Transverse mesocolon

The mesenteric organ comprises the small intestinal mesentery (distal to the duodenojejunal flexure), the right mesocolon, the transverse mesocolon, the left mesocolon, the mesosigmoid and mesorectum. The gastrointestinal margin of the mesentery is that edge to which the gastrointestinal tract is attached. The region of gastrointestinal tract attached permits the naming of respective regions of the mesenteric organ. The transverse mesocolon is that region of the mesentery for which the transverse colon is attached to its gastrointestinal margin. The mesosigmoid is that region of the mesenteric organ to which the sigmoid colon is attached at the gastrointestinal mesenteric margin.

A plane is defined as the interface between two contiguous structures.^[1] For over a century, colorectal surgeons have exploited the plane formed by the mesentery and underlying fascia (i.e. Toldt's fascia) to separate the mesentery from the retroperitoneum.^[2] Despite the central importance of this plane in gastrointestinal surgery (and the central importance of mesenteric and peritoneal anatomy) this field is frequently described as difficult and has remained largely misunderstood.

Recent observations related to mesenteric and peritoneal anatomy confirmed a far simpler structure than was heretofore thought. In the first instance, the mesenteric organ is contiguous from duodenojejunal flexure to mesorectal level and not fragmented (as depicted by Frederick Treves).^[2][3][4][5] This realization recently provided significant educational, clinical and scientific opportunities. From a surgical perspective the concept of mesenteric contiguity greatly simplifies processes involved in resectional colorectal surgery by providing a clear anatomical roadmap for these. In addition it has permitted the canonical generation of nomenclature based entirely on anatomical observations (e.g. "total mesocolic excision").^[6][7] In the following we briefly describe classic appraisals of mesenteric and peritoneal fold anatomy and the basis thereof.

Historical background

The classic interpretation of mesenteric and peritoneal fold anatomy stemmed from descriptions by Sir Frederick Treves in 1889 and depicted a fragmented structure that was vestigial at the level of the right and left colon.^[3] Treves described the small bowel mesentery as inserting or attaching into the retroperitoneum along a line extending from the duodenojejunal flexure to the ileocaecal junction. The right and left mesocolon regressed and became vestigial as per Treves such that several commentators described these features as "abnormal as a cleft palate".^[8][9] Their absence coupled by the presence of a transverse and sigmoid mesocolon meant that the mesenteric organ was fragmented, a description at odds with the surgical approach to colonic mobilisation and resection. In addition Treves described the transverse mesocolon as inserting or attaching along a line extending from hepatic to the splenic flexure. He described the mesosigmoid as inserting along a V-shaped line where the apex occurs approximately at the bifurcation of the common iliac artery. These descriptions have been indoctrinated in most reference anatomic, embryologic, surgical, and radiologic texts to the present day.^[10][11][12]

Approximately 10 years prior to the descriptions of Treves, Carl Toldt described the right and left mesocolon as persisting into adulthood.^[13][14] In addition, Toldt described a fascial layer between mesocolon and retroperitoneum and that it was this particular layer that maintained the mesocolon and retroperitoneum separate. Toldt’s findings were largely ignored as those of Treves were indoctrinated into mainstream literature. In 1942 Edward Condon repeated Toldt’s findings demonstrating the presence of a fascial layer between a persistent right (or left) mesocolon and retroperitoneum.^[15] In 1986, the radiologist William Dodds suggested that the entire mesocolon is not secondarily retroperitoneal, but remains extra- retroperitoneal.^[16] Dodds postulated that and extra-retroperitoneal mesocolon and colon explains several radiologic anomalies. Dodds’s prescient suggestion and hypothesis were again largely ignored until recently when Culligan et al. confirmed that, in the adult, the entire small bowel and mesentery as well as colon and mesocolon, universally remain extra-retroperitoneal.^[2][17][18]

Recent clarification of anatomy

Mesentery

Recent findings have demonstrated that the structure of the mesenteric organ is far simpler than previously thought and, by extension, relations with peritoneal folds simpler and better understood than before. For example, the small bowel mesentery does not insert or attach to the posterior abdominal wall along a line from the duodenojejunal flexure to the ileocaecal junction. Instead the small bowel mesentery is contiguous with the right mesocolon and it curves downward from its mobile portion to become apposed to the posterior abdominal wall as the right mesocolon. The right mesocolon is not vestigial but is a distinct anatomic entity and at the level of the hepatic flexure where it is contiguous with the transverse mesocolon. The transverse mesocolon in turn is contiguous at the medial aspect of the splenic flexure with the left mesocolon. The left mesocolon continues distally as the apposed region of the mesosigmoid. The mesosigmoid does not attach along a V-shaped line. Its attachments are best understood by conceptualizing it as comprising contiguous mobile and apposed components.^{[1][2][17][18]} The mobile component curves down onto the posterior abdominal wall in the left iliac fossa and continues apposed to the posterior abdominal wall towards the midline. The medial and lateral borders of the mesosigmoid then converge at the rectosigmoid junction to continue distally into the pelvis as the mesorectum.^{[1][2][17][18]} The mesorectum ends by coning towards the anorectal junction at the distal level of the anal canal.

The principle of mesenteric contiguity helps conceptualize several heretofore unresolved anatomic issues (i.e. the anatomic origin of the mesoappendix). In most patients, the mesoappendix arises from undersurface of that region of ileocaecal mesentery where the small bowel mesentery is contiguous with the right mesocolon. This likely explains the retrocaecal position of the appendix in most individuals.^[2][18]

In addition flexural anatomy has always been regarded as complex, with few if any surgical texts directly describing the anatomic basis of flexure mobilization. Flexural anatomy is greatly simplified by considering each flexure as centered on a mesenteric contiguity. For example, the ileocaecal flexure occurs where the ileum is continuous with the caecum around the ileocaecal mesenteric flexure.^{[1][2][17][18]} In addition, the hepatic flexure comprises a mesenteric confluence between right mesocolon and transverse mesocolon. The colonic component of the hepatic flexure is draped around this mesenteric confluence. The final components of the hepatic flexure include the lateral peritoneal fold and hepatocolic ligament which will be described below. In addition, the splenic flexure can be more readily conceptualized in terms of a mesenteric confluence between the transverse and left mesocolon. The colonic component of the splenic flexure occurs lateral to the mesenteric confluence with the remaining flexural components including the lateral peritoneal fold and the splenocolic fold.^{[1][2][17][18]}

Peritoneal folds

As mesenteric organ anatomy is now better understood, this provides an opportunity to clarify anatomy of the peritoneal folds. A mesenteric fold (the small intestinal mesenteric fold) occurs where the small intestinal mesentery folds onto the posterior abdominal wall and continues laterally as the right mesocolon. By coming through this fold surgically, one accesses the interface (i.e. surgical plane) between the small intestinal mesentery and the retroperitoneum (as is done during medial to lateral mobilisation). In the region of the ileocaecal junction this fold has been arbitrarily termed the "ileocaecal mesenteric fold". The latter continues around this flexure and is contiguous with the peritoneal fold in the right paracolic gutter (i.e. the right lateral peritoneal fold). During lateral to medial mobilization this fold is divided permitting the surgeon to serially lift the right colon and associated mesentery off the underlying fascia and retroperitoneum. At the hepatic flexure the right lateral peritoneal fold turns and continues medially to form the hepatocolic peritoneal fold. Division of the fold in this location permits separation of the colonic component of the hepatic flexure and mesocolon off the retroperitoneum.

Interposed between the hepatic and splenic flexures the greater omentum adheres to the transverse colon along a further band or fold of peritoneum. This band has not been formally described in anatomic texts. On dissection through this, the surgeon gains access to the cephalad aspect of the transverse mesocolon. On the left side a similar anatomic arrangement of folds occurs in that the splenic peritoneal fold (cephalad to the gastrointestinal component of the splenic flexure) is contiguous with the left lateral peritoneal fold. Division of the latter similarly permits the surgeon to separate the left colon and associated mesentery off the underlying fascia and hence free it from the retroperitoneum. The left lateral peritoneal fold in turn continues distally at the lateral aspect of the mobile component of the mesosigmoid.

Adhesions and the greater omentum

Finally, a rationalization of mesenteric and peritoneal fold anatomy permits the surgeon to differentiate both from intraperitoneal adhesions (also called "congenital adhesions"). These occur in several locations and are highly variable between patients. For example, congenital adhesions occur between the lateral aspect of the peritoneum overlying the mobile component of the mesosigmoid, and the parietal peritoneum in the left iliac fossa. During the lateral to medial approach of mobilizing the mesosigmoid, these must be divided first before the peritoneum proper can be accessed. Similarly, focal adhesions occur between the undersurface of the greater omentum and the cephalad aspect of the transverse mesocolon. These can be accessed after dividing the peritoneal fold that links the greater omentum and transverse colon. Adhesions here must be divided in order to separate the greater omentum off the transverse mesocolon and gaining access to the lesser sac proper.

Conclusion

Mesenteric and peritoneal fold anatomy have recently been greatly simplified. The single most important observation is that the mesenteric organ (i.e. small intestinal mesentery, right, transverse, left mesocolon, mesosigmoid and mesorectum) is a contiguous entity from duodenal flexure to mesorectal levels.

References