View source for Ileum

{{Short description|Final section of the small intestine}}
{{distinguish|Ilium (disambiguation){{!}}Ilium}}
{{Infobox anatomy
| Name        = Ileum
| Latin       = ileum
| Image       = Illu small intestine.jpg
| Caption     = Small intestine
| Width       =
| Image2      = Gray1045.png
| Caption2    = The cecal fossa. The ileum and [[cecum]] are drawn backward and upward.
| Precursor   = [[Midgut]]
| System      =
| Artery      = [[Ileal arteries]], [[ileocolic artery]]
| Vein        = [[Ileal veins]]
| Nerve       = [[Celiac ganglia]], [[vagus]]<ref>{{cite book| title= Essentials of Human Physiology| first= Thomas M. |last= Nosek| chapter=Section 6/6ch2/s6ch2_30 |chapter-url=http://humanphysiology.tuars.com/program/section6/6ch2/s6ch2_30.htm |archive-url=https://web.archive.org/web/20160324124828/http://humanphysiology.tuars.com/program/section6/6ch2/s6ch2_30.htm|archive-date=2016-03-24}}</ref>
| Lymph       =
}}
{{Gastrointestinal tract sidebar}}
The '''ileum''' ({{IPAc-en|ˈ|ɪ|l|i|əm}}) is the final section of the [[small intestine]] in most [[higher vertebrates]], including [[mammals]], [[reptiles]], and [[birds]]. In [[fish]], the divisions of the small intestine are not as clear and the terms '''posterior intestine''' or '''distal intestine''' may be used instead of ileum.<ref name=fish_feeding_book>
{{cite book
 | last=Guillaume
 | first=Jean
 |author2=Praxis Publishing |author3=Sadasivam Kaushik |author4=Pierre Bergot |author5=Robert Metailler
  | title=Nutrition and Feeding of Fish and Crustaceans
 | url=https://books.google.com/books?id=As0flTZo_EAC&q=fish+cytology+jejunum+duodenum&pg=PA31
 | page=31
 | year=2001
 | access-date=2009-01-09
 | publisher=Springer
 | isbn=1-85233-241-7 }}
</ref> Its main function is to absorb [[vitamin B12|vitamin B<sub>12</sub>]], [[bile salts]], and whatever products of digestion that were not absorbed by the [[jejunum]].

The ileum follows the [[duodenum]] and [[jejunum]] and is separated from the [[cecum]] by the [[ileocecal valve]] (ICV). In humans, the ileum is about 2–4 m long, and the [[pH]] is usually between 7 and 8 (neutral or slightly [[base (chemistry)|basic]]).

''Ileum'' is derived from the Greek word εἰλεός (eileós), referring to a medical condition known as [[ileus]].{{Citation needed|date=September 2024}}

==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>

==Function==
The main function of the ileum is to absorb [[vitamin B12|vitamin B<sub>12</sub>]], [[bile salts]], and whatever products of digestion were not absorbed by the jejunum.  The wall itself is made up of folds, each of which has many tiny finger-like projections known as [[Intestinal villus|villi]] on its surface. In turn, the epithelial cells that line these villi possess even larger numbers of [[microvilli]]. Therefore, the ileum has an extremely large surface area both for the [[adsorption]] (attachment) of [[enzyme]] molecules and for the [[Absorption (chemistry)|absorption]] of products of [[digestion]]. The DNES (diffuse neuroendocrine system) cells of the ileum secrete various hormones ([[gastrin]], [[secretin]], [[cholecystokinin]]) into the blood. Cells in the lining of the ileum secrete the [[protease]] and [[carbohydrase]] enzymes responsible for the final stages of [[protein]] and [[carbohydrate]] digestion into the [[Lumen (anatomy)|lumen]] of the intestine. These enzymes are present in the [[cytoplasm]] of the [[epithelial cells]].

The villi contain large numbers of capillaries that take the amino acids and glucose produced by digestion to the [[hepatic portal vein]] and the liver. [[Lacteals]] are small lymph vessels, and are present in villi. They absorb [[fatty acid]] and [[glycerol]], the products of fat digestion. Layers of circular and longitudinal [[smooth muscle]] enable the [[chyme]] (partly digested food and water) to be pushed along the ileum by waves of muscle contractions called [[peristalsis]]. The remaining chyme is passed to the [[Colon (anatomy)|colon]].

==Clinical significance==
The ileum is of importance in medicine as it can be affected in a number of diseases,<ref>{{Cite journal  | last1 = Cuvelier | first1 = C. | last2 = Demetter | first2 = P. | last3 = Mielants | first3 = H. | last4 = Veys | first4 = E.M. | last5 = De Vos | first5 = M | title = Interpretation of ileal biopsies: morphological features in normal and diseased mucosa. | journal = Histopathology | volume = 38 | issue = 1 | pages = 1–12 |date=Jan 2001 | doi =  10.1046/j.1365-2559.2001.01070.x| pmid = 11135039 | s2cid = 28873753 }}</ref> including:
* [[Crohn's disease]]
* [[Tuberculosis]]
* [[Lymphoma]]
* [[Neuroendocrine tumor]]s (carcinoid)

==Other animals==
In veterinary anatomy, the ileum is distinguished from the jejunum by being that portion of the jejunoileum that is connected to the [[caecum]] by the [[ileocecal fold]].

The ileum is the short termi of the small intestine and the connection to the large intestine.  It is suspended by the caudal part of the mesentery (mesoileum) and is attached, in addition, to the cecum by the ileocecal fold.  The ileum terminates at the cecocolic junction of the large intestine forming the ileal orifice.  In the dog the ileal orifice is located at the level of the first or second lumbar vertebra, in the ox in the level of the fourth lumbar vertebrae, in the sheep and goat at the level of the caudal point of the costal arch.<ref>Nickel, R., Shummer, A., Seiferle, E. (1979) The viscera of the domestic mammals, 2nd edn. Springer-Verlag, New York, USA.{{page needed|date=June 2015}}</ref> By active muscular contraction of the ileum, and closure of the ileal opening as a result of engorgement, the ileum prevents the backflow of ingesta and the equalization of pressure between jejunum and the base of the cecum.  Disturbance of this sensitive balance is not uncommon and is one of the causes of colic in horses. During any intestinal surgery, for instance, during appendectomy, distal 2 feet of ileum should be checked for the presence of Meckel's diverticulum.

==References==
{{Reflist}}

==External links==
{{Commons}}
{{Wiktionary}}
* {{SUNYAnatomyLabs|37|11|01|01}} – "Abdominal Cavity: The Jejunum and the Ileum"
* {{SUNYAnatomyImage|7|7|87}}
* {{SUNYAnatomyImage|8|7|55}}
* {{BUHistology|12001oca}}
* [https://web.archive.org/web/20120218235831/http://www.endoatlas.com/co_ge_15.html Ileal Villi at endoatlas.com]
* [https://web.archive.org/web/20051029223728/http://science.nhmccd.edu/biol/digestive/ileum.htm Ileum Microscopic Cross Section at nhmccd.edu]
* [https://web.archive.org/web/20060315183719/http://www.deltagen.com/target/histologyatlas/atlas_files/digestive/ileum_20x.htm Ileum 20x at deltagen.com]

{{Digestive tract}}
{{Authority control}}

[[Category:Small intestine]]