Poriferan tissue and skeleton

The mesohyl is an extracellular matrix that contains specialized and pluripotent sponge cells, symbiont prokaryotes and other endobionts,
as well as organic material (mainly proteins such as collagen fibrils and sometimes spongin), spicules, inorganic foreign material, and inorganic material produced by sponge cells. Most sponges possess skeletons composed of spicules usually organized in architectural networks. In some groups, they are arranged into rigid (fused) skeletal structures. Siliceous spicules are biocomposites that incorporate organic material in their structure—a special protein complex forming an axial filament situated in the interior of most siliceous spicules (Ehrlich et al., 2022; Görlich et al., 2020; Uriz, 2006; Uriz et al., 2003). Collagen or chitin matrix may be also present inside the silica spicules (Ehrlich et al., 2007, 2017, 2018; Fromont et al., 2019; Pisera et al., 2021). So far, the axial filament has never been observed in the spicules of Calcarea (Uriz, 2006). Calcareous spicules, despite the absence of an axial filament, show low amounts of an organic matrix between the crystals and an organic envelope surrounding the spicules (Rossi et al., 2014). Some sponges belonging to both Demospongiae and Calcarea also have a notable rigid, massive aragonite or calcite basal skeleton (Hajdu & van Soest, 2002; Manuel et al., 2002). These massive structures are not composed of spicules, but spicules can be associated with this kind of skeleton, being present in the mesohyl. In lithistids, an informal group within Demospongiae, a hypersilicified skeleton occurs (Pisera & Lévi, 2002; Pisera, 2003).