Sea cucumbers are well known for their ability to regenerate their digestive system as they can eviscerate and then regenerate their internal organs, including the gut. After evisceration the remaining mesenteries play a key role in gut regeneration. The laboratory of Jose García-Arrarás has published some reports describing how during regeneration an intestinal primordium develops from a thickening of the mesenterial edge. With time this thickening grows up to the formation of a tube that will become the regenerated gut. One of the components of the sea cucumber gut is the enteric nervous system (ENS) that innervates the gastrointestinal tract. Now, a recent paper from this same laboratory describes for the first time in detail the regeneration of the enteric ENS in the sea cucumber Holothuria glaberrima (http://onlinelibrary.wiley.com/doi/10.1002/reg2.15/abstract).
Despite some studies have reported that miss-function of the ENS can cause several neuropathies both neurodegenerative and inflammatory in mammals, very few studies have analyzed the regenerative potential of this system under those contexts. Here, the authors use the sea cucumber to study how the ENS regenerates de novo together with the new gut after evisceration. In a first set of experiments, the authors use a collection of antibodies to label specific neural projections and cells of this ENS. Although, as the authors point out, it is not clear what are the antigens recognized by these antibodies, they are useful to consistently label specific neural fibers and cell populations and see how this pattern is restored during regeneration.
The main components of the ENS were divided depending on their location within the mesothelial layer, the connective tissue and the luminal layer. In the mesothelium they found a fiber network within the muscle layer that innervates the visceral muscle. There is a second mesothelial plexus formed by large nerve bundles that run along the longitudinal axis of the gut, and mostly parallel to the longitudinal muscle fibers. Within the connective tissue there are thick fibers that would correspond to nerves connecting the mesothelial plexus with the connective and luminal layers. There is also a network of small neurons and fine fibers all throughout this connective layer. Finally, the luminal plexus is formed by neuroendocrine-like cells distributed among the luminal epithelial cells. Occasionally, some of these cells display fiber projections extending towards the mesothelium.
Here, the authors used their markers to follow the regeneration process of all these components of the ENS at 3, 5, 7, 10, 14, 21, 28 and 35 dpe (days post evisceration). Summarizing all their stainings ENS regeneration could be divided into several stages: 1) initially there was a neurodegeneration stage consisting in the degradation of the preexisting fibers within the mesentery edge that will give rise to the intestinal primordium (5-7 dpe); 2) during the first days, then, the intestinal primordium lacked any ENS innervation (8-10 dpe); 3) then, re-innervation of this primordium started by 14 dpe. Here, new fibers appeared, mainly proximally in the mesothelium from an extrinsic source, that is, from cells in the mesentery. Moreover, new cells (of intrinsic origin) appeared also within the connective tissue, also mainly in proximal regions; 4) the fourth stage at around 21 dpe, was characterized by the differentiation of large fibers crossing from the mesothelium to the connective tissue as well as for the intrinsic differentiation of new neural cells within the lumen epithelium; 5) by 28 dpe most of the mesothelium was innervated and no differences between proximal and distal areas were observed; finally 6) by 35 dpe the ENS pattern in the regenerated intestine was similar to normal non-eviscerated intestine.
It is important to point out that ENS regeneration occurred in parallel to other important events. Thus, for example, the initial neurodegeneration coincided with the remodeling of the extracellular matrix. Also, fiber regeneration coincided with myogenesis and the incoming neural fibers were possibly re-innervating the newly differentiated muscle fibers. An important question to answer in future experiments concerns the origin of the new ENS cells. The authors discuss that they could originate from the dedifferentiation of muscle cells or coelomic epithelial cells or, alternatively from enteric stem cells or glia present in the intestine or mesentery. In the case of the neuroendocrine cells they seemed to differentiate from the luminal epithelial cells.
In summary, the authors describe here the different stages that characterize the regeneration of the ENS in sea cucumbers. As some events are conserved in mammals following lesions or inflammatory responses as, for example, the observed degeneration-regeneration stages, H. glaberrima can be a good model to understand ENS plasticity in other models. Moreover, there is an obvious interest for bioengineers trying to obtain intestines for transplantation and, as the authors state, studying ENS regeneration could provide insights into the type of cells and timing at which ENS precursors should be added in order to make a properly functional intestine.