Cell and tissue patterning starfish arm regeneration: a microscopic overview

Purpose: to provide a comprehensive overview of cell and tissue patterning of starfish regenerative processes Methods: Two ecologically different starfish species (Marthasterias glacialis and Echinaster sepositus) were subjected to traumatic arm tip amputation and left to regenerate for prefixed periods (up to 16 weeks). In E. sepositus experiments on isolated arm explants (double-amputated) were also performed. Regenerating samples were analysed by light- and electron microscopy analyses. Results: In both species, in both species, arm tip regeneration follows three main phases: a) a repair phase resembling the vertebrate’s one and characterized by haemostasis, tissue remodelling, rapid re-epithelialization and “oedema” (granulation-like tissue) formation; b) an early regenerative phase, where no localized blastema is observed and extensive recruitment/recycling of myocytes occur in parallel to first differentiation events and regrowth of the main “pilot” structures,, i.e. radial nerve cord and coelomic canals; c) an advanced regenerative phase characterized by complete differentiation, morphogenesis and growth of the regenerate following a distalization-intercalation model. The arm explant model, although both amputated ends undergo partial tissue regeneration, emphasizes a polarized regenerative growth in proximal-distal direction. Conclusions: Our results confirm that starfish regeneration relies on a remodelling and recycling of the existing tissues (mainly myocytes) without any blastemal formation. This morphallactic regenerative process follows a distalization-intercalation mechanism according to proximal-distal growth patterning, as observed in other regenerative animal models (e.g. planarians, amphibians). Overall this overview underlines also that starfish can be valid regeneration models to be compared to vertebrates.