The enteric nervous system of echinoderms: unexpected complexity revealed by neurochemical analysis

Echinoderms are one of the most important groups of metazoans from the point of view of evolution, ecology and abundance. Nevertheless, their nervous system has been little studied. Particularly unexplored have been the components of the nervous system that lie outside the ectoneural and hyponeural divisions of the main nerve ring and radial nerve cords. We have gathered information on the nervous components of the digestive tract of echinoderms and demonstrate an unexpected level of complexity in terms of neurons, nerve plexi, their location and neurochemistry. The nervous elements within the digestive system consist of a distinct component of the echinoderm nervous system, termed the enteric nervous system. However, the association between the enteric nervous system and the ectoneural and hyponeural components of the nervous system is not well established. Our findings also emphasize the importance of the large lacunae in the neurobiology of echinoderms, a feature that should be addressed in future studies.

Regeneration of the enteric nervous system in the sea cucumber Holothuria glaberrima

Among higher metazoans, echinoderms exhibit the most impressive capacity for regeneration. Holothurians, or sea cucumbers, respond to adverse stimuli by autotomizing and ejecting their visceral organs, which are then regenerated. Neuronal fibers and cell bodies are present within the viscera, but previous regeneration studies have not accounted for the nervous component. We used light microscopic immunocytochemistry and ultrastructural studies to describe the regeneration of the enteric nervous system in the sea cucumber Holothuria glaberrima. This study provides evidence that the enteric nervous system of this echinoderm regenerates after evisceration and that in 3-5 weeks the regenerated system is virtually identical to that of noneviscerated animals. The regeneration of the enteric nervous system occurs parallel to the regeneration of other organ components. Nerve fibers and cells are observed within the mesenterial thickenings that give rise to the new intestine and within the internal connective tissue prior to lumen formation. We also used bromodeoxyuridine incorporation to show that proliferation of the neuronal population occurs in the regenerating intestine. The regeneration of the nervous system commands high interest because members of the closely related phylum Chordata either lack or have a very limited capacity to regenerate their nervous system. Thus, holothurians provide a model system to study enteric nervous system regeneration in deuterostomes.