• To discuss basic topics of embryology and anatomical contemporary classification of the mesentery.

• To expose differences between the traditional and the new proposal of mesenteric anatomy.

• To illustrate main patterns of mesenteric pathologies, which can be divided into primary or secondary diseases.

• To provide tips for the correct diagnostic investigation of such pathologies and exclusion of the main differential diagnoses.

Basic topics of embryology and anatomical contemporary classification of the mesentery and the main differences between the traditional and the new proposal of mesenteric anatomy

Although recent data have revolutionized the concepts about the mesenteric anatomy, anatomical and embryologic books and publications continue to present classic anatomical descriptions. Thus, our focus will not be on the classical concepts: we will be addressing the proposed new concepts.

Recent data suggest a change in the formerly known anatomical model of mesentery, now proving its continuity and defending his new classification as an organ due to its uniqueness, function and role in disease.

The classic concept of mesenteric fragmentation was overcome: it was previously believed that the mesentery of the ascending and descending colon region (right and left mesocolon) did not exist in the adult due to regression or resorption mechanism. Today, it is known that the mesentery is a continuous structure from distal to the duodenojejunal flexure to the mesorectal level, including the normal presence of the right and left mesocolon.

Important concepts about the embryology of the mesentery:

Embryology – The classic concepts and the new model

Classic concepts

The classic model of mesenteric embryology was based on the anatomical concept of mesenteric discontinuity. According to this model, the small bowel mesentery, as well as that of the transverse colon and sigmoid, persists into adulthood. However, the right and left mesocolon regress, which would make mesentery discontinuous and fragmented. Theories were developed to try explain right and left mesenteric involution, but none of them were widely accepted. Despite the mesenteric fragmentation theory, the start and end points for each mesenteric region have never been identified, which corroborates the lack of consistency of this observation. Nowadays we know such points do not exist, since the mesentery has been proven as a continuous structure.

New model

Once mesenteric continuity has been proven, its anatomical structure in the adult is much simpler than previously thought. Although the new model simplifies its anatomy, the embryological development of the mesentery, peritoneal reflection and fascia needs to be systematically studied, as it no longer fit the previously described model.

The intestine develops from the endodermal germ layer, while the mesentery derives from the mesodermal germ layer.

Mesentery: Relatively little is known regarding the cellular events involved in mesenteric development. It is suggested that rotation, anchorage, elongation, and attachment are important and interrelated processes. The mesenteric embryology process can be simplified in a set of key processes:

• suspension at points of vascular connectivity;
• differential elongation of regions of the intestine and mesentery with a resultant counter-clockwise rotation of both;
• mesenteric flattening against the posterior abdominal wall;
• development of Toldt’s fascia and the peritoneal membrane to maintain attachment in this conformation.

          It is important to define the terms “attachment” and “suspension.”

• “Attachment” refers to the flattening of the mesentery against the posterior abdominal wall so that it becomes apposed to the retroperitoneum. There is no “insertion” into the posterior abdominal wall in any location.

• “Suspension" refers to the suspension of the mesentery to the posterior abdominal wall at vascular pedicles. The mesentery fans out from the “root region” where the superior mesenteric artery suspends it to the posterior abdominal wall. From this point, the mesentery expands, being mobile in some regions while in others it is attached to the posterior abdominal wall. The continuous mesentery spans the intestine from duodenojejunal to anorectal junction.

Peritoneum: The embryological development of peritoneal reflections is also little elucidated. It is suggested that the peritoneal reflection develops after the development and final positioning of the gastromesenteric complex and that it serves to secure the gastromesenteric complex in position. A secondary benefit is that it limits the spread of diseases.

Important concepts about the anatomy of the mesentery:

Anatomy – The classic concepts

According to the classical anatomical model, mesentery would be a fragmented structure. It was previously considered to be a double layer of peritoneum that encloses the intestines and attaches them to the posterior abdominal wall. Its main fragments were:

• small bowel mesentery (mesentery proper) suspends the jejunum and the ileum
• greater omentum: connects the stomach to the colon
• lesser omentum: connects the stomach to the liver
• mesoappendix: peritoneum of the vermiform appendix
• transverse mesocolon: peritoneum of the transverse colon
• sigmoid mesocolon: peritoneum of the sigmoid colon

Anatomy – New Model

Mesentery: The current concept is that it is a continuous structure from distal to the duodenojejunal flexure to the mesorectal level. However, it can be subdivided into the following regions for didactic purposes:

• Mesenteric root
• Small bowel mesentery
• Ileocolic pedicle territory
• Right mesocolon
• Transverse mesocolon
• Left mesocolon
• Mesosigmoid
• Mesorectum

Fig. 1: The new mesentery

Peritoneum: The peritoneal reflections develop to bridge any space between the intestine, mesentery, and abdominal wall. The following peritoneal reflections are highlighted:

• Hepatocolic peritoneal reflection
• Splenocolic peritoneal reflection
• Small bowel peritoneal reflection
• Ileocaecal peritoneal reflection
• Right peritoneal reflection
• Left peritoneal reflection
• Mesosigmoidal peritoneal reflection
• Pararectal peritoneal reflection

Toldt's fascia: Toldt’s fascia is the connective tissue layer that occurs between attached regions of mesentery and retroperitoneum (or pelvis). It occupies a potential space between the mesentery and retroperitoneum. The peritoneal reflection is the anatomic limit between this space and the fascia. Whenever two mesothelial surfaces come into direct and prolonged contact, Toldt’s fascia develops between and bridges both.