The gastrointestinal wall surrounding the lumen of the gastrointestinal tract is made up of four layers of specialised tissue – from the lumen outwards:

• Mucosa.
• Submucosa.
• Muscular layer.
• Serosa (if the tissue is intraperitoneal) / Adventitia (if the tissue is retroperitoneal) -- these last two tissue types differ slightly in form and function according to the part of the gastrointestinal tract they belong to (see the section below for more detail).