Mesenteries are a double layer of peritoneum in the abdominal cavity and is a continuation of the visceral and parietal peritoneum with the serous membranes adhered back to back so that the outer mesothelium secretes serous fluid into the peritoneal cavity. This decreases the friction between the adjacent visceral surfaces and allows some movement of the organs that occur during digestion. A thin layer of connective tissue is contained within the two layers of peritoneum and provides a passageway for lymphatics, nerves, arteries and veins to reach the viscera, allowing communication between the body wall and internal organs. Mesenteries are also important as they suspend or hold the organs in place to the posterior abdominal wall.
Those that are totally suspended in the cavity (i.e. are covered in visceral peritoneum) are known as intraperitoneal organs (such as the liver, ileum and stomach) whereas those that lie posterior to the peritoneum and are only covered by the visceral peritoneum on the anterior surface are known as retroperitoneal organs. Retroperitoneal organs include the distal part of the duodenum, abdominal aorta and pancreas. The mesenteries also store fat as a means of padding. There are also other specialty peritoneal folds named the greater and lesser omenta derived from the original dorsal and ventral mesenteries, respectively, in development.
In the fully formed abdominal cavity, mesenteries are found dorsally and adhere the viscera to the posterior wall. There are three mesenteries: the mesentery of the small intestine or mesentery proper, transverse mesocolon and sigmoid mesocolon, all named after their organ attachments in the abdominal cavity.
Mesentery of the small intestine (mesentery proper)
The mesentery of the small intestine is a large and broad fan-shaped mesentery that is attached to the jejunum and ileum of the small intestine, connecting them to the posterior abdominal wall. Superiorly, the mesentery of the small intestine is attached to the end of the duodenum/beginning of the jejunum (duodenojejunal junction) just to the left of the 2nd lumbar vertebra. It runs obliquely down to terminate and attaches at the end of the ileum/beginning of the cecum (ileocecal junction) by the right sacro-iliac joint.
The blood vessels, lymphatics and nerves that are required to supply the jejunum and ileum are found between the two layers of the peritoneum that make up the mesentery of the small intestine.
The transverse mesocolon is attached to the transverse colon of the large intestines, attaching it to the posterior wall. From the posterior wall, the mesentery of the transverse mesocolon lies anteriorly across the front of the head and body of the pancreas to enclose around the transverse colon. The transverse mesocolon also divides the abdominal cavity into supracolic and infracolic compartments (superior and inferior to the transverse mesocolon, respectively).
The supracolic compartment contains:
whereas the infracolic compartment contains:
- small intestine
- ascending and descending colon of the large intestine
The infracolic compartment is further divided into left and right infracolic spaces by the mesentery of the small intestine. There is communication between the supracolic and infracolic compartments by the paracolic gutters.
The structures in between the peritoneal layers are the blood vessels, nerves and lymphatics that are responsible for supplying the transverse colon. The transverse mesocolon is also continuous with the greater omentum.
The sigmoid mesocolon is an inverted V-shaped attachment of the sigmoid colon of the large intestines to the abdominal wall. The apex of the V is attached by the bifurcation point of the internal and external branches of the common iliac artery. The descent of the left branch of the V-shaped sigmoid mesocolon goes along the medial border of the left psoas major muscle. The right side of the sigmoid mesocolon travels down into the pelvis, ending anteriorly around the level of the 3rd sacral vertebra.
The structures passing through the peritoneal layers are the nerves and lymphatics associated with the sigmoid colon and the superior and sigmoid rectal blood vessels.
Mesenteric fibromatosis is a rare occurrence involving a benign fibrous lesion found in the mesentery of the small intestine or in the retroperitoneum. Like other fibromatoses, it does not metastasize but can spread locally. It can occur as a primary mesenteric fibromatosis or it can arise from factors that predispose the person to mesenteric fibromatosis such as trauma, Gardener’s syndrome or long term estrogen intake. Patients with mesenteric fibromatosis present with an abdominal mass that is often accompanied with weight loss and pain. Treatment of mesenteric fibromatosis is usually by surgical removal which often involves resection of the small intestines and associated mesentery.
Intestinal volvulus (where a part of the bowel is twisted around the root of the mesentery) can be caused by mesenteric defects during the formation of the midgut (which later becomes the intestines). During development, there are three stages in intestinal rotation. Firstly the embryonic gut begins as a straight tube and then starts to bend, forming an S-shape that protrudes out of the abdominal cavity. During the second stage, the midgut is back in the abdominal cavity as the size of the fetuses body catches up in size and as a result, the intestine completes a 270° counterclockwise turn which gives it the final orientation in the peritoneum. At the last stage, there is little rotation occurring but this stage is important as this is when the mesentery fuses with the posterior abdominal wall. This allows the small intestine to develop into a wide secondary attachment along the root of the mesentery. There are two main types of mesenteric defects that can occur: basilar, where the base of the mesentery is involved and fails to fuse in the third stage or segmental, which only affects isolated parts of the mesentery but both can lead to volvulus. The severity of symptoms depends on the extent of the defect and the amount of small intestine that is involved in the volvulus but it can lead to intestinal obstruction and/or intestinal atresia.