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Ileum: want to learn more about it?

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Ileum

The small intestine is composed of three distinct parts, the last one being the ileum. At the distal end, the ileum is separated from the large intestine, into which it opens, by the ileocecal valve. The ileum itself is very rich in lymphoid follicles and is attached to the abdominal wall by the mesentery. Its vascular supply is provided by the ileal arteries and its innervation via the coeliac and superior mesenteric plexi.

In terms of histology, the mucosa of the ileum consists of simple columnar epithelium comprising of enterocytes and goblet cells. A characteristic histological feature of the ileum are Peyer's patches.

Key facts about the ileum
Definition Last of the three parts of the small intestine, found between the jejunum and large intestine
Blood supply

Straight arteries (branches of the superior mesenteric artery)

Superior mesenteric vein

Innervation

Coeliac plexus

Superior mesenteric plexus

Vagus nerve

Histology

Mucosa: Simple columnar epithelium; also contains Peyer's patches

Submucosa: Contains neurovasculature

Tunica muscularis: Circular and longitudinal muscle layers

Tunica serosa: Simple squamous epithelium

Function Enzymatic digestion of nutrients
Absorption of vitamin B12, fats and bile salts
Immunological function

This article will explain both the anatomy and histology of the ileum, and provide you with a quick summary at the end to emphasize the important elements that you should remember for your next exam about the small intestine.

Anatomy of the ileum

Characteristics and 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.

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.

Why not test your knowledge of the iluem with some quiz questions? They're the secret to your success!

Blood supply and innervation

About twelve ileal arteries called straight 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 which accompany the arteries and empty into the superior mesenteric vein.

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.

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).

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).

For more information about the anatomy and histology of the ileum, take a look below:

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)

Terminal ileum, cecum and appendix in a cadaver: The terminal ileum joins the cecum approximately at right angles. The wall of the ileum is thinner than the wall of the jejunum. This distinguishing feature is helpful in cadaveric dissections.

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

Ileum: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

Sign up for your free Kenhub account today and join over 1,164,375 successful anatomy students.

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

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 and layout:

  • Achudhan Karunaharamoorthy
  • Christopher A. Becker

Illustrators:

  • Terminal ileum, cecum and appendix in a cadaver (image) - Prof. Carlos Suárez-Quian
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