Nervous system and arm regeneration in crinoids

The crinoid Antedon mediterranea has remarkable regeneration abilities, making it a valuable model for the study of these post-embryonic developmental phenomena. Indeed, it can rapidly and completely regenerate arms, pinnules, cirri and the visceral mass, after autotomy or experimentally induced amputations. During regeneration a variety of neural regulatory molecules (neurotransmitters, neuropeptides and growth factors) are thought to play important signalling roles. Here we have analysed the ultrastructural features and some molecular aspects of the nervous system of A. mediterranea in normal and regenerating arms. Animals were collected from Baia delle Grazie in the Gulf of La Spezia (Ligurian Sea, Italy) and maintained in artificial sea-water. Experimental amputations were carried out mimicking natural conditions and regeneration was examined at early (72h post-amputation (pa)) and advanced stages (up to 4 weeks pa). Some tissue samples were processed for standard histological and ultrastructural studies whereas others were prepared for analysis of neuropeptide expression using mRNA in situ hybridization and immunocytochemical techniques. In crinoid arms the main nervous component is the brachial nerve formed by a medulla mainly composed of neural processes and sparse chains of perikarya and an external cortex of perikarya. There is also some evidence of well-defined groups of perikarya at the level of muscle and ligament innervation, that could potentially be considered as a basic form of ganglia thus hypothetically suggesting metamerism of the nervous system. However, this “perikarya clustering” needs to be confirmed by further studies. In the early regenerative phase, the nerve has not yet fully regrown within the regenerate but a continuity can be observed between chains of nervous cells and the undifferentiated cells filling the blastema that is the characteristic structure of this phase. The advanced regenerative phase is characterized by the regeneration of the brachial nerve and the progressive differentiation of other tissues. Analysis of A. mediterranea transcriptome sequence data has enabled identification of transcripts encoding precursors of a variety of neuropeptides (SALMFamides, calcitonin-type, vasopressin-type) and on-going studies are investigating the expression of these and other neuropeptides, which will provide a basis for investigation of the physiological roles of neuropeptides in both non-regenerating and regenerating arms.