Comparative study of spiculogenesis in demosponge and hexactinellid larvae

Abstract

Spicule deposition was studied by electron microscopy in fixed embryos and larvae of the haplosclerid sponge Reniera sp. and the hexactinellid Oopsacas minuta. Spicules form in centrally located vacuoles within cells and within syncytia, as in the adult sponges. In Reniera, scleroblasts differentiate from micromeres prior to gastrulation. At gastrulation the scleroblasts migrate to the periphery of the embryo and commence spicule deposition around a hexagonal axial filament. Sclerocytes have numerous pseudopodia and migrate to the posterior pole where they become aligned along the antero-posterior axis in the free-swimming larva. Reniera larvae possess some 40-50 oxeas, each 70-75 microm long and 1 microm wide. Mature Oopsacas larvae have up to 14 stauractin spicules, which are produced on a rectangular axial filament in tissues that lie under the smooth epithelium at the posterior pole of the larva. Young sclerocytes have many pseudopodia. The 4-rayed spicules elongate along both the antero-posterior and medial axes, until the longitudinal rays become anchored in a lipid-filled cytoplasm at the anterior of the larva and the lateral rays intersect around the midline. The length of the transverse rays of the stauractins in free-swimming larvae are 27-45 microm each, while the length of the two longitudinal rays is 40-80 microm. Although spicule deposition begins in cells with a similar morphology in both cellular and syncytial sponges, the elaboration and organization of the spicules differ markedly in cellular and syncytial sponges and appear to be an outcome of the very distinct cellular differentiation and larval morphogenesis that occur in each of these groups.

New functional insights into the internal architecture of the laminated anchor spicules of Euplectella aspergillum

“…Some architectural features of biological structures are merely a consequence of the growth processes through which the structures are formed and have no obvious functional implications, e.g., growth rings in fish scales (34). Despite previous efforts (35), knowledge regarding the detailed mechanisms underlying hexactinellid spicule formation is still incomplete and therefore, at this stage, it cannot be ruled out whether other factors, such as growth processes, are also responsible for the spicule's decreasing thickness lamellar structure. Even more importantly, many biological skeletal elements are inherently multifunctional and have evolved the ability to perform a variety of tasks in addition to their mechanical ones.…”

Mini-opinion From unicellularity to multicellularity - molecular speculations about early animal evolution

“…Demosponges have been overlooked for developmental studies because their adult body plan appears simple and primitive, with little in common with the more advanced Metazoa (Brusca and Brusca, 2003). Only recently have embryogenesis and gastrulation shown startling similarities and homologies with other Metazoa (Leys, 2003a(Leys, ,b, 2004Leys and Degnan, 2002;Maldonado, 2004). Although adult sponges did not seem to have any obvious body axes, nor do they even have tissue-level organization, reassessment of their morphology proves that they possess rudimentary axes and cell types for tissue organization (Bavestrello et al, 1998;Borchiellini et al, 2001;BouryEsnault et al, 2003).…”

Analysis of silicatein gene expression and spicule formation in the demosponge Amphimedon queenslandica

“…Many sponge larvae are known to possess spicules (Leys 2003), including the demosponge Amphimedon queenslandica which start to fabricate spicule early in development . The spicules present in larvae usually differ in size and in shape from the adult sponge; larval spicules tend to be smaller.…”

“…The spicules present in larvae usually differ in size and in shape from the adult sponge; larval spicules tend to be smaller. Indeed, the different types of spicule present in the adult are usually absent in larvae, especially the microscleres (Leys 2003). Amphimedon queenslandica larvae and adult both lack microsclere and only possess megascleres (Hooper and Van Soest 2006).…”

“…Fabrication and secretion of spicules can occur at different developmental stages of the sponge, either during embryogenesis, larval phase and metamorphosis, through to the development of the juveniles and growth of the adult. However, specific details on the ontogenetic basis of spicule formation are scant (Berquist and Sinclair 1968;Leys 2003;Maldonado et al 1997;Woollacott 1990). Therefore, to better understand the process of spiculogenesis during natural sponge development-embryonic, larval and post-larval stages -using a sexual reproductive system, spicule formation and silicatein expression were analysed in the demosponge A. queenslandica.…”