CorpusHuman Anatomy

Wall of the Gastrointestinal Tract

Transverse section of the small intestine, a magnified wedge of its wall, and peristalsis

The left panel is a transverse section of the small intestine, hung from the posterior abdominal wall by the mesentery. From the lumen outwards it shows the mucosa (villi on circular folds), submucosa, muscularis externa (inner circular and outer longitudinal layers, with the myenteric plexus between them) and serosa. This four-layer plan runs from the oesophagus to the anal canal. The dashed wedge is shown magnified on the right, rotated so that the lumen is at the top. The magnified view shows the epithelium with goblet cells, the lamina propria with capillaries and lacteals, the crypts with stem and Paneth cells at their base, a lymphoid follicle, the muscularis mucosae, and the submucosa with its vessels, gland and Meissner plexus. Below these are the muscle coats and the mesothelium of the serosa.

The bottom strip animates peristalsis in a longitudinal section. A ring of circular-muscle contraction travels aborally behind the bolus while the segment ahead relaxes, pushing the bolus along. The myenteric plexus coordinates this reflex. In the magnified panel, blood flows in the villous capillaries, chyle drains down the lacteals and impulses travel through the myenteric plexus.

Step-by-step

  1. The four-layer plan

    From the oesophagus to the anal canal the wall has four tunics: mucosa, submucosa, muscularis externa and serosa (or adventitia where the gut is fixed). The section is of jejunum, suspended by the mesentery.

  2. Mucosa: epithelium and villi

    Simple columnar epithelium of absorptive enterocytes, with a microvillous brush border, covers finger-like villi on the circular folds. Goblet cells scattered through it secrete protective mucus.

  3. Lamina propria and crypts

    The connective-tissue core of each villus carries a capillary loop, which takes sugars and amino acids to the portal vein, and a central lacteal, which takes chylomicrons to the lymph. Between the villi, crypts of Lieberkühn hold the stem cells that renew the epithelium every 3–5 days and Paneth cells that secrete defensins. Lymphoid follicles (MALT) sit in the lamina propria.

  4. Muscularis mucosae

    A thin double layer of smooth muscle forms the deep boundary of the mucosa. It moves the mucosa and villi locally and squeezes secretions out of the crypts. Invasion through it defines invasive carcinoma.

  5. Submucosa

    Dense irregular connective tissue that makes the strongest layer of the gut wall. It carries the larger blood and lymphatic vessels, the submucosal (Meissner) plexus, which controls secretion and blood flow, and, in the duodenum only, Brunner glands.

  6. Inner circular muscle

    Contraction of the circular layer narrows the lumen. It produces segmentation and the constricting ring of peristalsis, and the layer thickens to form the gut sphincters.

  7. Myenteric (Auerbach) plexus

    Enteric ganglia between the two muscle layers coordinate motility, working with the pacemaker interstitial cells of Cajal. They can produce peristalsis even when the extrinsic nerves are cut.

  8. Outer longitudinal muscle

    Contraction of the longitudinal layer shortens the segment. In the colon this layer forms the taeniae coli.

  9. Serosa and mesentery

    Visceral peritoneum, a mesothelium on thin connective tissue, covers the intestine and continues as the mesentery, which carries vessels, lymphatics and nerves to the gut.

  10. Peristalsis

    A ring of circular-muscle contraction forms behind the bolus while the segment ahead relaxes, driving the bolus aborally. This is the myenteric reflex, the "law of the intestine".

Labelled parts