Microscopic anatomy of arm regeneration in the red starfish echinaster sepositus

Starfish are armed echinoderms well-known for their striking regenerative ability. Despite this, the process of arm regeneration is still not well defined in terms of both morphological and molecular aspects. Indeed, also considering the limited number of species examined so far, a comprehensive account of the variety of possible regenerative models is still lacking. In this work, arm regeneration was studied in the red starfish Echinaster sepositus until 16 weeks post-amputation (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 in other starfish, the regenerative process could be divided into three main phases. The repair phase (1 h-1 W p.a.) was characterized by a strong constriction of the stump sealing off the perivisceral coelom and a wound healing by stretched stump epithelial cells providing a thin epidermal monolayer covering the injury. Beneath the wound epithelium a network-like syncytium of phagocytes was detectable at 24 h p.a., whereas an oedematous area consisting of different cytotypes (including dedifferentiating myocytes) widespread in the newly-deposited extracellular matrix was evident between 72 h p.a. and 1 W p.a. The early regenerative phase (1 W-6 W p.a.) was characterized by further stump tissue rearrangement (mainly muscle components) and first signs of regenerative processes mainly involving the radial nerve cord, the ossicles and the terminal tube foot. This latter can be considered as the \u201cdriving distal structure\u201d of the subsequent regenerative process: indeed, the new elements, such as ossicles, muscles and tube feet, were developed by gradual intercalation between the terminal tube foot and the stump with a proximo-distal gradient. The advanced regenerative phase (6 W-16 W p.a.) was characterized mainly by skeletogenesis, myogenesis and differentiation of neural structures (optic cushion) leading to the re-growth of a complete miniaturized arm-tip. Overall, E. sepositus arm regeneration p.a. can be considered mainly a morphallactic process during which the pre-existing tissues are rearranged and used as source of cells/materials to re-grow the new structures. Our findings indicate the distalization-intercalation model proposed by Agata and co-workers (2007) as the most appropriate mechanism to describe the regenerative processes in this species