– In step seven, the prisms continue to grow layer per layer, according to the same
process described here above. Competition for growth is extremely marginal and
most of the prisms grow in parallel. The growth is definitively stopped when the
organic interface between the prismatic and the nacreous layers covers the
inward surface of the prisms.
In this general scheme, very acidic proteins like caspartin and calprismin may
display different functions, and their role is coordinated with that of the hydrophobic constituents of the periprismatic walls. We are fully aware that the steps
described above are speculative, and that the true process may be completely
different.
13.5.2 The Nacreous Layer
Similarly to the formation of prisms, the synthesis of nacre in P. nobilis gives rise to
a broad range of speculations, which cannot be easily verified. Indeed, our knowledge of the nacre constituents of P. nobilis is limited to one single nacre-specific
protein, mucoperlin, and most of the knowledge accumulated on nacre derives from
other models, such as Nautilus, Pinctada, or Haliotis. Thus, we cannot guarantee
that the different aspects of the nacre formation referred below can strictly apply the
nacre of P. nobilis: as described in Sect. 3.5, the nacre of P. nobilis is indeed rather
atypical in the bivalvian world, since it represents the row-stack type, and not the
brickwall type.
A paper published few years ago by Addadi and coworkers (2006) gives an
excellent review of critically important elements for nacre formation. These
elements can be summarized as follows: chitin is the structural polymer that preorganizes the interface between the mineralization front and the mantle cells, before
mineralization. In nacre, chitin is supposed to define the interlamellar template on
which nacre tablets grow. Between successive chitin sheets, a silklike gel composed
of hydrophobic proteins is present. This gel also contains acid-soluble proteins that
may work as mineral nucleators.
Concerning the process of nacre growth itself, the paper from Addadi et al.
(2006) underlines the following aspects: nacre tablets grow from their center, which
is enriched in polyanionic polymers (carboxylate-rich, or sulfate-rich). Carboxylate
groups may be involved in the nucleation itself, while sulfate groups may concentrate calcium ions at the vicinity of the carboxylate groups (Addadi et al. 1987;
Nudelman et al. 2006). The initial minerals formed are amorphous, but they convert
to aragonite. Nacre tablets grow vertically, then laterally. By doing so, they entrap
acidic proteins and push laterally the hydrophobic gel. When neighboring nacre
tablets come in confluence, the squeezed gel polymerizes and forms the intertabular
insoluble matrix. Whether nacre tablets grow by heteroepitaxy or by mineral
bridges is still unclear and may depend on the studied model.
386
F. Marin et al.
process described here above. Competition for growth is extremely marginal and
most of the prisms grow in parallel. The growth is definitively stopped when the
organic interface between the prismatic and the nacreous layers covers the
inward surface of the prisms.
In this general scheme, very acidic proteins like caspartin and calprismin may
display different functions, and their role is coordinated with that of the hydrophobic constituents of the periprismatic walls. We are fully aware that the steps
described above are speculative, and that the true process may be completely
different.
13.5.2 The Nacreous Layer
Similarly to the formation of prisms, the synthesis of nacre in P. nobilis gives rise to
a broad range of speculations, which cannot be easily verified. Indeed, our knowledge of the nacre constituents of P. nobilis is limited to one single nacre-specific
protein, mucoperlin, and most of the knowledge accumulated on nacre derives from
other models, such as Nautilus, Pinctada, or Haliotis. Thus, we cannot guarantee
that the different aspects of the nacre formation referred below can strictly apply the
nacre of P. nobilis: as described in Sect. 3.5, the nacre of P. nobilis is indeed rather
atypical in the bivalvian world, since it represents the row-stack type, and not the
brickwall type.
A paper published few years ago by Addadi and coworkers (2006) gives an
excellent review of critically important elements for nacre formation. These
elements can be summarized as follows: chitin is the structural polymer that preorganizes the interface between the mineralization front and the mantle cells, before
mineralization. In nacre, chitin is supposed to define the interlamellar template on
which nacre tablets grow. Between successive chitin sheets, a silklike gel composed
of hydrophobic proteins is present. This gel also contains acid-soluble proteins that
may work as mineral nucleators.
Concerning the process of nacre growth itself, the paper from Addadi et al.
(2006) underlines the following aspects: nacre tablets grow from their center, which
is enriched in polyanionic polymers (carboxylate-rich, or sulfate-rich). Carboxylate
groups may be involved in the nucleation itself, while sulfate groups may concentrate calcium ions at the vicinity of the carboxylate groups (Addadi et al. 1987;
Nudelman et al. 2006). The initial minerals formed are amorphous, but they convert
to aragonite. Nacre tablets grow vertically, then laterally. By doing so, they entrap
acidic proteins and push laterally the hydrophobic gel. When neighboring nacre
tablets come in confluence, the squeezed gel polymerizes and forms the intertabular
insoluble matrix. Whether nacre tablets grow by heteroepitaxy or by mineral
bridges is still unclear and may depend on the studied model.
386
F. Marin et al.