Re-growth, morphogenesis, and differentiation during starfish arm regeneration.

The red starfish Echinaster sepositus is an excellent model for studying arm regeneration processes following traumatic amputation. The initial repair phase was described in a previous paper in terms of the early cicatrisation phenomena, and tissue and cell involvement. In this work, we attempt to provide a further comprehensive description of the later regenerative stages in this species. Here, we present the results of a detailed microscopic and submicroscopic investigation of the long regenerative phase, which can be subdivided into two subphases: early and advanced regenerative phases. The early regenerative phase (1-6 weeks p.a.) is characterized by tissue rearrangement, morphogenetic processes and initial differentiation events (mainly neurogenesis and skeletogenesis). The advanced regenerative phase (after 6 weeks p.a.) is characterized by further differentiation processes (early myogenesis), and obvious morphogenesis and re-growth of the regenerate. As in other starfish, the regenerative process in E. sepositus is relatively slow in comparison with that of crinoids and many ophiuroids, which is usually interpreted as resulting mainly from size-related aspects and of the more conspicuous involvement of morphallactic processes. Light and electron microscopy analyses suggest that some of the amputated structures, such as muscles, are not able to replace their missing parts by directly re-growing them from the remaining tissues, whereas others tissues, such as the skeleton and the radial nerve cord, appear to undergo direct re-growth. The overall process is in agreement with the distalization-intercalation model proposed by Agata and co-workers. Further experiments are needed to confirm this hypothesis.