MASTER THESIS Biomimetic potential of sponge ... - IAP/TU Wien

step mechanism. The first two steps catalyze the di-/ trimerization of silicic acid in a
nucleophilic substitution reaction, i.e., two and later three silicic acid molecules are bonded
together. The third step consists in the cyclization of the trimer. Since this reaction pathway
involves hydrolysis steps, water molecules, which are cleaved off the initial building blocks
(silicic acid), are released. These water molecules have to be removed during a maturation
process (see below).
How the circular silica trimers synthesized in this reaction further proceed
(polycondensation, linking individual trimers) is less well known, but silintaphins are believed
to guide the assembly in silica into nanospheres (Schröder et al., 2010) (Wang et al., 2012b).
This mechanism allows for the deposition of silica beads onto the filament within the
silicalemma of sclerocytes. Using the common precursor tetraethoxysilane (TEOS) as an
analogue of the naturally used silicic acid, the deposition of silica beads onto silicateincontaining
filaments could be observed (Figure 13g) (Müller et al., 2005).
Appositional and axial growth
At the beginning of the second stage, the primordial spicule is extruded to the
extracellular space. Interestingly, the spicules then re-attach to a sclerocyte and remain in
close association throughout axial and appositional growth (Wang et al., 2011c). In a
remarkable form of contact between spicule and the sclerocytes, the cell evaginates
(protrudes) into the axial canal of the becoming spicule (Figure 14a-f). As can be seen in
Figure 14b and f, the evaginations of the cell do not extend all the way to the closed apex of
the axial canal (Figure 14c). At the boundary between intracellular space (ics/cp) and
extracellular space (ecs), the release of silicasomes (sis) from the evaginations can be
observed. These provide the building blocks, silicateins and silica for the intra-spicular
biosilica deposition. By this process, the innermost silica-structure surrounding the axial
filament (the mantel) is synthesized and the diameter of the avail canal decreases (Wang et
al., 2011c). Since the apex of the axial canal is closed, longitudinal growth can only occur at
the open, basal end of the axial canal, where the evagination of the sclerocyte enters (Wang
et al., 2012b). Observing the underlying mechanism of the evagination, it becomes evident
that hydrodynamic forces push the cell membrane outwards into the spicular axial canal.
These forces gradually displace the spicule outwards while in the growth region (gr, Figure
14a,d) at the sclerocyte cell-body more silica is laid down. In effect, the elongation of the
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