Wall of the alimentary canal
Also known as: gut wall, GI wall layers, tunics of the GI tract, layers of the gut
From the oesophagus to the anal canal the gut wall has the same four concentric layers: mucosa, submucosa, muscularis externa and serosa or adventitia. Two enteric nerve plexuses lie within it.
Overview
From the lumen outwards the four layers (tunics) are the mucosa (epithelium, lamina propria and muscularis mucosae), which secretes, absorbs and protects; the submucosa, dense connective tissue carrying larger vessels, lymphatics and the submucosal (Meissner) plexus; the muscularis externa, inner circular and outer longitudinal smooth muscle with the myenteric (Auerbach) plexus between them, which mixes and propels; and the serosa (visceral peritoneum) over intraperitoneal parts, or a fibrous adventitia where the gut is fixed in the neck, thorax or retroperitoneum.
Each region adapts this plan to its function. Epithelium is protective stratified squamous in the oesophagus and lower anal canal, secretory simple columnar in the stomach, and absorptive simple columnar with villi in the small intestine. Glands occur in the submucosa only in the oesophagus and duodenum (Brunner). The muscularis is skeletal in the upper oesophagus and external anal sphincter, has an extra oblique layer in the stomach, and is gathered into taeniae coli in the colon. Lymphoid tissue (MALT) is spread through the lamina propria and gathered into Peyer patches in the ileum and follicles in the appendix.
The enteric nervous system has two plexuses. The myenteric plexus mainly controls motility, and the submucosal plexus mainly controls secretion and blood flow. Interstitial cells of Cajal, associated with the myenteric plexus, generate the slow-wave pacemaker rhythm.
- Location
Along the whole alimentary canal, from the oesophagus to the anal canal. The oral cavity and pharynx differ, lacking a typical muscularis mucosae and having skeletal muscle.
Parts & subdivisions
- Epithelium, lamina propria and muscularis mucosae.
- Thin smooth-muscle layer at the base of the mucosa.
- Connective tissue with vessels, glands and the submucosal plexus.
- Circular and longitudinal smooth muscle with the myenteric plexus.
- Outer covering: visceral peritoneum or fibrous adventitia.
- Enteric ganglia in the submucosa. Control secretion, absorption, local blood flow and the movements of the muscularis mucosae.
- Enteric ganglia between the circular and longitudinal muscle. Control peristalsis, segmentation and sphincter tone.
- Diffuse lymphocytes and plasma cells in the lamina propria, solitary follicles, Peyer patches and appendiceal follicles.
Anatomy
- Histology
Mucosa → submucosa → muscularis externa (inner circular, outer longitudinal) → serosa or adventitia. Ganglia lie in the submucosa and between the muscle layers.
- Development
The epithelium and glands come from endoderm. The lamina propria, muscle and serosa come from splanchnic mesoderm. Enteric neurons and glia come from neural crest cells that migrate along the gut (vagal crest cells colonise craniocaudally, and sacral crest cells contribute to the hindgut).
Function
- Secretion, absorption and barrier protection (mucosa).
- Vascular, lymphatic and neural distribution (submucosa).
- Mixing and propulsion (muscularis externa).
- Low-friction movement and anchorage (serosa and adventitia).
- Intrinsic neural control (enteric plexuses).
Clinical significance
Depth of disease
Ulcerative colitis is confined to the mucosa and submucosa. Crohn disease is transmural, which explains its fistulae and strictures. A true diverticulum (Meckel) contains all layers. A false diverticulum (colonic, Zenker) is mucosa and submucosa herniating through the muscularis.
T staging of GI cancers
Staging is by depth of invasion. Tis is intramucosal. T1 invades the submucosa, where the risk of nodal spread rises. T2 invades the muscularis propria. T3 reaches the subserosa or adventitia. T4 penetrates the serosa or invades adjacent organs. Endoscopic resection is possible only for superficial lesions.
Hirschsprung disease
Absence of ganglion cells in both the submucosal and myenteric plexuses of the distal bowel leaves a tonically contracted segment and a dilated proximal colon. It is diagnosed on rectal suction biopsy.
Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.