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The Ileum

Anatomy of the Ileum

Characteristics & Features

The ileum is the last of the three parts of the small intestine. The transition from the jejunum to the ileum is not sharply marked, while at the distal end, the ileum opens into the large intestine. At the junction between the ileum and the caecum lies the ileocecal valve (ileal ostium), a functional sphincter formed by the circular muscle layers of both the ileum and cecum. It prevents a reflux of the bacteria-rich content from the large intestine into the small intestine.

Ileum - ventral view

Ileum - ventral view

The ileum makes up about 3/5 of the total length of the small intestine (2.5 to 3.5 meters). Compared to the jejunum, the parallel running circular folds in the mucosa (valves of Kerckring) are less prominent. In contrast it is rich in lymphoid follicles. Similar to the jejunum, the ileum is attached to the posterior wall of the abdomen by the mesentery and therefore lies flexibly in the abdominal cavity.

Mesentery - ventral view

Mesentery - ventral view

Blood Supply

About twelve ileal arteries (branches of the superior mesenteric artery) supply the ileum with arterial blood. These form arcades with the other arteries of the small intestine. The venous blood flows from the correspondent veins into the inferior mesenteric vein.

Ileal arteries - ventral view

Ileal arteries - ventral view

Innervation

Analogous to the jejunum both the coeliac plexus and the superior mesenteric plexus innervate the ileum sympathetically, while the vagus nerve (cranial nerve X) parasympathetically.

Superior mesenteric plexus - ventral view

Superior mesenteric plexus - ventral view

Histology of the Ileum

Histologically, the ileum has the same basic structure as the jejunum:

  • mucosa
  • submucosa
  • muscularis
  • serosa

The mucosa is lined by simple columnar epithelium (lamina epithelialis) comprising enterocytes and goblet cells. Underneath lies a connective tissue layer (lamina propria) and a muscle layer (lamina muscularis mucosae). Compared to the rest of the small intestine the circular folds are rather flat and the villi relatively short. The submucosa contains blood vessels, lymph nodes and the Meissner’s plexus. The muscularis consists of an inner circular and outer longitudinal muscle layer. The ileum is entirely covered by serosa from the outside. It is made up of simple squamous epithelium and a connective tissue layer underneath (lamina propria serosae).

Ileum - histological slide

Ileum - histological slide

A characteristic feature of the ileum is the Peyer’s patches lying in the mucosa. It is an important part of the GALT (gut-associated lymphoid tissue). One patch is around 2 to 5 centimeters long and consists of about 300 aggregated lymphoid follicles and the parafollicular lymphoid tissue. The dome-like bulge above one follicle is called dome area. M cells (microfold cells) are found in the dome epithelium which are counted among the FAE cells (follicle-associated epithelial cells). Their function is to pick up antigens from the intestinal lumen and transport them to the antigen-presenting cells (APC).

Peyer's patch - histological slide

Peyer's patch - histological slide

Function

The main tasks of the ileum are:

  • enzymatic cleavage of nutrients
  • absorption of vitamin B12 (with intrinsic factor from the stomach), fats (especially fatty acids and glycerol) and bile salts
  • immunological function (access and transfer of antigens)

Summary

Anatomy

The ileum is the last of the three parts of the small intestine. The transition from the jejunum to the ileum is not sharply marked, while at the distal end the ileum opens into the large intestine. At the junction between the ileum and the caecum lies the ileocecal valve (ileal ostium). The ileum is attached to the posterior wall of the abdomen by the mesentery.

Its arterial supply is via the ileal arteries, while the innervation is provided by the coeliac and superior mesenteric plexi (sympathetic), together with the vagus nerve (parasympathetic).

Histology

Histologically, the ileum has the same basic structure as the jejunum:

  • mucosa, lined by simple columnar epithelium and containing Peyer's patches
  • lamina propria
  • lamina muscularis mucosae
  • submucosa, containing blood vessels, lymph nodes and the Meissner's plexus
  • muscularis consisting of an inner circular and outer longitudinal muscle layers
  • serosa, consisting of a simple squamous epithelium
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Show references

References:

  • D. Drenckhahn/J. Waschke: Taschenbuch Anatomie, 1.Auflage, Urban & Fischer Verlag/Elsevier (2008), S.266-267
  • U. Welsch: Lehrbuch Histologie, 2.Auflage, Urban & Fischer Verlag/Elsevier (2006), S.378-380
  • M. Schünke/E. Schulte/U. Schumacher: Prometheus – LernAtlas der Anatomie – Innere Organe, Thieme Verlag (2009), S.224-228
  • M. Müller: Chirurgie – für Studium und Praxis 2012/13, Medizinische Verlags- und Informationsdienste (2011), S.188

Author & Layout:

  • Achudhan Karunaharamoorthy
  • Christopher A. Becker

Illustrators:

  • Ileum - ventral view - Irina Münstermann
  • Mesentery - ventral view - Irina Münstermann
  • Ileal arteries - ventral view - Irina Münstermann
  • Superior mesenteric plexus - ventral view - Irina Münstermann
  • Ileum - histological slide - Smart In Media
  • Peyer's patch - histological slide - Smart In Media
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

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