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. 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.
About 12 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. Analogous to the jejunum both the coeliac plexus and the superior mesenteric plexus innervate the ileum sympathetically, the vagus nerve (cranial nerve X) parasympathetically.
Histologically, the ileum has the same basic structure as the jejunum: mucosa, submucosa, muscularis and 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).
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)