Sponges (phylum Porifera) represent the oldest metazoans. Their characteristic metazoan adhesion molecules and transcription factors enable them to establish a complex "Bauplan"; three major differentiated cell types (epithelial cells, skeletal cells/sclerocytes, and contractile cells) can be distinguished. Since no molecular markers are as yet available to distinguish these somatic cells or the corresponding embryonic cells from which they originate, we have selected the following three genes for their characterization: noggin (a signaling molecule in development), a caspase that encodes an apoptotic molecule, and silicatein. Silicatein is an enzyme that is involved in the synthesis of siliceous spicules and can hence be considered as a marker for scleroblasts. We have used the demosponge Suberites domuncula as a model system. During the hatching of the gemmules (asexual reproduction bodies) of S. domuncula, the expression of both noggin and caspase increases, whereas no transcripts for silicatein can be detected, irrespective of the presence of silicate or ferric iron (Fe3+) in the medium. In contrast, in adult specimens, silicate/Fe3+ cause an increased expression of these genes. In situ analysis has revealed that the first cells that express noggin, caspase, and silicatein lie in the epithelial layer of the pinacoderm. In a later phase, the noggin- and silicatein-positive cells migrate into the mesohyl, where they are found in association with spicules. Thus, the pinacoderm of sponges contains cells that have a differentiating capacity and from which somatic cells, such as skeletal cells/sclerocytes, derive.
Copyright 2004 Springer-Verlag