lateral view of a cross break shows the solid dimension of a lamella. Inside of the
axial canal, the axial filament is located (Fig. 9.7g).
Studies, to obtain an insight into the structural organization of the spicules at the
nm scale, can be obtained by partial and limited dissolution of the silica using HF
with the limitations described (Simpson et al. 1985). A rapid dissolution results in
the removal of the inorganic scaffold, while a less-intense exposure of the spicules
to HF vapor releases the organic matrix from within the lamellae. Gentle exposure
of cross breaks of the spicules to HF vapor results in the dissolution of the silica
material under release of the organic component of the lamellae (M€ uller et al.
2008a, d). The proteinaceous palisade-like scaffold is uncovered; it is composed of
fibrous structures, into which holes formed by interconnecting fibers are
interspersed. Under avoidance of shear forces during the HF treatment, the complete proteinaceous layer of one lamella can be obtained. At higher magnification, it
becomes overt that the rim of each hole is reinforced by densely arranged 10–15 nm
large spheres. The protein fibers that are attaching the silica scaffold do not contain
any banding pattern reminiscent of collagen.
9.6 Phases of Silica Deposition During Spicule Formation
Along the Proteinaceous Filament
Taking into account the data collected (reviewed in M€ uller et al. 2006b), the process
of spicule formation can be divided into phases; the initial intracellular steps and the
extracellular final growth and shaping phases.
Intracellular phase in the sclerocytes: Silica is taken up actively by a Na
+
/
HCO 3
À [Si(OH) 4 ] cotransporter (Schr€ oder et al. 2004). In the first steps, silicatein is
synthesized as a proenzyme (signal peptide–propeptide–mature enzyme: 36.3 kDa)
and processed via the 34.7 kDa form (propeptide–mature enzyme) to the 23 kDa
mature enzyme. Very likely during the transport through the endoplasmic reticulum
and the Golgi complex, silicatein undergoes phosphorylation and is transported into
vesicles where it forms rods, the axial filaments. After assembly to filaments, the
first layer(s) of silica is (are) formed. Silica deposition occurs in two directions; first
from the axial canal to the surface (centrifugal orientation) and second from the
mesohyl to the surface of the spicule (centripedal). Finally, the spicules are released
into the extracellular space where they grow in length and diameter by appositional
growth; Fig. 9.8.
Extracellular phase (appositional growth): Silicatein is present also in the
extracellular space. It came surprising that also there the silicatein molecules are
organized to larger entities. The immunogold electron microscopical analysis
showed that the silicatein molecules are arranged along strings, which are organized
in parallel to the surfaces of the spicules (Schr€ oder et al. 2006). In the presence of
Ca
2+ , silicatein associates with galectin and allows the appositional growth of the
spicules. Since the surface of a new siliceous spicule is also covered with silicatein,
9 The Unique Invention of the Siliceous Sponges
267
axial canal, the axial filament is located (Fig. 9.7g).
Studies, to obtain an insight into the structural organization of the spicules at the
nm scale, can be obtained by partial and limited dissolution of the silica using HF
with the limitations described (Simpson et al. 1985). A rapid dissolution results in
the removal of the inorganic scaffold, while a less-intense exposure of the spicules
to HF vapor releases the organic matrix from within the lamellae. Gentle exposure
of cross breaks of the spicules to HF vapor results in the dissolution of the silica
material under release of the organic component of the lamellae (M€ uller et al.
2008a, d). The proteinaceous palisade-like scaffold is uncovered; it is composed of
fibrous structures, into which holes formed by interconnecting fibers are
interspersed. Under avoidance of shear forces during the HF treatment, the complete proteinaceous layer of one lamella can be obtained. At higher magnification, it
becomes overt that the rim of each hole is reinforced by densely arranged 10–15 nm
large spheres. The protein fibers that are attaching the silica scaffold do not contain
any banding pattern reminiscent of collagen.
9.6 Phases of Silica Deposition During Spicule Formation
Along the Proteinaceous Filament
Taking into account the data collected (reviewed in M€ uller et al. 2006b), the process
of spicule formation can be divided into phases; the initial intracellular steps and the
extracellular final growth and shaping phases.
Intracellular phase in the sclerocytes: Silica is taken up actively by a Na
+
/
HCO 3
À [Si(OH) 4 ] cotransporter (Schr€ oder et al. 2004). In the first steps, silicatein is
synthesized as a proenzyme (signal peptide–propeptide–mature enzyme: 36.3 kDa)
and processed via the 34.7 kDa form (propeptide–mature enzyme) to the 23 kDa
mature enzyme. Very likely during the transport through the endoplasmic reticulum
and the Golgi complex, silicatein undergoes phosphorylation and is transported into
vesicles where it forms rods, the axial filaments. After assembly to filaments, the
first layer(s) of silica is (are) formed. Silica deposition occurs in two directions; first
from the axial canal to the surface (centrifugal orientation) and second from the
mesohyl to the surface of the spicule (centripedal). Finally, the spicules are released
into the extracellular space where they grow in length and diameter by appositional
growth; Fig. 9.8.
Extracellular phase (appositional growth): Silicatein is present also in the
extracellular space. It came surprising that also there the silicatein molecules are
organized to larger entities. The immunogold electron microscopical analysis
showed that the silicatein molecules are arranged along strings, which are organized
in parallel to the surfaces of the spicules (Schr€ oder et al. 2006). In the presence of
Ca
2+ , silicatein associates with galectin and allows the appositional growth of the
spicules. Since the surface of a new siliceous spicule is also covered with silicatein,
9 The Unique Invention of the Siliceous Sponges
267