Post-traumatic nervous regeneration is a rarely seen phenomenon in
the central nervous system (CNS) of adult higher vertebrate animals.
Organisms with CNS injuries can suffer the loss of motor activity and
sensory perception and, as a result, develop limitations or
disabilities. Some animal species, on the other hand, are capable of
functional regeneration after an injury to their CNS. In these cases,
severed neuronal fibers can find their targets through axonal path
finding and neurogenesis can be activated. Efforts in different fields
have generated a large amount of information as to what directs or
limits the potential of neuronal cells to achieve proper functional
regeneration. However, continued research and new approaches are still
needed before we fully understand what governs the dynamics of injured
nervous tissues and its regeneration.
One of the approaches that have been used to study nerve
regeneration is the comparative analysis to determine the differences
between those species that readily regenerate their CNS and those where
regeneration is more limited [1-4]. The regeneration capabilities of the CNS have been said to decrease as we move higher in the phylogenetic tree [4,5].
Thus, relatively simple organisms such as Hydra and Planaria, can
regenerate their neuronal network after amputation of a body part .
Similarly, studies with mollusks and annelids (including leeches) have
shown that they are capable of regeneration after a crush-type injury
to one of the CNS tracts .
However, the assumption that organisms in the lower phylogenetic scale
are more able to regenerate their nervous system is not always true.
This is highlighted by the fact that crustaceans, as well as two of the
most studied invertebrate model animals, C. elegans and Drosophila, show limited CNS regeneration [5,7,8].
Among deuterosotomes, most of the studies on CNS regeneration have
been done on the spinal cord of vertebrates. Here again, organisms can
be grouped into good regenerators and poor regenerators. Among the
former are lampreys, fishes, and urodele amphibians, the latter include
most amphibians, reptiles, birds and mammals [4,9-11].
Some fish species have served as models to study axonal regeneration in
the CNS of vertebrates, as these animals are capable of functional
regeneration after a CNS injury [2,11,12].
Urodeles are among the most gifted to regenerate their CNS; following
an injury they can regenerate a spinal cord with some functional
recovery at any recovery time during their life cycle [3,13,14]. Other amphibians are also capable of regrowing spinal nerves after amputation of the tail, but only in an immature state [15,16].
Even though these models have provided detailed descriptions of the
problems and limitations that neuronal tissues must overcome before
functional regeneration, still a great deal of basic knowledge is
missing to understand how this phenomenon could be regulated and
neuronal regeneration enhanced.
CNS regeneration has been little studied in lower deuterostomes. In
ascidians, the regeneration of the neural complex has been shown to
occur after complete ablation [17,18],
but no study of nerve transection or partial injury is available. There
is no information on CNS regeneration in hemichordates or
cephalochordates. In echinoderms, organisms with an amazing
regenerative capacity, a few publications provide some information on
CNS regeneration, mainly describing what occurs following arm
amputation in asteroids and crinoids [19-22].
However, these experiments provide a brief description of nerve
regeneration that is part of an overall description of how the new arm
forms or focus on some specific processes such as cell proliferation
during regeneration. Only recently has a study on CNS regeneration in
echinoderms been done, but it is limited to transmission electron
microscopy . Our work confirms some of their findings and greatly expands the analysis of CNS regeneration in echinoderms.
In this study we use the sea cucumber Holothuria glaberrima to study nervous system regeneration. H. glaberrima,
is an echinoderm of the class Holothuroidea, and as such, is an
invertebrate deuterostome, placing this organism in close evolutionary
relationship with vertebrates. Moreover, these organisms have been
known for a long time to possess tremendous capabilities to
functionally regenerate most of their organs and tissues in a very
short time [24,25]. We have previously used this model system to study intestinal regeneration , the regeneration of the enteric nervous system  and the wound healing of the body wall .
The holothurian CNS main component is an anterior nerve ring, at the
base of the tentacles, from which 5 radial nerve cords (RNCs) exit and
travel within the bodywall, spanning the length of the animal and
ending blindly at the posterior end [24,29,30].
RNCs are subdivided into two components, the ectoneural (EN) or
hyponeural (HN) band which are separated by a band of connective tissue
(see Fig. 1A).
In general terms the HN component is thought to be primarily motor,
while the EN has both sensory and motor functions. RNCs are
ganglionated nerves, meaning that they have neurons throughout their
length. The neurons can be found in the periphery of both HN and EN
components with the central portion, or neuropile, made up of nerve
fibers. Innervation of organs occurs by peripheral nerves that emerge
from the RNCs. We have developed a technique that allows for an
accurate injury to be made to the RNC and are now able to study the
regenerative potential of the holothurian CNS. We report here that
following transection, re-construction of the nerve stumps occurs by 12
dpi, as a result of axonal growth; afterwards the RNC goes through a
remodeling phase to achieve its original morphology. Other events
associated with tissue development and regeneration were also observed,
including cell division, migration and apoptosis. With this work we set
the foundations for further studies on nervous regeneration by showing
that this animal model carries out the two most important processes for
CNS regeneration, explicitly neurogenesis and axonal pathfinding. More
importantly, these occur as a natural response to the injury, allowing
for future characterization of a permissive environment for possible