Regeneration of the radial nerve cord in a holothurian: a promising new model system for studying post-traumatic recovery in the adult nervous system.

After a complete transection, the radial nerve cord (RNC) of the adult sea cucumber Eupentacta fraudatrix quickly regrows and reconnects. The description of the major cellular events that accompany this regeneration is derived from light and transmission electron microscopy. Shortly after lesioning, the extensive nerve fiber degeneration and neuronal apoptosis occur. The gap in the cord created by the transection is rapidly bridged, at first by connective tissue and subsequently by regenerating nerve tissue. On either side of the wound, the ectoneural and hyponeural components of the injured RNC form separate tubular rudiments, whose epithelial walls are composed mostly of dedifferentiated glial cells, capable of mitotic division, but also contain some nerve fibers and occasional neuronal perikarya. It is suggested that the glial cells play a crucial role in regeneration not only by providing the supporting guiding scaffold for regrowing nerve fibers, but also by producing new neurons. Other mechanisms of post-traumatic neurogenesis may involve proliferation and/or migration of existing perikarya. The anterior and posterior regenerates grow towards one another and eventually fuse to restore the anatomical continuity of the RNC. Re-differentiation of gliocytes and accumulation of nerve cells in the newly formed regions of the nervous tissue make histological organization of the fully regenerated RNC indistinguishable from that of the intact cord. The authors suggest that the holothurian RNC provides a valuable experimental model, which opens new possibilities for exploring the fundamental mechanisms underlying regeneration of the nervous system in deuterostomes.