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Published in final edited form as: Semin Cell Dev Biol. 2018 Sep 6;92:45–54. doi: 10.1016/j.semcdb.2018.09.001

THE MESENTERY AS THE EPICENTER FOR INTESTINAL REGENERATION

José E García-Arrarás 1,*, Samir A Bello 1, Sonya Malavez 1
PMCID: PMC6401350  NIHMSID: NIHMS1506277  PMID: 30193995

Abstract

The mesentery, a newly minted organ, plays various anatomical and physiological roles during animal development. In echinoderms, and particularly in members of the class Holothuroidea (sea cucumbers) the mesentery plays an additional unique role: it is crucial for the process of intestinal regeneration. In these organisms, a complete intestine can form from cells that originate in the mesentery. In this review, we focus on what is known about the changes that take place in the mesentery and what has been documented on the cellular and molecular mechanisms involved. We describe how the events that unfold in the mesentery result in the formation of a new intestine.

Keywords: regeneration, dedifferentiation, echinoderm, mesentery, invertebrate

1. Introduction

Adult sea cucumbers can undergo a process of evisceration where they eject their internal organs as a response to predation or stressful environmental conditions. These animals have the remarkable ability to regenerate the lost organs. As for example, it has been known for almost a century now that regenerating intestine can arise at the tip of the mesentery [1, 2]. Our lab has been studying this phenomenon for the last two decades. Our results point out to central role for the mesentery in the regeneration process. In this review we will describe (i) histology of the holothurian mesentery, (ii) cellular processes that take place within the mesentery during regeneration and (iii) molecular changes that underlie the regenerative event. However, as holothurians are not broadly referenced in the biomedical research literature, we will begin with a short description of these organisms and of others in the phylum Echinodermata.

2. Echinoderms as model animals

Sea cucumbers or holothurians belong to the Class Holothuroidea, one of five Classes that make up the phylum Echinodermata. Other classes include sea urchins (Echinoidea), sea stars (Asteroidea), brittle stars (Ophiuroidea) and the lesser-known crinoids (Crinoidea). Echinoderms are grouped in the Deuteromata evolutionary branch of animals and, as such, are phylogenetically closely related to vertebrates (FIG. 1). This relationship is mainly observed during embryological development and is less evident in adults due to the radial symmetry of their body plans. Adult sea cucumbers are a useful model for organ regeneration. They can be induced to eject their digestive tract by injecting potassium chloride into the coelomic cavity [3–6]. Evisceration follows a fixed pattern that is highly reproducible compared with other models of regeneration (i.e. surgical amputation or wounds). Sea cucumbers have the extraordinary capacity to regenerate their digestive tract, and some species can achieve this in less than four weeks post-evisceration. Moreover, some species have properties that provide additional opportunities for biomedical studies. In Holothuria glaberrima (the model system we use) these include: (i) a small scale allowing animals to be maintained in indoor aquaria but large enough to obtain the tissue in quantities needed for molecular experiments, (ii) they are abundant in local coastal waters and (iii) the regeneration of the intestine takes place at the tip of the mesentery (i.e. spatially isolated from other tissues or organs) [7].

Figure 1:

Figure 1:

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Phylogenetic relationship of echinoderms. The phylum Echinodermata represents a group of invertebrate deuterostomes along the basal branch leading to the Chordata.

Many cellular processes involved in the regeneration of the intestine have been described [7–10]. Additionally, multiple genes that are associated with the intestinal regeneration have been identified using high throughput sequencing and microarrays [11–13]. Thus, the sea cucumber H. glaberrima provides an excellent model to study digestive organ regeneration in adult animals.

3. Anatomical description of the holothurian mesentery

Holothurian digestive systems are mainly formed by a long intestine that can loop several times within the body cavity. Some species have a slender esophagus or an enlarged muscular region corresponding to a stomach [14, 15]. The digestive tube of H. glaberrima is composed of a pharynx, a short esophagus and looping small and large intestines which open into a cloaca [7] (Fig. 2). The initial descending and ascending segments are considered to be small intestine equivalents while the second descending portion is considered to be equivalent to the large intestine. The intestine is attached to the body wall by a mesentery that is continuous but varies in its attachment to the body wall: the mesentery of the descending small intestine body wall, while that of the large intestine attaches to the ventral body wall. Thus, the attaches to the dorsal body wall, the one of the ascending small intestine attaches to the lateral digestive tract mesentery presents an “S”-shaped organization that spirals from the anterior end (esophagus) to the posterior end (cloaca). As will be evident below, there are strong similarities between the mesentery of the holothurian digestive tract and that of mammals, suggesting that they also share some of the functions that are now being ascribed to what has been recently described as a “novel” organ [16].

Figure 2.

Figure 2.

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Schematic diagram of the internal anatomy of the sea cucumber Holothuria glaberrima. A. The digestive tract of H. glaberrima is composed of a short esophagus (E), an anterior descending intestine (ADI), a medial ascending intestine (MAI) and a posterior descending intestine PDI), ending in the cloaca (C). The first two intestinal regions correspond to the small intestine, while the third section corresponds to the large intestine. The middle intestinal section is associated with the hemal system and one respiratory tree. B. The regenerating intestinal rudiment forms as a thickening of the tip (Arrows) of the ventral and medial mesenteries.

Evisceration causes a rupture of the intestine from the esophagus and the cloaca, at the anterior and posterior ends, and from the attached mesentery, leaving the torn edges of the mesentery free in the body cavity [6]. The level at which the mesentery separates from the intestine can differ depending on the species. In Holothuria scabra, autotomy occurred in the mesentery close to the body wall [17]. This differs from H. glaberrima where it occurs at a site in the mesentery that lies close to the intestine and from Thyone briareus and Eupentacta quinquesemita where it occurs at the junction with the intestine [7, 18]. There have been some attempts at trying to determine anatomical features that pinpoint the area of the mesentery that autotomizes, though none has been found [17].

Significant macroscopic changes can be observed in the conformation of the mesentery during regeneration. The most prominent involves a change in the length of the attachments of the mesentery to the dorsal wall. These changes eliminate mesenteric looping and the “S” shape of the intestinal mesentery observed in normal animals reverts to an almost straight conformation from esophagus to cloaca [3–5].

Intestinal regeneration begins with a wound healing of the torn edge of the mesentery, during the first 24 hrs following injury (Fig. 3). It is in this area, at the tip of the mesentery (previously attached to the eviscerated organ) where the new intestine will form. Most studies of intestinal regeneration in holothurians focus on the formation of the rudiment that will eventually give rise to regenerated intestine, but few delve into the changes at the level of the mesentery.

Figure 3.

Figure 3.

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Autotomy and regeneration of intestinal tissues in holothurians. A. The digestive tract is attached to the body wall by a long mesentery (shown here in an abridged form for the sake of illustration). B. Following autotomy or evisceration the intestine ruptures from the mesentery, which remains attached to the body wall at one end and free within the coelomic cavity at the other. C. Regeneration begins with a thickening of the mesenterial tip. D. This thickening continues growing, forming a solid rod of tissue that extends from the esophagus to the cloaca. E. Eventually, this tissue is invaded by cells from the esophageal and cloacal luminal epithelium forming the intestinal lumen.

Little data is available regarding the histology of the mesentery, other than to say it comprises three layers [14, 19, 20]. The mesentery is composed of two different tissue layers, the outermost mesothelium and an inner connective tissue, whereas the digestive tube consists of three layers, the innermost luminal epithelium (digestive epithelium), the mesothelium and the connective tissue layer between the two epithelia (Fig. 4). The two layers of mesothelium that surround y is continuous with the connective tissue of the digestive tract (submucosa) at one end anthe connective tissue of the mesentery are continuous with the mesothelium of the digestive senterd with the connective tissue of the body wall at the other. As described by sevtract and also with the coelomic epithelia of the body cavity. Likewise the connective tissue of the meeral authors, the histological continuity between the mesentery and body wall, or visceral organs, is conserved in other deuterostomes up to and including Homo sapiens. [21–23]. However, this continuity has received little attention.

Figure 4.

Figure 4

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Histology of the holothurian mesentery. A. Diagramatic representation of the mesentery. The mesentery is composed of two mesothelial layers with a middle layer of connective tissue. The mesothelium itself is made up of coelomic epithelial cells or peritoneocytes (dark blue) and muscle cells (light blue), both lying on top of the basement lamina that separates them from the connective tissue. Neuronal cells and fibers (green) are found within both the connective and mesothelial layers although they are much more abundant in the latter. B. Histological section of the mesentery labelled with immunohistochmeistry, shows the presence of cell nuclei (DAPI nuclear stain-blue), muscle fibers (Fluorescent phalloidin staining-red) and the separating basement membrane (green). Bar = 40μm

The mesothelium is a complex tissue separated from the underlying connective tissue by a basal lamina and composed of different cell types: coelomic cells (peritoneocytes), myoepithelial cells which form a muscular layer, and neurons which are part of the visceral nervous plexus. Mesenteric mesothelium can be considered to be a pseudostratified myoepithelium [24–27]. When viewed in detail, the peritoneocytes form the apical (outer) cell layer. These cells have heterochromatic nuclei and cilia. They extend projections that form intercellular junctions among them. Myocytes form the basal layer and are distributed along the axis of the mesentery. Their nuclei are usually euchromic, located apically and elongated along the axis of the muscle fiber. Both myocytes and peritonoeocytes are attached to the basal lamina that separates the mesothelium from underlying connective tissue.

The connective tissue layer of the mesentery is populated by mesenchymal cells with varying phenotypes. These resemble cells in intestinal connective tissue. Various extracellular matrix (ECM) components have been identified, including collagen, fibronectin, laminin, proteoglycans and some still undetermined components [23, 28, 29].

Nervous elements in both mesothelium and connective tissue layers of the mesentery 21, 30, 31]. In the latter, the nervous component is made up of isolated small neuron-like cells with long and fine neurites. They resemble neuron-like cells found in intestinal connective tissue. The latter have been previously described by Díaz-Balzac and colleagues [32]. In contrast, the nervous component of mesothelium is made up of large and abundant fiber bundles that run from the body wall to the intestine. Neuronal cells are often found in the fiber network, and extensions from these bundles innervate the mesothelial muscle layer.

Finally, in contrast to the vertebrate mesentery, the holothurian organ lacks lymphatic or vascular components.

4. Cellular processes during regeneration

Many cellular events are observed during the regeneration of the digestive tract in sea cucumbers [7]. However, the initial, and most important processes involve a substantial reorganization and remodeling of mesenteric tissues. This involves extensive dedifferentiation of mesothelium and remodeling of the connective tissue. These initial changes are thought to “prepare” cells for migration to the free mesenterial tip where the new intestinal rudiment will form. Dedifferentiated cells then re-enter the cell cycle to provide the cells that populate the intestine. Next, some of the cells covering the free edge of the mesentery undergo an epithelial to mesenchymal transition and invade the underlying connective tissue [8]. At 5 days post-evisceration (dpe) the distal edge of the mesentery has thickened and grown in size acquiring an elongated oval or tear morphology. At 7 dpe the intestinal rudiment is observed as a solid rod-like structure composed of a large number of cells within an inner connective tissue that is surrounded by a thin coelomic epithelium (Fig. 3). At 14 dpe the lumen has already formed and the mucosal epithelium has regenerated by the invasion of tubular outgrowths of mucosal epithelium from the esophagus and the cloaca. The basic structural format of the intestine has already been established within two weeks following evisceration. By the third week the intestine enlarges and start functioning.

Most of our studies have focused on the formation of the intestine as such. Nevertheless, it is important to highlight that the cells that form the intestine originate from the mesentery and possibly many of the extracellular signals that direct or modulate the process also arise within the mesenteric tissues. In this review we will focus on the pivotal roles played by the mesenteric tissues in intestinal regeneration. In particular, we will describe the dedifferentiation of cells, the remodeling of the ECM, the migration of cellular components and a possible source of modulating factors.

4.1. Cell dedifferentiation-Mesenterial origin of cells for blastema formation

Dedifferentiation is a process by which cells develop in reverse, from a more differentiated to a less differentiated state [33]. This transition involves changes at the gene, protein, morphological and function levels. Dedifferentiation has been observed during regeneration in different tissues and organs of plants, invertebrates, and vertebrates including mammals. In fact, cells of many vertebrate and invertebrate animals, particularly those in the deuterostome clade undergo dedifferentiation as a part of a regenerative event [33]. For example, limb regeneration in newts involves the phenotypic reversion of fully differentiated muscle, cartilage, and glial cells, among others, that once dedifferentiated they proliferate for a short time to form a mass of progenitor cells named a blastema. Cells within the blastema then redifferentiate to build a replica of the lost part of the limb [34].

During intestinal regeneration in the sea cucumber H. glaberrima, mesothelial cells of the mesentery dedifferentiate [8, 27]. The two main cell types of the mesenteri, c mesothelium, peritoneocytes and muscle cells, dedifferentiate. The former have been studied using electron microscopy where it has been shown that they cleave their long bundles of intermediate filaments into smaller fragments [26]. The muscle cells have been studied more extensively, since their actin bundles can be labeled using fluorescent phalloidin (Fig. 5). These cells degrade their contractile apparatus and the myofilaments are encased in membrane bound structures named “spindle-like structures” (SLSs) [35]. SLSs are eventually ejected from the cells and either end up in the coelomic cavity or in the ECM where they are degraded by phagocytic ameobocytes. SLSs are sometimes also found within the cytoplasm of peritoneocytes. Dedifferentiated mesothelial cells remain connected to each other by intercellular junctions, but the underlying basal lamina disappears. (Fig. 2B) [26, 28, 36]. Changes in the shape of mesothelial cells occurs concomitantly with the remodeling of the cytoskeleton. These cells develop pseudopodium-like protrusions and invade the underlying connective tissue [8]. Following the dedifferentiation, cells with a high nucleus-to-cytoplasm ratio appear in the connective tissue (Fig. 6) [27, 37].

Figure 5.

Figure 5.

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Muscle cell dedifferentiation. During regeneration the mesenterial muscle dedifferentiates forming spindle-like structures (SLSs) (green) that contain the contractile apparatus. Both the muscle fibers in the proximal mesentery (PM) and the SLSs in the mesentery adjacent (AM) to the regenerating rudiemnt are labeled with fluorescent phalloidin (green). A-B. The differentiation of muscle cells occurs in a gradient, starting close to the tip of the mesentery (B) while muscle near the body wall (A) retain its fiber morphology (nuclei-red). B’ is an insert showing a magnification of B and specifically a large number of SLSs . BW-body wall, PM-proximal mesentery, MM-medial mesenetery, AM-adjacent (to the rudiment) mesentery. Bar = 60 μm (A&B), 20 μm (B’).

Figure 6.

Figure 6.

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Electron microscopy of small undifferentiated cell. These cells showing a round nuclei and little cytoplasm are found in the mesentery following muscle dedifferentiation. They are thought to be precursors of the cells that form the new intestinal tissues. BL-basemente lamina, CT-connective tissue, CE-coelomic epithelium, UC-undifferentiated cells. Bar = 3μm (From Candelaria-Suria 2005).

Muscle cell dedifferentiation follows a spatio-temporal gradient (Fig. 5). In the early stages of regeneration, dedifferentiating cells are found in the area of the mesentery close to the tip where the intestine was previously attached [8]. Little or no dedifferentiation is observed in the rest of the mesentery. However, as regeneration progresses, muscle cells in the intermediate zone of the mesentery begin to dedifferentiate with little dedifferentiation observed close to the body wall. Thus, dedifferentiation occurs in a gradient where three areas can be observed within the regenerating mesentery: The first area is close to the regenerating rudiment, has no muscle fibers and few if any SLAs remain. The second area exhibiting dedifferentiation is in the mid-region of the mesentery, contains few muscle fibers but greater number of SLSs. The third area, where dedifferentiation is not seen, contains many muscle fibers, few SLSs and is adjacent to the body wall.

4.2. ECM Remodeling-Re-organization of the cellular/matrix scaffold

Mesothelium is not the only tissue undergoing important changes during regeneration. A dramatic remodeling of the connective tissue layer occurs [28, 38]. This involves a loss of fibrillar collagen and the concomitant activation of matrix metalloproteases (MMPs). Collagen is absent as shown using immunocytochemistry and western blot. These changes appear to result from dynamic cellular events involving phagocytic cells (Fig. 7).

Figure 7.

Figure 7.

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Loss of collagen expression in the regenerating mesentery. A. Histological section labeled with anti-collagen antibody (green). As the free end of the mesentery thickens, fibrillar collagen expression (green) is lost. Phase constrast microscopy together with the immunohistochemical analysis shows cells (arrows) that appear to be involved in the degradation of collagen are within the area of collagen dissapearance (Nuclei-blue). Bar = 100 μm (A), 15 μm (B)

Degradation of the ECM component (similar to cellular dedifferentiation) occurs in a distal-proximal gradient [28]. However, the timing is slightly later than cellular dedifferentiation [8]. Thus, little collagen degradation is observed at 3dpe. At 7 dpe, few if any collagen fibers are observed in mesenteric connective tissue adjacent to regenerating rudiment. At this stage, collagen fibers are abundant in the section of the mesentery and in the area adjacent the body wall. As time passes, collagen is lost from a larger portion of the mesentery, with some fibers still found in the area near the body wall.

It is suggested that some phagocytic cells involved in collagen decomposition, ingress from the mesothelial layer following dedifferentiation. Whatever their origin it is likely that phagocytic cells use MMPs in their degradation of collagen. The concomitant increase in MMP activity observed during regeneration. as the results of functional assays where MMP inhibitors are injected into regenerating animals are strong evidence for the role of MMPs in ECM remodeling. In support of this, intracoelomic injections of MMP inhibitors not only prevent the degradation of the collagen within the mesentery but also have an overall negative effect in the intestinal regeneration process [28]. Laminin and collagen share a similar fate during ECM remodeling while fibronectin and proteoglycans appear to remain unaltered [28, 29].

ECM remodeling usually takes place during regeneration and is regarded as essential for this to occur. It is suggested that remodeling provides an extracellular environment that enables cells to migrate and proliferate during tissue reorganization. For example, mesothelial cell migration an ingression toward connective tissue would require changes to the basal lamina. A further possibility is that ECM remodeling exposes or releases factors that control events such as survival, proliferation, migration and differentiation.

4.3. Cell division

An increase in cellular proliferation is observed to closely follow the dedifferentiation within the mesentery [7, 8]. Cell division in the mesothelium increases from 1% (at 3 dpe) to 19% in the area close to the regenerating rudiment and to 8% in the area in the middle of the mesentery (at 7 dpe). Similarly, cell division increases in the connective tissue of the mesentery, from no cell division to up to 30%. (It is important to remember that the number of cells within the connective tissue is rather small, thus an increase in percentage of cell division does not necessarily correlates with a large increase in total cell numbers). Cell division within the mesentery returns to normal levels at about 10–14 days. It is important to highlight that cell division within the mesothelium of the growing rudiment, by contrast, increases after the first week of regeneration and remains high as the new organ continues to grow in size. Cell division, similar to cellular dedifferentiation and ECM remodeling, follows a gradient where the higher levels of division are observed in the distal tip of the mesentery, that is, in the growing rudiment or in the region of the mesentery where it is attached.

4.4. Cell migration

Although the mesentery has been proposed as a route for cell migration during intestinal regeneration, there is little concrete evidence to validate this. For example, there is no in vivo microscopic evidence of the migration of individual cells. Until such direct evidence is available the migration of cells via the mesentery remains putative. Two main migration paths have been proposed. The first occurs in the mesothelium, dedifferentiated muscle and peritoneocyte cells form a regenerating epithelium that is thought to migrate as a sheet toward the mesentery tip. According to this model, cells remain connected in the sheet, but retain the ability to proliferate, as the sheet migrates to surround the connective tissue of the intestinal blastema. The second pathway involves individual cell movements through the connective tissue of regenerating mesentery. Various cell types have been proposed to migrate via this tissue including dedifferentiated muscle [27, 37], spherulocytes [22] and nervous system cells [21, 31].

The strongest evidence to support cell migration relates to a cell type similar to the vertebrate mast cell, called the spherulocyte. In echinoderms this cell type is found in connective tissue compartments and their release of spherules or vesicular content has been associated with the formation of connective tissue as well as with immune reactions [39]. During regeneration, their numbers in the mesentery increase transiently, supporting possible migration from the body wall to the intestinal rudiment [22]. Transit of spherulocytes via the mesenteric connective tissue is disrupted with injections of RGDS peptide [23]. This peptide binds to receptors in the cell membrane, such as integrins, thus disrupting the association of the cell with ECM molecules and thus, the migration mechanism. Injections of RGDs into regenerating animals decrease the number of putative migrating spherulocytes in the mesenteric connective tissue and concomitantly slow down the intestinal regenerative process.

4.5. The mesentery nervous system

In mesenteries of regenerating animals, large numbers of fiber persist that run from the body wall to the tip of the mesentery (Fig. 8). A small retraction is observed at the very tip, where a section of the mesentery adjacent to the growing rudiment is temporarily devoid of the nervous fibers. However, these grow back and enter the new intestinal rudiment during the second week of regeneration. Similarly, fibers are observed within the connective tissue layer throughout the regenerative process [31]. The main difference observed when compared to normal mesenteries is that the cell bodies of some neuronal cells are clearly visible and in some cases appear to ingress from the mesothelium into the connective tissue layer. Thus, in contrast to most other components of the mesentery, the nervous component undergoes little change during the regeneration process. This finding is rather surprising in view that the underlying muscle layer is lost by dedifferentiation in most of the mesentery and that the connective tissue, as well, is largely remodeled.

Figure 8.

Figure 8.

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Mesentery nerve bundles. Mesentery whole mounts labeled with an antibody against holothurian START-10 protein (Rosado-Oliveri et al., 2017) (green) label the mesothelial nerve bundles. The nerve bundles are slightly altered during regeneration, even in areas where the underlying muscle practically disappears and extensive changes in extracellular matrix are observed. Bar = 60 μm

Nonetheless, the nervous system within the mesentery may also play an important role in intestinal regeneration. First, it provides the extrinsic innervation of the intestine [30, 31]. Second, some enteric neurons might be generated by precursors or mature neurons migrating from the body wall (or the radial nerve cords) to the intestinal rudiment through the mesentery, similar to the trans-mesenteric migration of neural crest during development [40]. Dedifferentiated mesenteric mesothelium could represent an alternative source of enteric neurons. Dedifferentiated mesothelia have been proposed as precursors of most of the cell types observed in the regenerated intestine [8, 41]. β-tubulin (a neuronal cell marker) immunopositive cells arise in primary cultures from regenerating intestine. These are small and round, and their presence supports the suggestion that dedifferentiated mesenteric mesothelia may contribute to the enteric nervous system [42]. These cells are not found in non-eviscerated organisms or in advanced stages of regeneration.

Finally, it is also possible that the mesentery nervous system secretes growth and survival factors that modulate the regeneration of intestinal tissue components. This occurs in many animal systems in which it has been shown that the presence of nerves is required for regeneration to occur [43, 44].

5. Molecular aspects of regeneration

Concurrent with the cellular events observed in the mesentery during the intestinal regenerative process, major changes in gene expression appear to take place [12, 13, 45, 46]. Unfortunately, most gene expression comparisons have been made between regenerating and normal intestine, and the mesentery represents only a minor proportion of the tissues within the normal intestine. Only now are we in the process of comparing gene expression between mesenteries of normal and regenerating animals to specifically identify the gene activation and inhibition processes that take place.

In parallel, by focusing on specific gene candidates and determining their localization using immunohistochemistry or in situ hybridization, we have strong evidence that major changes in gene expression occur (Fig. 9). For example, the expression of gene products for genes encoding ubiquitin and protesome subunits is detectable in the mesentery adjacent to the intestinal rudiment [47]. Similarly, in situ detection of gene expression shows that genes such as survivin, mortalin, Myc, Oct1/2/11, Klf1/2/4, Wnt9, Bmp1/TII are expressed at higher levels in the mesentery near the growing intestinal rudiment that in the mesentery of normal, non-regenerating animals [11, 48, 49]. In some cases expression appears to be assymetrical, being higher on one of the mesothelial layers than on the other [48]. Most of these studies have focused on expression within the regenerating rudiment, and in these studies only the adjacent segment of the mesentery is examined. No study has focused on the expression changes within the mesentery as a whole. This is concerning given that, as shown above, many cellular processes within the mesenteric tissues occur along a spatial gradient with large differences observed when the region close to the body wall is compared to that adjacent to regenerating intestine. We would anticipate that the expression of many genes differentially expressed during regeneration, would follow the gradient observed for the cellular processes described above.

Figure 9.

Figure 9.

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Gene expression in the regenerating mesentery. In situ characterization of gene expression in the regenerating mesentery shows specific gene expression in the mesothelial cell layer for (A) Wnt9 and (B) TCTP at 7days of regeneration and (C) TCTP at 14 days of regeneration. Notice that the expression in some cases appears to be higher in one of the mesothelial layers than in the other. Bar = 130 μm (From Mashanov et al. 2012.)

6. Concluding remarks-

The role of the mesentery in regeneration of the digestive tract has not been well studied in other animals. The mesentery has been shown to play a role in regeneration of the pyloric caeca in members of at least one other echinoderm class, the sea star Henricia leviuscula [50, 51]. More recently, mesenterial cells were documented to participate in the regeneration of the crinoid digestive tract [52]. Outside of the Echinodermata, digestive tract evisceration has been documented in ascidians (Chordates), animals that also show a large regenerative capacity [53]. However, in these models it is not known how regeneration occurs and whether the mesentery is involved. In vertebrates, only one group [54, 55] adressed the role of the mesentery in intestinal regeneration and only then in an indirect way. This group studied intestinal regeneration in the transected intestine of the salamander and concluded the mesentery was not involved. Thus, the issue of whether the mesentery plays a role in the regenerative phenomenon in other animals, or whether it can be induced to play such a role is open to future investigation.

In addition, it would be interesting to determine if events observed in the holothurian mesentery also applied to the human mesentery in normality and disease. For example, the mesentery has recently been associated with intestinal inflammation in Crohn’s disease [56, 57]. Could it be possible that the ECM remodeling observed during regeneration can be somehow linked to the changes in ECM observed in Crohn’s and other inflammatory bowel diseases? [58, 59]. Another medical condition where the holothurian mesentery might shed some light is the formation of mesenteric adhesions. These are important clinical problems that usually occur following surgery [60]. Circumstantial evidence from our group and others suggest that the regenerating mesentery (probably the dedifferentiating mesothelium) becomes adhesive or can induce adjacent tissues to become adhesive. For example, Dawbin [4] reports of several cases of regenerating sea cucumbers (Stichopus mollis) that formed mesenteric adhesions during the process of intestinal regeneration. In addition, we have recently set up in vitro cultures for intestinal explants and have observed multiple cases where the regenerating end of the mesentery adheres to and fuses with another region of the mesentery that it comes in contact with. Thus, it is possible that the holothurian mesentery might offer insights not only to deciphering the process of regeneration but also to the etiology of some diseases of the human digestive tract.

Highlights.

  • Cellular changes in the mesentery occur soon after evisceration.

  • These changes give rise to cellular precursors that form the new intestinal rudiment.

  • Muscle dedifferentiation and cellular proliferation are the main mechanisms of the regeneration process.

  • Other mechanisms involve apoptosis, extracellular matrix remodeling and epithelial to mesenchymal transitions.

  • Major changes in gene expression profiles accompany the regeneration event.

Acknowledgements

The authors would like to thank Ms. Griselle Valentin-Tirado for technical help in the preparation of figures and for critical reading of the manuscript. This work was supported by NIH R15GM124595 and R21AG057974. We also acknowledge partial support from the University of Puerto Rico.

Footnotes

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