Regenerative potential following traumatic arm amputation in Echinaster sepositus

Although arm regeneration in Asteroids is a quite-well studied phenomenon, numerous important aspects have still to be defined in terms of both morphological and molecular patterns, and, due to the limited number of species examined so far, a comprehensive account of the variety of possible regenerative models is still lacking. In this work, the tissue and cellular aspects of the regeneration process following traumatic arm amputation (p.a.) were studied in the red starfish Echinaster sepositus until 16 weeks (W) p.a. by means of a microscopic approach, in order to deeply elucidate its cellular mechanisms and provide the necessary complement to the recently published molecular data related to this same species (Ben Khadra et al., 2014). As for other starfish the regenerative process can be subdivided into three main phases. The repair phase (1 h-1 W p.a.) is characterized by emergency reaction and wound healing: closure of the coelomic canals is performed mainly by coelomic muscle contraction, whereas re-epithelialization is due to the stump epidermis which spreads out providing a thin epidermal covering. In some cases, a phagocyte syncytium is detectable beneath the wound epithelium at 24 h p.a., whereas an edema area consisting of different cytotypes widespread within the newly-deposed extracellular matrix is evident between 72 h and 1 W p.a. The early regenerative phase (1 W-6 W p.a.) is characterized by stump tissue rearrangement (mainly muscle bundles/layers) and first signs of regenerative processes involving specific structures such as the radial nerve cord, the ossicles and the terminal tentacle. This latter can be considered as the \u201cdriving distal structure\u201d of the subsequent proper regenerative process: indeed, the new structures, such as tube feet, ossicles and muscles, are developed by gradual intercalation between the terminal tentacle and the stump with a proximal-distal gradient. The advanced regenerative phase (6 W-16 W p.a.) is characterized mainly by processes of skeletogenesis, myogenesis and neural differentiation (optic cushion, etc.) leading to the re-growth of a complete arm. Overall, E. sepositus arm regeneration following traumatic amputation can be considered mainly a morphallactic process during which the pre-existing tissues are rearranged and used as source of cells to re-grow the new structures. Our findings indicate the distalization-intercalation model as the most appropriate mechanism to describe the regenerative processes in this species