Secondary mesenchyme cells as potential stem cells of the sea urchin embryo.

In the last years, stem cells have been the subject of great interest. Although considerable progress has been made in this field, the signaling pathways and related molecules controlling stem cells behaviour still remain unclear. Among marine invertebrates, echinoderms have the attracting ability to regenerate parts of their bodies, involving stem cells recruitment, and thus providing themselves as excel-lent models for studies on stem cells in adult organisms. On the contrary, hardly any research focused on embryonic stem cells has been performed using echinoderm embryos, although they have been utilised extensively and with noteworthy results, for example, for studies on basic developmental biology. Indeed, the great amount of data accumulated over the years, the availability of new genomic and proteomic research tools, together with the advantage of experimental manipulation support the sea urchin embryo as a good candidate for detailed studies on embryonic stem cells. Here, we review fundamental findings concerning the two distinct populations of mesodermal cells in the sea urchin embryo, discussing the possibility to identify some of them as embryonic stem cells. In particular, secondary mesenchyme cells (SMCs) are a heterogeneous population of cells with several different fates and behaviours. Taken together, a number of evidence indicates that SMCs function as multipotent stem cells, thus sharing some features with vertebrate embryonic stem cells, including the neural crest cells. Our aim is to address new viewpoints for forthcoming studies on SMCs as well as to open new directions for research on fundamental mechanisms of stem cell biology exploiting the sea urchin embryo as a model system.