13.5.1 The Prismatic Layer
As we have seen in Sect. 3.4, the prisms of Pinna nobilis have a paradoxical status:
on one side, they behave like monocrystals, having a single extinction under
polarized, analyzed light and a single crystal diffraction pattern; on the other side,
they are complex biominerals that exhibit a structural hierarchy. The basic elements
are mineral nanograins, which are spatially extremely well oriented, and which
form, together with intracrystalline protein an intimate organo-mineral association.
A great part of these two contradicting views disappears when considering the
formation of calcitic prisms of P. nobilis via a “mesocrystal” pathway.
The concept of “mesocrystal” was introduced by C€ olfen and Antonietti (2005)
and by C€ olfen (2007). The formation of mesocrystals typically follows a
“nonclassical”crystallization pathway. In short, the starting point is identical to
that of classical chemical crystallization pathway, ions, which, by concentration,
form nucleation clusters. In natural environments, these clusters can grow or
disintegrate again. When they grow, they can reach the size of the critical crystal
nucleus. In the case of nonclassical crystallization pathways, the formed primary
nanoparticles are temporarily stabilized by organic polymers, which adsorb on their
faces. The later stage implies that the nanoparticles, on which organic polymers are
adsorbed, assemble and orient in a superstructure, a mesocrystal, which, by fusion
of its oriented nanoparticles, becomes a “single” crystal. The polymers associated
to this “crystal” remain entrapped after the fusion. When applied to the calcitic
prisms of P. nobilis, the concept of mesocrystal conciliates the apparent simplicity
of each prism with their truly ultrastructural complexity.
How are the calcitic prisms of P. nobilis formed? In natural environments,
prismatic crystalline structures can be produced through a completely abiotic
pathway, which has been described for a long time. Grigor’ev (1965) explained
the growth of prismatic structures via a process of competition for space. On an
uneven surface, spherulites can be more or less regularly seeded; when they grow in
a centrifugal manner, they enter into contact with neighbor spherulites, and consequently compete for space. They tend to grow in one direction, the fast-growing
spherulites “absorbing” the slow-growing ones. After a while, the growth of the
elongated structures is unidirectional and perpendicular to the initial plane where
spherulites were seeded.
Up to a certain point, such a description might apply to the prisms of P. nobilis,
and, more generally, to prismatic microstructures in bivalve shells (Ubukata 1994).
Several analogies may be found between the two processes. First, similarly to the
abiotic competition process, prisms of P. nobilis grow in one direction (inward)
from an uneven surface, the periostracal layer. Their growth is perpendicular to that
surface. Second, Cuif and coworkers (1983a, b) observed that the early stage of
prisms formation at the edge of the shell was characterized by the formation of
nodules (spherulites) that concentrically develop from a center and completely fill
the polygonal spaces delimited by the periprismatic sheaths that form prior mineral
deposition. In juvenile specimens of P. nobilis, we fully confirm, for each newly
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
383
As we have seen in Sect. 3.4, the prisms of Pinna nobilis have a paradoxical status:
on one side, they behave like monocrystals, having a single extinction under
polarized, analyzed light and a single crystal diffraction pattern; on the other side,
they are complex biominerals that exhibit a structural hierarchy. The basic elements
are mineral nanograins, which are spatially extremely well oriented, and which
form, together with intracrystalline protein an intimate organo-mineral association.
A great part of these two contradicting views disappears when considering the
formation of calcitic prisms of P. nobilis via a “mesocrystal” pathway.
The concept of “mesocrystal” was introduced by C€ olfen and Antonietti (2005)
and by C€ olfen (2007). The formation of mesocrystals typically follows a
“nonclassical”crystallization pathway. In short, the starting point is identical to
that of classical chemical crystallization pathway, ions, which, by concentration,
form nucleation clusters. In natural environments, these clusters can grow or
disintegrate again. When they grow, they can reach the size of the critical crystal
nucleus. In the case of nonclassical crystallization pathways, the formed primary
nanoparticles are temporarily stabilized by organic polymers, which adsorb on their
faces. The later stage implies that the nanoparticles, on which organic polymers are
adsorbed, assemble and orient in a superstructure, a mesocrystal, which, by fusion
of its oriented nanoparticles, becomes a “single” crystal. The polymers associated
to this “crystal” remain entrapped after the fusion. When applied to the calcitic
prisms of P. nobilis, the concept of mesocrystal conciliates the apparent simplicity
of each prism with their truly ultrastructural complexity.
How are the calcitic prisms of P. nobilis formed? In natural environments,
prismatic crystalline structures can be produced through a completely abiotic
pathway, which has been described for a long time. Grigor’ev (1965) explained
the growth of prismatic structures via a process of competition for space. On an
uneven surface, spherulites can be more or less regularly seeded; when they grow in
a centrifugal manner, they enter into contact with neighbor spherulites, and consequently compete for space. They tend to grow in one direction, the fast-growing
spherulites “absorbing” the slow-growing ones. After a while, the growth of the
elongated structures is unidirectional and perpendicular to the initial plane where
spherulites were seeded.
Up to a certain point, such a description might apply to the prisms of P. nobilis,
and, more generally, to prismatic microstructures in bivalve shells (Ubukata 1994).
Several analogies may be found between the two processes. First, similarly to the
abiotic competition process, prisms of P. nobilis grow in one direction (inward)
from an uneven surface, the periostracal layer. Their growth is perpendicular to that
surface. Second, Cuif and coworkers (1983a, b) observed that the early stage of
prisms formation at the edge of the shell was characterized by the formation of
nodules (spherulites) that concentrically develop from a center and completely fill
the polygonal spaces delimited by the periprismatic sheaths that form prior mineral
deposition. In juvenile specimens of P. nobilis, we fully confirm, for each newly
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
383