Stem cells in aquatic invertebrates: Common premises and emerging unique themes

The remarkable ability to generate an embryo from a single fertilized oocyte and to regenerate tissues that are injured or going through natural physiological turnover, is the direct result of stem cells, nature's gift to multicellular organisms. Only recently, studies on marine invertebrates have revealed the diversity of phenomena involved with these cells and their importance to the biology of these organisms. We present an overview on the stem cell biology of four aquatic invertebrate groups: urochordate ascidians, cnidarians, echinoderms and platyhelminthes. While most studies outlined here are based on descriptive data, we found that aquatic invertebrates exhibit multiple cell types with stem cell atributes. Studies revealed that, in contrast to the prevalence of diverse oligopotent and unipotent stem cells in vertebrates, invertebrates appear to display the communal spread of multipotency and pluripotency, with stem cells that give rise to cell lineages characteristic to more than a single germ layer, sometimes with somatic and germ line potentials. The cumulative data further indicate that in contrast to vertebrate systems, stem cells from aquatic invertebrates are disseminated and widespread i.e. not associated with a regulatory microenvironment (niche). We also notice that transdifferentiation is ubiquitous to both anatomically simple and highly evolved invertebrates. These observations delineate common and unique properties to stem cells, posibly tailored, to suit the varied life history and developmental modes characteristic of aquatic invertebrates.