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Mesentery reclassified as single, continuous organ
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The mesentery, long considered to be a fragmented and complex structure connecting the intestine to the abdomen, has been reclassified as a single, continuous organ by researchers at the University of Limerick, Ireland, prompting the latest edition of Gray’s Anatomy to adopt this view.
“The anatomic description that had been laid down over 100 years of anatomy was incorrect. This organ is far from fragmented and complex. It is simply one continuous structure,” J. Calvin Coffey, FRCSI, the Foundation Chair of Surgery at UL’s Graduate Entry Medical School and University Hospitals Limerick, said in a press release.
J. Calvin Coffey
“Clarification of structure and function provides us with the evidence to support its designation as an organ,” Coffey told Healio Gastroenterology. “This is important because it helps us in exploring the mesentery in a systematic (i.e. scientific) manner. The scientific exploration of the mesentery has been very slow up to now.”
Along with his colleague D. Peter O’Leary, PhD, Coffey outlined the evidence for designating the mesentery as a simplified, contiguous structure in their peer reviewed paper published in the Lancet Gastroenterology and Hepatology. In it, they argued their discovery justifies approaching “mesenteric science” as its own field of medical study, which could lead to advances in gastrointestinal surgery and in treating diseases like inflammatory bowel disease, colorectal cancer, diverticular disease, cardiovascular disease, diabetes, obesity and metabolic syndrome.
“This is relevant universally as it affects all of us,” Coffey said in the press release. “Up to now there was no such field as mesenteric science. Now we have established anatomy and the structure. The next step is the function. If you understand the function you can identify abnormal function, and then you have disease. Put them all together and you have the field of mesenteric science ... the basis for a whole new area of science.”
Coffey and O’Leary noted that Leonardo Da Vinci depicted the mesentery as a continuous, centrally converging structure centuries ago, and this view persisted in medical literature until Sir Frederick Treves concluded in 1885 that the mesentery was fragmented across the small intestine and colon.
It is now known that because the mesentery is folded in a compact spiral in the peritoneal cavity, its continuity can only be seen when exposed in a particular way.
“When peeled away from the fascia, the mesentery emerges as a discrete entity. Repeating this process from duodenum to rectum reveals the entirety of the mesentery,” the investigators wrote. “The shape of the mesentery is remarkable [and] can be viewed as similar to a handheld fan.”
In terms of its basic function, the mesentery connects the intestines to the abdominal wall and prevents them from collapsing when standing upright to maintain intestinal transit, but many questions about its function remain.
“Clarification of structure means that we can suggest functions and it would seem that some of its functions are very important — it keeps the intestine connected to the body, but it keeps the intestine at a distance from the main frame of the body. This is crucial as the intestine needs to contract and relax in order to propel its contents forward,” Coffey told Healio Gastroenterology. “The conformation of the mesentery is an adaptation that appears to have developed and that maintains the conformation of the intestine, with the small intestine in the middle of the abdomen, and the colon draped around at the sides. This is important because it means the intestine doesn’t twist on its blood supply, and it doesn’t fall into the pelvis whenever we stand upright.”
However, Coffey and O’Leary wrote that it is currently unclear whether the mesentery should be viewed as part of the intestinal, vascular, endocrine, cardiovascular, or immunological systems, though it has important roles in all of them. In addition, research on the mesentery’s effects at the hematological, immunological, endocrine, metabolic and other levels are ongoing.
It is known, however, that mesenteric nodes can sample intestinal bacterial components to regulate immune responses, although these mechanisms are not fully understood. The mesentery also produces C-reactive protein, which regulates glycemic and lipid metabolism, and the mesenteric mesothelium may be implicated in tissue repair and several disorders like hernia and adhesion formation.
Role in disease
Clarification of the normal mesenteric shape will enable better identification of abnormalities and related diseases, Coffey and O’Leary wrote.
“We now have a new anatomic model or frame of reference,” Coffey told Healio Gastroenterology, “This provides us with a new perspective or fresh viewpoint, from which to further explore intestinal diseases that affects the small and large intestine.”
Mesenteric abnormalities are relevant to several common disorders. Primary mesenteropathies include volvulus, non-rotation, superior mesenteric artery thrombosis, sclerosing mesenteritis and mesenteric cysts, and secondary mesenteropathies including mesenteric involvement in intestinal malignancy or inflammation, Crohn’s disease (although this could be a primary mesenteropathy, they noted), and possibly obesity, diabetes, atherosclerosis and metabolic syndrome.
Understanding the contiguous anatomy of the mesentery may also enable noninvasive or minimally invasive diagnosis of these mesenteric abnormalities through endoscopic examination, the investigators wrote. Because the mesentery is anatomically remote, it can currently only be radiologically or surgically evaluated.
“Endoscopic visualization can be used to map the mesentery and could facilitate transintestinal biopsy in a similar way to transrectal prostate biopsy,” they wrote.
This new understanding of the mesentery also sheds light on flexural anatomy, greatly simplifying certain technical aspects of colorectal surgery. A key benefit in colorectal surgery is a standardized nomenclature, which will enable systematic investigation, education and performance of mobilization and resection. This could have important implications for surgeries in Crohn’s disease, they added.
“Recommendations that the mesentery should be included in resections for Crohn’s disease have been largely ignored due to the dangers (eg, extensive hemorrhage) associated with division of the Crohn’s mesentery,” they wrote. “Reoperation rates remain as high as 40% after resection for Crohn’s disease. However, data suggest that margin positivity and reoperation rates can be substantially reduced if resection is guided by mesenteric-based strategies.”
While the authors expect this development will lead to several benefits to gastroenterology therapeutics and diagnostics and abdominal surgery, it most importantly “provides a platform from which to direct future investigations across natural and applied sciences,” they concluded. – by Adam Leitenberger
Disclosures: The researchers report no relevant financial disclosures.
Editor's Note: This article was updated on Jan. 9 with additional information from the study author.Perspective
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Amy L. Lightner, MD
The mesentery, or fatty tissue connecting the small and large intestine to the rest of the body, is the single largest expanse of mesothelium in the body. Despite its milleau of vital structures including vasculature, lymphatics, immunomodulatory cells, and mesenchymal stem cells, the role of the mesentery in various disease processes remains poorly understood. Only recently has there been a shift in the perspective from the mesentery acting as an innocent bystander to the idea that the mesentery is an active player, or indeed a leader, in the pathogenesis of various disease states.
Surgery is the only means to anatomically alter the mesentery. From the surgical literature, we know that performing a mesorectal excision in patients with rectal cancer decreases local recurrence and improves survival. In colon cancer, it is the standard of care to perform a high ligation of the feeding vessel and its associated lymph nodes in order to improve staging and enhance survival. This concept of surgically altering the mesentery at the time of oncologic resection is based on studies which documented that lymphatic spread to mesenteric nodes is an important component of systemic spread of metastatic disease.
To date, Crohn's disease has been classically viewed as a disease of the intestine, and the mesentery was an innocent bystander secondarily affected. More recently, there is emerging evidence suggesting that the mesentery may play an active role in Crohn's disease. This makes sense since the classic features of Crohn's disease include creeping fat, thickened mesentery, enlarged lymph nodes, involvement of the small or large bowel, and transmural inflammation of the bowel wall. This transmural feature of Crohn's disease facilitates increased bacterial translocation into the creeping fat which then activates adipocytes which have complex metabolic and immunologic functions. While the interplay of adipokines, neuropeptides, and vascular and lymphatic endothelia affect tissue remodeling of the mesentery, the mechanism of how the mesentery affects the disease state remains largely unknown.
The current surgical management in Crohn's disease is to spare the mesentery, or leave it in situ when performing a bowel resection, unlike resections for adenocarcinoma of the colon. Unfortunately, despite removing diseased intestine, patients experience high rates of endoscopic and clinical recurrence following resections for Crohn's disease with up to one third of patients requiring subsequent surgery. Perhaps this is related to leaving diseased mesentery in situ. Investigation to compare rates of disease recurrence when an increased volume of mesentery is resected at the time of operation for Crohn’s disease may be interesting and practice changing.
As the mesentery becomes increasingly seen as its own unique organ, the role of the various components of the mesentery, (cellular, connective tissue, lymphatic, vascular and nervous systems) will be delineated and potential disease mechanisms revealed. The next decade of research should determine the role of the mesentery in disease states; its central place in the abdomen underscores the possibility that the mesentery is an active participant in Crohn's disease of both the small bowel and colon.
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