Editing Ileum (section)

==Structure==
The ileum is the third and final part of the small intestine. It follows the [[jejunum]] and ends at the ileocecal junction, where the [[wikt:terminal|terminal]] ileum communicates with the [[cecum]] of the large intestine through the [[ileocecal valve]]. The ileum, along with the jejunum, is suspended inside the [[mesentery]], a [[peritoneum|peritoneal]] formation that carries the blood vessels supplying them (the [[superior mesenteric artery]] and [[superior mesenteric vein|vein]]), lymphatic vessels and nerve fibers.<ref name="moore">{{cite book |vauthors=Moore KL, Dalley AF, Agur AM |date=2013 |title=Clinically Oriented Anatomy, 7th ed. |publisher=Lippincott Williams & Wilkins |pages=241–246 |isbn=978-1-4511-8447-1 }}</ref>

There is no line of demarcation between the jejunum and the ileum. There are, however, subtle differences between the two:<ref name="moore" />
* The ileum has more fat inside the mesentery than the jejunum.
* The diameter of its [[lumen (anatomy)|lumen]] is smaller and has thinner walls than the jejunum.
* Its [[circular folds]] are smaller and absent in the terminal part of the ileum.
* While the length of the intestinal tract contains [[lymphoid tissue]], only the ileum has abundant [[Peyer's patches]], unencapsulated [[lymphoid nodule]]s that contain large numbers of [[lymphocyte]]s and other cells of the [[immune system]].

===Histology===
{{Main|Gastrointestinal wall}}

The four layers that make up the wall of the ileum are consistent with those of the [[Human gastrointestinal tract|gastrointestinal tract]]. From the inner to the outer surface, these are:<ref name="ross">{{cite book | title=Histology: A Text and Atlas. Sixth edition | publisher=Lippincott Williams & Wilkins |vauthors=Ross M, Pawlina W | year=2011 | isbn=978-0-7817-7200-6}}</ref>{{rp|589}}
* A [[mucous membrane]], itself formed by three different layers:
** A single layer of [[simple columnar epithelium|tall cells]] that line the [[Lumen (anatomy)|lumen]] of the organ. The [[epithelium]] that forms the innermost part of the mucosa has five distinct types of cells that serve different purposes: [[enterocytes]] with [[microvilli]], which digest and absorb nutrients; [[goblet cell]]s, which secrete [[mucin]], a substance that lubricates the wall of the organ; [[Paneth cell]]s, most common in the terminal part of the ileum, are only found at the bottom of the [[intestinal glands]] and release antimicrobial substances such as [[alpha defensin]]s and [[lysozyme]];<ref name="santaolalla">{{cite journal | title=Innate immunity in the small intestine. |vauthors=Santaolalla R, Fukata M, Abreu MT | journal=Current Opinion in Gastroenterology | year=2011 | volume=27 | issue=12 | pages=125–131 | doi=10.1097/MOG.0b013e3283438dea | pmid=21248635 | pmc=3502877}}</ref> [[microfold cells]], which take up and transport [[antigen]]s from the lumen to lymphatic cells of the lamina propria; and [[enteroendocrine cell]]s, which secrete [[hormones]].
** An underlying [[lamina propria]] composed of [[loose connective tissue]] and containing [[germinal center]]s and large aggregates of lymphoid tissue called [[Peyer's patches]], which are a distinctive feature of the ileum.<ref name="ross" />{{rp|589}}
** A thin layer of [[smooth muscle]] called [[muscularis mucosae]]
* A [[submucosa]] formed by [[dense irregular connective tissue]] that carries the larger [[blood vessels]] and a nervous component called [[submucosal plexus]], which is part of the [[enteric nervous system]]
* An [[Muscularis externa|external muscular layer]] formed by two layers of [[smooth muscle]] arranged in circular bundles in the inner layer and in longitudinal bundles in the outer layer. Between the two layers is the [[myenteric plexus]], formed by nervous tissue and also a part of the [[enteric nervous system]].
* A [[serosa]] composed of [[mesothelium]], a single layer of [[simple squamous epithelium|flat cells]] with varying quantities of underlying [[connective tissue|connective]] and [[adipose tissue]]. This layer represents the [[peritoneum|visceral peritoneum]] and is continuous with the [[mesentery]].<ref name="ross" />{{rp|571}}
<gallery mode=packed>
 File:Gut wall.svg|General structure of the gut wall. Brunner's glands are not found in the ileum, but are a distinctive feature of the [[duodenum]].
 File:Gobletcell.jpg|[[Goblet cell]]s in the wall of an ileum vili. At its sides, [[enterocytes]] are visible over a core of [[lamina propria]].
 File:Peyer's patch (improved color).jpg|Cross section of ileum with a [[Peyer's patch]] circled.
File:Cross-section histology of small intestinal villi of the terminal ileum.jpg|Cross-section histology of [[intestinal villi]] of the human terminal ileum.
</gallery>

===Development===
The [[small intestine]] develops from the [[midgut]] of the [[primitive gut tube]].<ref name=LARSENS2009>{{cite book|first1=Gary C. |last1=Schoenwolf |first2=Steven B. |last2=Bleyl |first3=Philip R. |last3=Brauer |first4=Philippa H. |last4=Francis-West |title=Larsen's human embryology|date=2009|publisher=Churchill Livingstone/Elsevier|location=Philadelphia |isbn=9780443068119 |chapter=Development of the Urogenital system |edition=4th |page=237}}</ref> By the fifth week of [[embryo]]logical life, the ileum begins to grow longer at a very fast rate, forming a U-shaped fold called the [[primary intestinal loop]]. The [[proximal]] half of this loop will form the ileum. The loop grows so fast in length that it outgrows the abdomen and protrudes through the [[Navel|umbilicus]]. By week 10, the loop retracts back into the abdomen. Between weeks six and ten the small intestine rotates anticlockwise, as viewed from the front of the embryo. It rotates a further 180 degrees after it has moved back into the abdomen. This process creates the twisted shape of the [[large intestine]].<ref name=LARSENS2009 />

In the [[fetus]] the ileum is connected to the [[navel]] by the [[vitelline duct]]. In roughly 2−4% of humans, this duct fails to close during the first seven weeks after birth, leaving a remnant called [[Meckel's diverticulum]].<ref name="Sagar">{{cite journal |author1=Sagar J. |author2=Kumar V. |author3=Shah D. K. | year = 2006 | title = Meckel's diverticulum: A systematic review | journal = Journal of the Royal Society of Medicine | volume = 99 | issue = 10| pages = 501–505 | doi = 10.1177/014107680609901011 | pmid = 17021300 | pmc = 1592061 }}</ref>