The mesentery and the mesenteric model of abdominal compartmentalization

Abdominal compartmentalization

The peritoneal model of abdominal compartmentalization has long been the starting point for descriptions of abdominal anatomy. According to this model, abdominal digestive organs are organized along peritoneal lines and the position of each is described using the terms ‘intra’-, ‘extra’-, or ‘retro’-peritoneal ( ). However, some organs have both intra- and retro-peritoneal regions, a feature that is not explained by conventional embryological models of abdominal development. For example, the duodenum in the adult is ‘retro’-peritoneal in position while the rest of the small intestine is ‘intra’-peritoneal. The peritoneal landscape appears complex and regionalized into sacs, fossae and pouches that are described using an array of terms including fold, ligament, membrane and reflection. This heterogeneous terminology makes understanding abdominal anatomy challenging.

Abdominal organs are usually described as being individually connected to the rest of the body, typically mediated by a network of vascular and peritoneal links. The small intestine and the transverse and sigmoid colons are exceptions to this pattern and are conventionally described as linked to the rest of the body via their respective ‘mesenteries’ ( ). The peritoneal and fascial models that have been generated by the concept of multiple mesenteries are dauntingly complex. Fasciae within the abdomen are described as individual entities, a practice that arose in part because they were described separately, e.g. Toldt’s, Denonvilliers’ and Waldeyer’s fasciae ( , ).

Mesenteric anatomy distal to the duodenojejunal flexure was clarified by the demonstration that the mesentery is continuous and connects the small and large intestines to the rest of the body ( ). This perspective also explains peritoneal and fascial anatomy distal to the duodenojejunal flexure ( ). Further research has now clarified the organization of the entire abdominal mesentery and all abdominal digestive organs, peritoneum and fasciae, on the basis of a mesenteric- rather than a peritoneal-based model of abdominal anatomy and compartmentalization.

Mesenteric model of abdominal compartmentalization

The abdominal mesentery is a collection of tissues that supports the growth of all abdominal digestive organs during embryological development and connects those organs to systems of the body throughout life. It is a continuous organ, beginning and ending at the oesophagogastric and anorectal junctions, respectively; all abdominal digestive organs are connected to it ( ). The mesentery can be subdivided into three regions (upper, mid and lower) using two axes: the coeliac trunk represents the inferior boundary of the upper region and the upper limit of the mid-region ( Fig. 8.1.1 ), and the superior mesenteric axis is the distal limit of the mid-region and the proximal limit of the lower region (see Fig. 8.1.1 ).

Emerging data indicate that the abdomen and its contents are compartmentalized along mesenteric lines. According to the mesenteric-based model of abdominal anatomy and compartmentalization, the abdomen is divided into two principal domains, mesenteric and non-mesenteric. The mesenteric domain contains all abdominal digestive organs positioned on the mesentery. The non-mesenteric domain comprises all non-digestive organs (i.e. the genitourinary organs) positioned on the musculoskeletal mainframe of the abdomen. The domains are connected, attached and linked anatomically. They are connected at the arterial inflow and venous drainage of the mesentery. The arterial inflow occurs at the coeliac trunk, and superior and inferior mesenteric arteries. Venous drainage occurs at the junction between the hepatic veins and the inferior vena cava. Both domains are attached , which means the posterior surface of the mesenteric domain is apposed to the anterior surface of the non-mesenteric domain: there is a fascia between apposed surfaces ( ). The junction between fascia and the overlying mesentery is a conceptual zone that surgeons access when they detach the mesentery from its attachment to the posterior abdominal wall. At the periphery of the mesenteric domain, the two domains are linked by the peritoneal reflection, a serous mesothelial membrane that bridges the parietal peritoneum lining the non-mesenteric domain and the visceral peritoneum of the mesenteric domain.

The mesenteric-based model of abdominal compartmentalization explains several aspects of abdominal anatomy. Whilst the peritoneal landscape is generally regarded as extremely complex, it can be understood readily if approached with reference to the mesentery. From this perspective, the lining of organs of the mesenteric domain corresponds to ‘visceral’ peritoneum. The lining of the non-mesenteric domain corresponds to ‘parietal’ peritoneum. At the periphery of the mesenteric domain, parietal peritoneum detaches from the non-mesenteric domain and is reflected onto the mesenteric domain, where it continues as the lining of the latter ( Fig. 8.1.2 ). The regions, sacs, pouches and cavities of the peritoneal landscape are regions of this model. Folds, ligaments and membranes are terms applied to different regions of peritoneal reflection.

Since all abdominal digestive organs are directly connected to the mesentery, it provides a reference frame for describing the positional anatomy of each. For example, the head of the pancreas is located in the mid-region of the mesentery whilst its neck and body are located in the upper region ( Fig. 8.1.3 ). The location of each of the arteries supplying individual organs can also be explained in mesenteric terms. The same principle applies to the extrahepatic portal venous system. For example, the inferior mesenteric vein travels in the left mesocolon to reach the central zone of the mesentery, where it merges with the splenic and/or superior mesenteric vein to generate the hepatic portal vein ( Fig. 8.1.4 ). The latter utilizes the mesoduodenum posterior to the pancreas to reach the liver. The shape of the periphery of the mesentery also explains the double spiral trajectory of the abdominal digestive tract ( Fig. 8.1.5 ).

Mesenteric-based compartmentalization helps to clarify the organization of the fasciae of the abdomen. Where surfaces of the mesenteric and non-mesenteric domains are apposed, there is a fascia between them: the fasciae originally described by separate investigators are now perceived as different regions of the same tissue layer.

Mesenteric-based compartmentalization explains how abdominal digestive organs are linked to systems of the body. They are directly connected to the mesentery and, in turn, the mesentery collectively connects all of them to each system (and hence to the body). This perspective represents a radical departure from conventional descriptions, which hold that all abdominal digestive organs are centrally connected via a network of peritoneal derivatives.