents the most remarkable example of a biologically controlled mineralization produced outside living tissues by a
calcifying epithelium (Simkiss and Wilbur 1989).
The organ responsible for the mineral deposition and shell formation is the
mantle, the thin epithelium that envelopes the soft body of molluscs, and which is
in contact with the growing shell. More precisely, according to classical views, the
calcification takes place in a closed space, the extrapallial space, located at the
interface between the mantle tissues, the leathery periostracum and the growing shell
itself. The shell emerges from a subtle chemistry between the precursor mineral ions,
i.e., calcium, bicarbonate, and minor elements such as magnesium or strontium that
are extruded in the extrapallial space, and the numerous macromolecular extracellular components produced by the mantle epithelial cells, i.e., proteins and
polysaccharides. All these components react in a well-coordinated manner. This
self-assembling process produces solid and compact microstructures such as nacre,
prisms, crossed-lamellar, foliated, or homogeneous textures (Carter 1990).
In the recent years, numerous new data have completed this classical view. The
most recent advances deal more particularly with the structure of shell biominerals
at the nanoscale (Oaki and Imai 2005), and with the identification of an increasing
number of shell matrix proteins (Marin et al. 2008). Although the organic matrix
represents usually less than 5% of the shell weight, it is however the major
component that controls different aspects of the shell formation processes (Addadi
354
F. Marin et al.
produced by the mantle epithelial cells during mineralization, is predominantly
composed of proteins, glycoproteins, acidic polysaccharides, and chitin that precisely regulate the deposition of calcium carbonate outside the mantle cells. In the
present paper, we focus on the shell of Pinna nobilis, the giant Mediterranean fan
mussel, usually considered as a model for studying molluscan biomineralization
processes. P. nobilis exhibits indeed a nacro-prismatic shell, the outer layer of
which is constituted of the so-called “regular simple calcitic prisms,” according to
Carter and Clark (1985). We review here the microstructural characteristics of the
prisms and nacre and the biochemical properties of their associated matrices.
In particular, the calcitic prisms of P. nobilis are characterized by a cortege of
unusually acidic intraprismatic proteins, while the ones of the nacreous layer seem
less acidic. A brief description of the molecular characterization of three acidic
proteins, caspartin, calprismin and mucoperlin, is given. In particular, we show that
extremely acidic intracrystalline proteins such as caspartin interact with calcium
carbonate at different scales, from micrometric to crystal lattice levels.
13.1 Biomineralization of the Molluscan Shell, a Brief
Overview of the Mechanism
In the metazoan world, molluscs are often considered as the master controllers of
biomineralization processes, and their shell represents the most remarkable example of a biologically controlled mineralization produced outside living tissues by a
calcifying epithelium (Simkiss and Wilbur 1989).
The organ responsible for the mineral deposition and shell formation is the
mantle, the thin epithelium that envelopes the soft body of molluscs, and which is
in contact with the growing shell. More precisely, according to classical views, the
calcification takes place in a closed space, the extrapallial space, located at the
interface between the mantle tissues, the leathery periostracum and the growing shell
itself. The shell emerges from a subtle chemistry between the precursor mineral ions,
i.e., calcium, bicarbonate, and minor elements such as magnesium or strontium that
are extruded in the extrapallial space, and the numerous macromolecular extracellular components produced by the mantle epithelial cells, i.e., proteins and
polysaccharides. All these components react in a well-coordinated manner. This
self-assembling process produces solid and compact microstructures such as nacre,
prisms, crossed-lamellar, foliated, or homogeneous textures (Carter 1990).
In the recent years, numerous new data have completed this classical view. The
most recent advances deal more particularly with the structure of shell biominerals
at the nanoscale (Oaki and Imai 2005), and with the identification of an increasing
number of shell matrix proteins (Marin et al. 2008). Although the organic matrix
represents usually less than 5% of the shell weight, it is however the major
component that controls different aspects of the shell formation processes (Addadi
354
F. Marin et al.