The regenerative potential of starfish arm explants

Starfish are known for their amazing regenerative capabilities. As other echinoderms, they are unique in the animal kingdom in simultaneously displaying: 1) a striking regenerative potential; 2) a key phylogenetic position (they are much closer to vertebrates than other invertebrate models employed for regeneration studies, such as planarians and Hydra); and 3) an impressively rich diversity of regeneration models amenable for an experimental approach. Although starfish arm regeneration has been well characterized, the regenerative potential of isolated arm explants has been explored only in other echinoderm classes (i.e. crinoids and ophiuroids). Therefore, this study aims to characterize the cell and tissue patterns of arm explant regeneration using the starfish Echinaster sepositus as model species. Explants were obtained by double chopping adult starfish arms. After pre-fixed regeneration time-points, they were processed for a histological analysis. Our results indicate that starfish explants not only can survive for several months but also undergo regeneration. In contrast to other echinoderms, both proximal and distal explant ends regenerate, even though with differences in timing and degree of tissue differentiation (polarity). This difference is more evident during the advanced morphogenesis: in fact, at the distal end both distalization (differentiation of terminal structures i.e. terminal tube foot and ossicle) and intercalation of new structures and tissues (i.e. ossicles, muscles, tube feet) occur, whereas at the proximal end only distalization and limited intercalation (i.e. some ossicles) are detected. Overall, starfish arm explant regeneration mainly relies on massive cell dedifferentiation and migration of adult tissues, underlining the extreme plasticity potential of their tissues. Although the polarity of tissue regrowth indicates the existence of a “proximal-distal memory”, the regenerative development of the explant proximal end differs significantly from other echinoderms. These preliminary findings represent the basis for future studies addressed to evaluate ultrastructural and molecular differences and/or similarities in regenerative capabilities among echinoderm classes and between echinoderms and vertebrates.