Formation of Siliceous Spicules in Demosponges: Example Suberites domuncula

Silica is a major constituent of sponge spicules in the classes Demospongiae and Hexactinellida. The spicules of these sponges are composed of hydrated, amorphous, non-crystalline silica. In the case of the marine demosponge Suberites domuncula, the initial secretion of spicules has been shown to occur in specialized cells, the sclerocytes, where silica is deposited around an organic filament. Subsequently, the spicules are extruded and completed extracellularly within a galectin/ collagen lattice/scaffold. A major step in elucidating the formation of siliceous spicules on the molecular level was the finding that the ''axial organic filament'' of siliceous spicules is an enzyme, silicatein, which mediates the apposition of amorphous silica and hence the formation of spicules. The formation of siliceous spicules is certainly genetically controlled; this process initiates the morphogenesis phase and involves, in addition to silicatein, galectin and collagen, other molecules. The aim of this chapter is to provide an understanding of spicule formation and to outline the application of the basic biological strategies of the controlled mineralization for nanobiotechnology.