Model of spicule formation in S. domuncula This starts intracellularly by processing of silicatein to a mature and phosphorylated 23-kDa form, which assembles to axial filaments and starts to template silica deposition around itself. The spicules are extruded from the cells and mature in size in the extracellular stage with the help of the 34.7 kDa prosilicatein (lacking the signal peptide). No mature silicatein has so far been identified outside the cell. Reproduced with permission from [110].
