Re-exploring epimorphosis vs morphallaxis in echinoderm regeneration

Abstract

In echinoderms regenerative processes are common in both the adults and larvae and are utilized to replace the loss of different body parts, or for asexual reproduction. Thus echinoderms provide valuable deuterostome models for an integrated approach exploring regeneration from tissue repair to asexual cloning. According to tradition, regeneration involves two alternative basic mechanisms, epimorphosis and morphallaxis. In epimorphosis, new tissues arise from a pool of progenitor cells which are recruited to develop a typical blastema: this discrete proliferation centre contains intrinsic morphogenetic information required to re-pattern the regenerating structure. In morphallaxis, remodelling of the lost part occurs through extensive phenomena of rearrangement/recycling from differentiated tissues, without any blastema formation: only limited and localized proliferation involves cells derived from existing tissues by de-differentiation and/or migration. In echinoderms it was frequently debated if regeneration processes follow morphallaxis or epimorphosis. This basic aspect deserves to be carefully reconsidered, since it implies many crucial questions related to 1) stemness properties of responsible cells (stem cells or reprogrammed cells), 2) activities (proliferation and/or migration), 3) plasticity and differentiation potential (derived cellular phenotypes). Current research is addressed to explore the molecular mechanisms including specific factors involved and differential gene expression. In the present work a comparison of the regeneration mechanisms in representative echinoderm models is provided, with particular reference to crinoids and asteroids: the results, obtained by employing an integrated in vivo and in vitro approach, provide an insight on specificity of mechanisms and processes governing large-scale pattern formation and cell-tissue information signalling. In spite of the schematic epimorphosis vs morphallaxis view, our results show evidence of an unexpected adaptability at the tissue/cellular level and the mechanisms appear rather plastic and largely overlapped to each other, their contribution being different in the different models and even in the same individual according to the specific condition. In addition, in terms of histogenetic potential, different types of tissues appear to be able to give rise to pluripotential cells responsible for development of specific tissues/organs independently of their embryonic origin