Arm-tip regeneration in the adult red starfish Echinaster sepositus (Echinodermata, Asteroidea) following traumatic amputation

Regeneration is a distinct type of developmental process during which a lost or severely injured part of the organism is reformed. It can occur at multiple levels of the biological organization, at different stages of the life cycle and can produce structures often quite similar to the original. Starfish (Echinodermata, Asteroidea) have remarkable regenerative capabilities: indeed, they can completely replace entirely functional arms lost after traumatic or self-induced mutilations. Aim of this research work is to describe the morphological aspects, namely the tissue and cellular patterning, of arm regeneration in the Mediterranean red starfish Echinaster sepositus by means of light and electron microscopy analyses. E. sepositus arm regeneration process can be subdivided into three overlapping phases: a) a repair phase (1 hour-1 week p.a.), characterized by wound sealing, healing (re-epithelialization), “edema” formation, but also first signs of neurogenesis; b) an early regenerative phase (1 week-3 weeks p.a.), during which dedifferentiation, rearrangement and beginning of first differentiation phenomena (such as skeletogenesis) occur and c) an advanced regenerative phase (after 3 weeks p.a.) characterized by massive cell recruitment from stump structures (e.g. podia), differentiation, morphogenesis (mainly myogenesis and skeletogenesis) and growth (all body systems, in particular nervous tissues and water vascular system), leading to the formation of a complete miniaturized arm-tip. Overall our findings indicate that regeneration in this species is mainly a morphallactic process which usually consists of a rearrangement of the existing tissues through cell dedifferentiation, differentiation and/or migration in order to regenerate the lost body structures, without formation of a localized undifferentiated blastema. Further deeper investigations (e.g. biochemical and molecular analyses) on this intriguing field are strongly necessary in order to confirm that morphallaxis is the main regenerative mechanism in Asteroids.