For the synthesis of the nacre of P. nobilis, some elements are seriously missing
to give the broad picture: we do not know whether chitin is present, and if so, where
it is located. We do not know whether the first minerals formed are amorphous. We
do not know whether the row-stack nacre tablets grow by heteroepitaxy or whether
they grow on each other via mineral bridges. What remains accurately established
concerns the fact that nacre tablets of P. nobilis are constituted of nano-grains, as
previously shown by Cuif and coworkers (1985). Again, similarly to what happens
in the case of the prism, nacre tablets can be considered as mesocrystals, formed by
the fusion of crystallographically oriented nanograins. Once more, the concept
developed C€ olfen seems to be valid and probably corresponds to a general law in
biomineralization.
Another important point that partly converges with the Addadi’s model
described above is the location of mucoperlin. As explained in Sect. 4.3.2,
mucoperlin is localized at the periphery of the tablets, in the intertabular matrix,
rather than in the interlamellar one. Thus, our explanation is that mucoperlin may be
a macromolecular constituent of the gel-like polymer mixture in which nacre tablets
grow. When the tablets extend laterally, they push aside mucoperlin, which is
consequently squeezed in the intertabular matrix. The single difference between
the model and our finding is that mucoperlin is primarily soluble and rather
hydrophilic. Thus, mucoperlin is markedly different from the silklike proteins
that are supposed to form the hydrophobic gel. Being acidic, one may expect that
mucoperlin is involved in nucleating nacre tablets. However, in none of our staining
did we observe that mucoperlin was localized within the nacre tablets. What
appears extremely consistent is that mucoperlin is specific of the nacreous layer.
Thus, we believe that, together with other components, mucoperlin may contribute
to the selection of the aragonite polymorph, among other putative functions. More
work has however to be done for testing in vitro this hypothesis.
13.6 Conclusion
In the present paper, we have tried, as far as we could, to synthesize all the data –
micro- and nanostructural and biochemical – on the shell of Pinna nobilis and on its
acidic macromolecular constituents. Although remarkable advances have been
recorded, our story is partial and “holey.” Fundamental researches have to be
Fig. 13.7 (continued) The polygonal shape of the periprismatic membranes suggests that they are
patterned by interfacial tension of an emulsion. If so, the periprismatic membranes may result from
the polymerization of the continuous phase of a liquid–liquid emulsion, consisting of an aqueous
discontinuous phase and a hydrophobic continuous one. Note that the periprismatic membranes are
formed prior to their mineralization. For each polygon, calcite crystals grow from one center and
expand laterally until reaching the periprismatic membranes. (b) later stage. The prisms grow in
thickness inward (toward the mantle epithelium), perpendicularly to the periostracal surface. (c)
advanced stage. The prisms continue to grow vertically. Locally, crystal competition may occur
◂
388
F. Marin et al.
to give the broad picture: we do not know whether chitin is present, and if so, where
it is located. We do not know whether the first minerals formed are amorphous. We
do not know whether the row-stack nacre tablets grow by heteroepitaxy or whether
they grow on each other via mineral bridges. What remains accurately established
concerns the fact that nacre tablets of P. nobilis are constituted of nano-grains, as
previously shown by Cuif and coworkers (1985). Again, similarly to what happens
in the case of the prism, nacre tablets can be considered as mesocrystals, formed by
the fusion of crystallographically oriented nanograins. Once more, the concept
developed C€ olfen seems to be valid and probably corresponds to a general law in
biomineralization.
Another important point that partly converges with the Addadi’s model
described above is the location of mucoperlin. As explained in Sect. 4.3.2,
mucoperlin is localized at the periphery of the tablets, in the intertabular matrix,
rather than in the interlamellar one. Thus, our explanation is that mucoperlin may be
a macromolecular constituent of the gel-like polymer mixture in which nacre tablets
grow. When the tablets extend laterally, they push aside mucoperlin, which is
consequently squeezed in the intertabular matrix. The single difference between
the model and our finding is that mucoperlin is primarily soluble and rather
hydrophilic. Thus, mucoperlin is markedly different from the silklike proteins
that are supposed to form the hydrophobic gel. Being acidic, one may expect that
mucoperlin is involved in nucleating nacre tablets. However, in none of our staining
did we observe that mucoperlin was localized within the nacre tablets. What
appears extremely consistent is that mucoperlin is specific of the nacreous layer.
Thus, we believe that, together with other components, mucoperlin may contribute
to the selection of the aragonite polymorph, among other putative functions. More
work has however to be done for testing in vitro this hypothesis.
13.6 Conclusion
In the present paper, we have tried, as far as we could, to synthesize all the data –
micro- and nanostructural and biochemical – on the shell of Pinna nobilis and on its
acidic macromolecular constituents. Although remarkable advances have been
recorded, our story is partial and “holey.” Fundamental researches have to be
Fig. 13.7 (continued) The polygonal shape of the periprismatic membranes suggests that they are
patterned by interfacial tension of an emulsion. If so, the periprismatic membranes may result from
the polymerization of the continuous phase of a liquid–liquid emulsion, consisting of an aqueous
discontinuous phase and a hydrophobic continuous one. Note that the periprismatic membranes are
formed prior to their mineralization. For each polygon, calcite crystals grow from one center and
expand laterally until reaching the periprismatic membranes. (b) later stage. The prisms grow in
thickness inward (toward the mantle epithelium), perpendicularly to the periostracal surface. (c)
advanced stage. The prisms continue to grow vertically. Locally, crystal competition may occur
◂
388
F. Marin et al.