During the last years, changes in muscle layers after massive small bowel resection have been controversially discussed. While several authors described a hypertrophy of intestinal muscle layers as a result of intestinal adaptation in SBS experiments on rats (Chen et al. 2012; Chen et al. 2015; Chen et al. 2013), others did not find evidence for a hypertrophy of the muscle layers (Martin et al. 2008).
Despite the fact that the ENS is crucial for bowel motility and its tighty connected to intestinal smooth muscles (Lotfollahzadeh et al. 2021), the changes in ENS after massive small bowel resection have not been published yet.
Here in this study we have analyzed the intestinal muscle layers and the ENS together as a neuro-muscular complex. In order to assess the plasticity of the ENS (Schafer et al. 2009) in the intestinal adaptation to SBS, we used nestin, a well-known ENS precursor cell marker (Vanderwinden et al, 2002(Grundmann et al. 2016), (Kulkarni et al. 2017), in both rats and humans samples after massive small bowel resection.
Our results show that the thickness of the small bowel muscle wall significantly increased in the SBS group in both proximal and distal segments. Many previous published human data on morphological changes of the small bowel in SBS was only focused on the mucosal layer (Tappenden 2014c, b; Doldi 1991; McDuffie et al. 2011; Joly et al. 2009). Besides, while previous studies, explored the changes in the muscle layers mainly in the proximal part (in the jejunum, proximal to the resection site) (Martin et al. 2008; Chen et al. 2012; Chen et al. 2015; Chen et al. 2013), here we have analyzed also the intestine distal parts (in the ileum, distal to the resection site), thus we can state that muscle hypertrophy occurs in both jejunum and ileum. Also the “Area of muscle circumference” in the SBS group was significantly increased in jejunum, as well as in the ileum compared to the healthy control group.
Altogether we demonstrated that muscle tissue increases significantly in both parts of the small bowel, jejunum and ileum, which results in an increased muscle thickness and an enlarged small bowel diameter .
We also evaluated whether the morphological changes in the muscle layers were a result of hypertrophy or hyperplasia. For that, a morphometric analysis of nuclei (size and density) of the muscle cells was performed. Our data revealed that in SBS, the nuclei of the muscle cells became bigger, but the density of nuclei was reduced, indicating that the leading pathophysiological mechanism of these changes is hypertrophy, which is supported by previous studies (Chen et al. 2015).
In all, our observations indicate that following massive small bowel resection, the intestine responds developing muscle hypertrophy in an effort to re-adapt itself to the new situation, where absortive capacity is strongly reduced.
Concerning the ENS plasticity, we found a higher nestin expression in the myenteric plexus in the remaining small bowel of the SBS samples, which demonstrates that not only the muscle wall but also the ENS responds to bowel resection as part of the adaptation process. These results point to a regenerative potential and plasticity of the ENS in the intestinal adjustment to SBS. Actually, a participation of the ENS in the intestinal epithelial growth and repair after SBS has previously been suggested (Toumi et al. 2003), (Haxhija et al. 2007).
Nestin expression was also detected in endothelial cells of newly formed blood vessels (Matsuda et al. 2013) and myofibroblasts (Beguin et al. 2012), demonstrating the regenerative character of nestin expression. So the higher number of nestin-expressing cells in the muscle layers that we found, could suggest that also complementary neuronal progenitor cells outside of myenteric plexus (ganglia) are stimulated during intestinal adaptation.
However, the nestin expression detected in SBS samples within the muscle layers was comparable to that observed in the myenteric plexus. This could point to an intensified influence of the ENS within the smooth muscle layers and intrinsic activation of neural precursors. It is discussed that after 5-HT4 activation, newly born neurons appeared in extra-ganglionic locations and migrated into MP (Liu et al. 2009). Thus, the ENS may execute a more substantial influence and intrinsic activation of neural precursors on the smooth muscle layers, as was previously suggested (Birbrair et al. 2013). Furthermore, the enteric neurons inside the myenteric plexus (ganglia) and neural precursors on the smooth muscle layers could activate or enhance the proliferation of the muscle cells.
In conclusion, our results show an upregulation of nestin in SBS and evidence that the ENS activates its neurogenic potential and participates in intestinal adaptation.
Moreover, we showed that smooth muscle hypertrophy has a strong positive correlation with the activation of nestin-positive cells in the myenteric plexus (r = 0.7524, p < 0.0001), as well as in muscle layers (r = 0.8065, p < 0.0001). A pronounced positive correlation between the proportion of stem cells in the myenteric plexus and smooth muscle hypertrophy allows us to consider the muscle layers of the small intestine and the ENS as a single neuromuscular complex, which plays an essential role in the functioning of the small intestine.
In summary, we have identified several readjustments in the gut following SBS, such us an increase in the diameter of the small intestine, hypertrophy of the small intestine muscle layers, and an increase in the proportion of neuronal stem cells in both the myenteric plexus (ganglia) and the small intestine muscle layers. These observations represent the most important pathophysiological mechanisms in SBS and occur in patients with this pathology. Finally, all these changes induce dilated areas of small bowel with muscular wall hypertrophy.
From a clinical point of view, this phenomenon suggests that the diameter of the small intestine must be measured regularly in patients with SBS. This step will make possible a timely recognition of the small bowel dilatation and the development of intestinal congestion, as well as enable future indications for intestinal lengthening surgeries in a prompt manner.
In conclusion, smooth muscle hypertrophy and neuroplasticity characterize the intestinal re-adaptation in SBS. The ENS activates its neurogenic potential through the neural stem cells and thereby participates in the intestinal adaptation. This process is tightly correlated with an increased expression of neural precursors in the ENS and an increase of the muscle tissue in muscle layers. Since the mechanism of intestinal adaptation is complex and the role of the ENS comprises neural stem cell regulation, further research on enteric glial and neuronal cells is necessary to elucidate the role of enteric neural stem cells in this process.