Nerve Reconnection After Transection in the Echinoderm Holothuria glaberrima

Abstract

Nerve regeneration and the necessary axonal pathfinding have been greatly studied; however there are still some hurdles that limit the growth of the field and the development of the right tools and approaches to achieve correct and accurate regeneration. We have now used a new model system, the echinoderm Holothuria glaberrima, that with its exceptional regenerative capacities and ganglionated nerve cord, provides an excellent system to study neurogenesis and axonal regeneration. The radial nerve was transected and allowed to regenerate for 2, 6, 12, 20 and 28 days. Spatial and temporal changes in cellular and tissue elements, including apopotosis and proliferation, were observed using classical dyes, immunohistochemistry, TUNNEL assay and BrdU incorporation. Regeneration began with growth of fibers exiting nerve stumps by day 6, connections between these stumps occurred by day 12. Afterwards, the regenerated segment continued increasing in thickness and acquired the typical arrangement of nuclei within the nerve by 28 days. Regeneration was accompanied by intense cellular division, especially during days 6–12. Also, apoptotic cells were seen throughout the regeneration process, even after reconnection. We conclude that H. glaberrima is able to regenerate its nerve cord and that this phenomenon involves cell division, apoptosis and directed axonal growth. This work sets the foundation for further studies on nerve regeneration in a model animal that promises to reveal information on the crucial differences that permit nerve regeneration to occur as a swift innate response in some organisms but not in others. Funded by NSF (IBN-0110692), NIH-MBRS (S06GM08102), NIH-RCMI (RRO-3641-01) and the UPR-RP MARC Program (5T34GM07821).