Regenerative potential of Echinaster sepositus (Retzius, 1783) arm explants: macro- and microscopic analyses

Although partial regeneration has been described in crinoid and ophiuroid arm explants, starfish are unique among the echinoderms in being able to regenerate a whole individual from single arm. The aim of this research was to provide a first description of the cell-tissue patterning occurring during regeneration of arm explants in the red starfish Echinaster sepositus. Specimens of E. sepositus were collected in the Marine Protected Area of Bergeggi Island (SV, Liguria). All the five arms of each animal were double amputated to obtain isolated explants. The latter were left to regenerate in the aquaria and collected at prefixed time-points (24 hours, 1, 3, 6 and 10 weeks post-amputation (p.a.)). The proximal and the distal side of each explant were photographed under a stereomicroscope to evaluate the general external anatomy and then processed for standard histological analyses. Our results indicate that, differently from crinoids and ophiuroids, in this species regenerative phenomena occur at both the proximal and the distal explant side, although differences in timing and extent of regeneration can be observed. Within the first week both sides accomplish the repair phase (wound healing). From 1 to 6 weeks p.a. early regenerative phenomena can be observed at the level of the terminal ossicle and tube foot (only drafted in the proximal side). After 10 weeks p.a. regeneration in the distal side has reached an advanced stage (miniaturized arm), whereas in the proximal side is delayed and partial (mainly aboral ossicles), the appearance of new metameric units (tube feet and associated ambulacral ossicles/muscles) being prevented. The distalization-intercalary regeneration model proposed for E. sepositus arm-tip regeneration [1] is appropriate to describe also explant regeneration in the distal side, whereas in the proximal one only distalization but not intercalation occurs. As in other Asteroids, the regenerative mechanism is mainly morphallactic; however, differently from arm-tip regeneration [1], in explants a massive cell recruitment apparently occur s from pyloric caeca rather than muscle bundles, thus underlining the notable plasticity of tissue remodelling exhibited by starfish. This work represents the cell-tissue basis for future studies addressed to analyse in detail explant regeneration and, in general, to shed light on the mechanisms underlining the remarkable regenerative abilities of starfish. [1] Ben Khadra et al., 2015. WRR 23(4), pp.623-634.