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Figure 6: Rod-like collagen-silica-based biomaterial derived in vitro (a) shows morphological similarity to M. chuni basal spicule (a, left). SEM image (b): nanoparticles of amorphous silica deposited in vitro from silicic acid solution on sponge collagen fibrils replicate the nanostructure of glass sponge spicules (Figure 2(e)). SEM micrograph (c) of the surface of silica-collagen hybrid material after 14 days of cultivation of human mesenchymal stem cells, which shows high biocompatibility on this substrate.

Contrary to the postulate that silicateins, as the major biosilica-forming enzymes present in demosponges [34], are responsible for the formation of silica-based structures in all sponges, we suggested that silicateins are associated with collagen [21]. From our point of view, silicateins resemble cathepsins, which are known to be collagenolytic and capable of attacking the triple helix of fibrillar collagens. Therefore, it is not unreasonable to hypothesize that silicateins are proteins responsible for the reconstruction of collagen to form templates necessary for the subsequent silica formation. According to a dynamic model proposed by Müller and his team [5], collagen guides the silicatein(-related) protein/lectin associates concentrically along the spicules of M. chuni. On the basis of the results presented in this paper, we propose a model for the structure of the spicules of Monorhaphis sponges, including micro- and nanoaspects, which can be seen in Figure 5. Recently, we confirmed that silicification of sponge collagen in vitro occurs via selfassembling, nonenzymatic mechanisms [15, 21]. To verify whether the collagenous matrix shapes the morphology of the spicules, we carried out invitro experiments in which we exposed collagen to silicic acid solution (Si(OH)4 ). We obtained rod-like structures of several mm in diameter and demonstrated their similarity to the sponge spicules (Figure 6(a)). The ultrastructural analysis of these selfassembled, collagen-silica composites demonstrates that amorphous silica is deposited on the surface of collagen fibrils in the form of nanopearl necklets (Figure 6(b)), closely resembling the nanoparticulate structure of natural M. chuni spicules (Figures 2(e) and 3(a)). Bridging the nano- and microlevel, we used diﬀerent techniques to create a wide spectrum of macroscopic silicacollagen-based hybrid materials. These are highly biocompatible, as demonstrated by the successful cultivation and os- teogenic diﬀerentiation of human mesenchymal stem cells on our materials (Figure 6(c)), and potentially useful for technical and biomedical applications. On the basis of the results reported above, we also developed an advanced procedure for the biomimetically inspired production of monolithic silica-collagen hybrid xerogels [16]. The disc-like samples showed convincing homogeneity and mechanical stability, enabling cell culture experiments for the first time on such materials.

4. CONCLUSION
Recently, interest in biomaterial properties of silicacontaining structures made by living sponges has grown. In order to exploit the mechanisms for the synthesis of advanced materials and devices, an investigation of the nanoscopic structure of the three-dimensional networks of these remarkable biomaterials needs to be performed [35–38]. Understanding the composition, hierarchical structure, and resulting properties of glass sponge spicules gives impetus for the development of equivalents designed in vitro. We showed for the first time that the silica skeletons of hexactinellids represent examples of biological materials in which a collagenous or chitinous organic matrix serves as a scaﬀold for the deposition of a reinforcing mineral phase in the form of silica. These findings allow us to discard diﬀerent speculations about materials, which have previously been defined as organic structures (layers, filaments, surfaces) of unknown nature, and open the way for detailed studies on sponge skeletons and spicules as collagen- and/or chitinbased nanostructured biocomposites with high potential for practical applications.