5Hermann Ehrlich et al.
Figure 4: (a) High-resolution transmission electron microscopy image of the fragment of M. chuni collagen microfibril; (b) the arrows indicate the presence of nanofibrillar structures with a diameter which corresponds to that of collagen triple helices (1.5 nm). The results of aminoacid analysis (right) of these microfibrills showed an aminoacid content typical for collagens isolated from different sources [21]. Collagen net
Figure 5: proposed model of micro- and nanostructural organisation of the basal spicule of M. chuni with respect to the organic matrix. (a) Collagen nets, surrounding the spicules, showed a tight mat of nanofibrils. Schematic view (b) shows a collagenous fibrillar matrix which could function as a glue between concentric layers. Image (c) represents the region of the axial canal and axial filament. The axial canal of M. chuni possesses a characteristic quadratic opening (c) and contains oriented bundles of unsilicified collagenous nanofibrils. The base material of the walls of the axial canal and concentric layers distributed above it consists of silicified collagen fibrils with a twisted plywood orientation. This kind of fibrillar architecture could be responsible for the remarkable micromechanical properties of the spicule as a biocomposite.
similar to that reported for lamellar bone and thus could also confirm the Girauld-Guille model in the case of biosilification in vivo. Correspondingly, this kind of collagen fibril orientation could explain why sponge spicules exhibit specific flexibility and can be bent even to a circle as reported previously [2, 4, 21]. From this point of view, basal spicules of Monorhaphis sponges could be also defined as natural plywood-like silica-ceramics organized similarly to the crossed-lamellar layers of seashells [33]. Thus, we suggest that the matrix of the M. chuni anchoring spicule is silificated fibrillar collagen rather than collagen-containing silica which is the reason for their remarkable mechanical flexibility.
