Editing Mesentery (section)

==Structure==
The mesentery of the [[small intestine]] arises from the '''root of the mesentery''' (or '''mesenteric root''') and is the part connected with the structures in front of the [[vertebral column]]. The root is narrow, about 15&nbsp;cm long, 20&nbsp;cm in width, and is directed obliquely from the [[duodenojejunal flexure]] at the left side of the second [[lumbar vertebra]] to the right [[sacroiliac joint]]. The root of the mesentery extends from the duodenojejunal flexure to the [[ileocecal valve|ileocaecal junction]]. This section of the small intestine is located centrally in the abdominal cavity and lies behind the transverse colon and the [[greater omentum]].

The mesentery becomes attached to the [[large intestine|colon]] at the gastrointestinal margin and continues as the several regions of the mesocolon. The parts of the mesocolon take their names from the part of the colon to which they attach. These are the transverse mesocolon attaching to the transverse colon, the sigmoid mesocolon attaching to the sigmoid colon, the mesoappendix attaching to the appendix, and the mesorectum attaching to the upper third of the rectum.

The mesocolon regions were traditionally taught to be separate sections with separate insertions into the posterior abdominal wall. In 2012, the first detailed observational and [[histology|histological]] studies of the mesocolon were undertaken and this revealed several new findings.<ref name="Culligan"/> The study included 109 patients undergoing open, elective, total abdominal [[colectomy]]. Anatomical observations were recorded during the surgery and on the post-operative specimens.

These studies showed that the mesocolon is continuous from the ileocaecal to the rectosigmoid level. It was also shown that a mesenteric confluence occurs at the ileocaecal and rectosigmoid junctions, as well as at the [[colic flexures|hepatic and splenic flexures]] and that each confluence involves peritoneal and omental attachments. The proximal rectum was shown to originate at the confluence of the mesorectum and mesosigmoid. A plane occupied by perinephric fascia was shown to separate the entire apposed small intestinal mesentery and the mesocolon from the [[retroperitoneum]]. Deep in the [[pelvis]], this fascia coalesces to give rise to [[presacral fascia]].<ref name="Culligan"/>

=== Flexural anatomy ===
Flexural anatomy is frequently described as a difficult area. It is simplified when each flexure is considered as being centered on a mesenteric contiguity. The ileocaecal flexure arises at the point where the ileum is continuous with the caecum around the ileocaecal mesenteric flexure. Similarly, the [[Colic flexures|hepatic flexure]] is formed between the right mesocolon and transverse mesocolon at the mesenteric confluence. The colonic component of the hepatic flexure is draped around this mesenteric confluence. Furthermore, the [[colic flexures|splenic flexure]] is formed by the mesenteric confluence between the transverse and left mesocolon. The colonic component of the splenic flexure occurs lateral to the mesenteric confluence. At every flexure, a continuous peritoneal fold lies outside the colonic/mesocolic complex tethering this to the posterior abdominal wall.<ref name="Coffey"/><ref name="Culligan"/>

===Mesocolon regions===
The '''transverse mesocolon''' is that section of the mesentery attached to the [[transverse colon]] that lies between the [[colic flexures]].

The '''sigmoid mesocolon''' is that region of the mesentery to which the [[sigmoid colon]] is attached at the gastrointestinal mesenteric margin.

The '''mesoappendix''' is the portion of the mesentery connecting the [[ileum]] to the [[Vermiform appendix|appendix]]. It may extend to the tip of the appendix. It encloses the [[appendicular artery]] and vein, as well as [[lymph]]atic vessels, nerves, and often a [[lymph node]].

The '''mesorectum''' is that part attached to the upper third of the rectum.

=== Peritoneal folds ===
Understanding the macroscopic structure of the mesenteric organ meant that associated structures—the peritoneal folds and congenital and omental adhesions—could be better appraised. The small intestinal mesenteric fold occurs where the small intestinal mesentery folds onto the posterior abdominal wall and continues laterally as the right mesocolon. During mobilization of the small intestinal mesentery from the posterior abdominal wall, this fold is incised, allowing access to the interface between the small intestinal mesentery and the retroperitoneum. The fold continues at the inferolateral boundary of the ileocaecal junction and turns {{linktext|cephalad}} as the right paracolic peritoneal fold. This fold is divided during lateral to medial mobilization, 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 as 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.<ref name="Coffey" /><ref name="Culligan" />

Interposed between the hepatic and splenic flexures, the greater omentum adheres to the transverse colon along a further band or fold of peritoneum. Dissection through this allows access to the cephalad (top) surface of the transverse mesocolon. Focal adhesions frequently tether the greater omentum to the cephalad aspect of the transverse mesocolon. The left colon is associated with a similar anatomic configuration of peritoneal folds; the splenic peritoneal fold is contiguous with the left lateral paracolic peritoneal fold at the splenic flexure. Division of the latter similarly allows for the separation of the left colon and associated mesentery off the underlying fascia and frees it from the retroperitoneum. The left lateral paracolic peritoneal fold continues distally at the lateral aspect of the mobile component of the mesosigmoid.<ref name="Coffey" /><ref name="Culligan" />

=== Microanatomy ===
Determination of the macroscopic structure of the mesenteric organ allowed a recent characterisation of the histological and electron microscopic properties.<ref name="pmid24441808">{{cite journal|vauthors=Culligan K, Walsh S, Dunne C, etal|date=January 2014|title=The Mesocolon: A Histological and Electron Microscopic Characterization of the Mesenteric Attachment of the Colon Prior to and After Surgical Mobilization|url=https://ulir.ul.ie/bitstream/10344/4895/1/Dunne_2014_surgical.pdf|journal=[[Annals of Surgery]]|volume=260|issue=6|pages=1048–56|doi=10.1097/SLA.0000000000000323|pmid=24441808|hdl=10344/4895|s2cid=23266182 |hdl-access=free}}</ref> The microscopic structure of the mesocolon and associated fascia is consistent from ileocecal to mesorectal levels. A surface [[mesothelium]] and underlying connective tissue is universally apparent. [[Adipocytes]] lobules within the body of the mesocolon are separated by fibrous [[Septum|septa]] arising from submesothelial connective tissue. Where apposed to the retroperitoneum, two mesothelial layers separate the mesocolon and underlying retroperitoneum. Between these is [[Toldt's fascia]], a discrete layer of connective tissue. Lymphatic channels are evident in mesocolic connective tissue and in Toldt's fascia.<ref name=pmid24441808/>

===Development===
[[File:Gray987.png|thumb|upright=1.4|Two of the stages in the development of the digestive tube and its mesentery]]

====Dorsal mesentery====
[[File:Gray1038 vector.svg|240px|thumb|left|Mesentery in red. Dorsal mesentery is the lower part of the circuit. The upper part is ventral mesentery.]]
[[File: Gray985.png |thumb|Abdominal part of digestive tube and its attachment to the primitive or common mesentery. Human embryo of six weeks.]]
[[File: Gray989.png |thumb|Schematic figure of the bursa omentalis, etc. Human embryo of eight weeks.]]
The [[primitive gut]] is suspended from the posterior abdominal wall by the '''dorsal mesentery'''. The gastrointestinal tract and associated dorsal mesentery are subdivided into [[foregut]], [[midgut]], and [[hindgut]] regions based on the respective blood supply. The foregut is supplied by the [[celiac trunk]], the midgut is supplied by the [[superior mesenteric artery]] (SMA), and the hindgut is supplied by the [[inferior mesenteric artery]] (IMA). This division is established by the fourth week of [[human embryogenesis|development]]. After this, the midgut undergoes a period of rapid elongation, forcing it to herniate through the [[navel]]. During herniation, the midgut [[intestinal rotation|rotates]] 90° anti-clockwise around the axis of the SMA and forms the midgut loop. The cranial portion of the loop moves to the right and the caudal portion of the loop moves toward the left. This [[intestinal rotation|rotation]] occurs at about the eighth week of development. The [[cranium|cranial]] portion of the loop will develop into the jejunum and most of the ileum, while the [[caudal (anatomical term)|caudal]] part of the loop eventually forms the terminal portion of the ileum, the [[ascending colon]] and the initial two-thirds of the transverse colon. As the foetus grows larger, the mid-gut loop is drawn back through the umbilicus and undergoes a further 180° rotation, completing a total of 270° rotation. At this point, about 10 weeks, the [[caecum]] lies close to the [[liver]]. From here it moves in a cranial to caudal direction to eventually lie in the lower right portion of the abdominal cavity. This process brings the ascending colon to lie vertically in the lateral right portion of the abdominal cavity apposed to the posterior abdominal wall. The descending colon occupies a similar position on the left side.<ref>{{cite journal |first1=Harold |last1=Ellis |first2=Vishy |last2=Mahadevan |date=April 2014 |title=Anatomy of the caecum, appendix and colon |journal=Surgery |volume=32 |issue=4 |pages=155–8 |doi=10.1016/j.mpsur.2014.02.001}}</ref><ref name=":7">Mitchell B, Sharma R. Embryology: An Illustrated Colour Text, 2e. Churchill Livingstone; 2 edition (June 22, 2009). {{ISBN|978-0702032257}}.{{page needed|date=July 2014}}</ref>

During these topographic changes, the dorsal mesentery undergoes corresponding changes. Most anatomical and embryological textbooks say that after adopting a final position, the ascending and descending mesocolons disappear during embryogenesis. ''Embryology—An Illustrated Colour Text'', "most of the mid-gut retains the original dorsal mesentery, though parts of the duodenum derived from the mid-gut do not. The mesentery associated with the ascending colon and descending colon is resorbed, bringing these parts of the colon into close contact with the body wall."<ref name=":7" /> In ''The Developing Human'', the author states, "the mesentery of the ascending colon fuses with the parietal peritoneum on this wall and disappears; consequently the ascending colon also becomes retroperitoneal".<ref name=":8">Moore KL, TPersaud TVN, Torchia MG. The Developing Human: Clinically Oriented Embryology with Student Consult Online Assess, 9th Edition. Saunders; {{ISBN|978-1437720020}}{{page needed|date=July 2014}}</ref> To reconcile these differences, several theories of embryologic mesenteric development—including the "regression" and "sliding" theories—have been proposed, but none has been widely accepted.<ref name=":7" /><ref name=":8" />

The portion of the '''dorsal mesentery''' that attaches to the [[greater curvature]] of the [[stomach]], is known as the '''dorsal mesogastrium'''. The part of the dorsal mesentery that suspends the [[large intestine|colon]] is termed the [[mesocolon]]. The dorsal mesogastrium develops into the [[greater omentum]].

====Ventral mesentery====
The development of the [[septum transversum]] takes part in the formation of the [[Thoracic diaphragm|diaphragm]], while the caudal portion into which the [[liver]] grows forms the '''ventral mesentery'''. The part of the ventral mesentery that attaches to the [[stomach]] is known as the '''ventral mesogastrium'''.<ref name=gray>Gray's anatomy</ref>

The [[lesser omentum]] is formed, by a thinning of the [[mesoderm]] or ventral mesogastrium, which attaches the stomach and [[duodenum]] to the anterior [[abdominal wall]]. By the subsequent growth of the liver, this leaf of mesoderm is divided into two parts – the [[lesser omentum]] between the stomach and liver, and the [[Falciform ligament|falciform]] and [[coronary ligament]]s between the liver and the abdominal wall and diaphragm.<ref name=gray/>

In the adult, the ventral mesentery is the part of the [[peritoneum]] closest to the [[navel]].