polymerizes in a solid and flexible honeycomb-like structure. To summarize, we
tentatively dissect the whole sequence of prism formation in P. nobilis in short
elementary chronological steps.
– In step one, the periostracal layer is secreted by the periostracal groove and
hardens when in contact with seawater. The hardening is performed via a
quinone-tanning process. Whether the internal surface of the periostracum is
“pre-patterned” or not for mineralization is not known. Maybe some spots are
“pre-designed” that correspond later to the nucleation centers from which
nodules/spherulites grow.
– In step two, a mixture of heterogeneous organic components – the prisms
matrix – is secreted together with the precursor fluid that contains mineral
ions, between the newly formed periostracum and the mantle. Because some
of these components are hydrophobic while the others are hydrophilic, a
liquid–liquid emulsion is formed, in close contact with the internal surface
of the periostracum. This labile structure, governed by interfacial tension,
self-organizes in a viscoelastic mesh composed of polygonal cells. It may be
transiently stabilized by acidic proteins at the interface between the two phases.
– In step three, the organic mesh polymerizes similarly to the periostracum,
becoming insoluble and flexible.
– In step four, crystal nucleation occurs within each individual polygon, and the
newly formed crystalline aggregates, which can be assimilated to nodules/
spherulites, expand concentrically (“hemispherically”) until reaching the polygonal boundaries. They form then flat “tiles.” Whether the step of crystallization
of calcite is preceded by a step of formation of amorphous mineral is not known
but can be suspected. The nucleating macromolecules may be acidic proteins
such as caspartin or calprismin, or sulfated polysaccharides.
– In step five, while the polygonal boundaries continue to grow in height, the
newly formed tiles elongate perpendicularly to the periostracal surface. The
extension of periprismatic boundaries may occur by successive accretion of
organic elements to their edge. The process involved might be contact recognition. The prism elongation is accomplished “layer-per-layer.” For each prism,
one “layer” results from the fusion of nano-grains, which are surrounded and
crystallographically oriented by acidic proteins (caspartin and calprismin),
according to the scheme of C€ olfen. Each “layer” can be consequently considered
as a flat mesocrystal, which itself fuses with the subjacent mineral receptacle.
Whether nano-grains together with their surrounding acidic macromolecules are
imported by exocytosis from vesicles of some mantle cells or whether they form
directly in the growing structure, at the interface between the mantle cells and
the periostracum, is not known.
– In step six, while the prisms continue to grow, competition for space – or
something, which looks like it – occurs in the range of 50–100 mm: some
slow-growing prisms are absorbed by their fast-growing neighbors. Because
the periprismatic boundaries are elastic and deformable, they accompany the
lateral expansion of the fast-growing prisms.
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
385
tentatively dissect the whole sequence of prism formation in P. nobilis in short
elementary chronological steps.
– In step one, the periostracal layer is secreted by the periostracal groove and
hardens when in contact with seawater. The hardening is performed via a
quinone-tanning process. Whether the internal surface of the periostracum is
“pre-patterned” or not for mineralization is not known. Maybe some spots are
“pre-designed” that correspond later to the nucleation centers from which
nodules/spherulites grow.
– In step two, a mixture of heterogeneous organic components – the prisms
matrix – is secreted together with the precursor fluid that contains mineral
ions, between the newly formed periostracum and the mantle. Because some
of these components are hydrophobic while the others are hydrophilic, a
liquid–liquid emulsion is formed, in close contact with the internal surface
of the periostracum. This labile structure, governed by interfacial tension,
self-organizes in a viscoelastic mesh composed of polygonal cells. It may be
transiently stabilized by acidic proteins at the interface between the two phases.
– In step three, the organic mesh polymerizes similarly to the periostracum,
becoming insoluble and flexible.
– In step four, crystal nucleation occurs within each individual polygon, and the
newly formed crystalline aggregates, which can be assimilated to nodules/
spherulites, expand concentrically (“hemispherically”) until reaching the polygonal boundaries. They form then flat “tiles.” Whether the step of crystallization
of calcite is preceded by a step of formation of amorphous mineral is not known
but can be suspected. The nucleating macromolecules may be acidic proteins
such as caspartin or calprismin, or sulfated polysaccharides.
– In step five, while the polygonal boundaries continue to grow in height, the
newly formed tiles elongate perpendicularly to the periostracal surface. The
extension of periprismatic boundaries may occur by successive accretion of
organic elements to their edge. The process involved might be contact recognition. The prism elongation is accomplished “layer-per-layer.” For each prism,
one “layer” results from the fusion of nano-grains, which are surrounded and
crystallographically oriented by acidic proteins (caspartin and calprismin),
according to the scheme of C€ olfen. Each “layer” can be consequently considered
as a flat mesocrystal, which itself fuses with the subjacent mineral receptacle.
Whether nano-grains together with their surrounding acidic macromolecules are
imported by exocytosis from vesicles of some mantle cells or whether they form
directly in the growing structure, at the interface between the mantle cells and
the periostracum, is not known.
– In step six, while the prisms continue to grow, competition for space – or
something, which looks like it – occurs in the range of 50–100 mm: some
slow-growing prisms are absorbed by their fast-growing neighbors. Because
the periprismatic boundaries are elastic and deformable, they accompany the
lateral expansion of the fast-growing prisms.
13 Acidic Shell Proteins of the Mediterranean Fan Mussel Pinna nobilis
385