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Gastrointestinal Tract Tunics
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Gastrointestinal Tract Tunics

4 tunics (layers) of the GI tract
From deep to superficial:
Mucosa
  • Lines the lumen of the GI tract.
  • The mucosa subdivides into
  • Epithelia comes into contact with contents of GI tract.
  • Lamina propria comprises loose connective tissue; in stomach, houses gastric glands.
  • Muscularis mucosae comprises an inner circular and outer longitudinal layer (note that it is different than the muscularis externa).
  • Mucin-secreting cells for lubrication.
Submucosa
  • Lies under the mucosa.
  • Inner surface follows the contours of the mucosa.
  • Neurovascular, glandular and lymphatic structures travel within it.
Muscularis externa (aka, muscularis)
  • Comprises two or more layers of muscle to propel substance through the GI tract
  • Inner circular layer comprises muscle fibers that encircle the diameter of the GI tract
  • Outer longitudinal layer comprises muscle fibers that run lengthwise along the GI tract.
  • Oblique layer in stomach, facilitates twisting of stomach and churning of stomach contents.
  • Myenteric nerve plexus lies between the inner and outer layers of the muscularis externa.
Adventitia or serosa
  • Outer protective layer.
  • It is categorized as adventitia or serosa depending on its position in the abdominal cavity.
— Serosa, which constitutes most of the GI tract, surrounds organs that are suspended in the abdomen by visceral peritoneum (such as the stomach). — Adventitia surrounds organs that adhere to the abdominal wall (such as the ascending colon). Otherwise, serosa is similar to adventitia.
GI Segments with Serosa
  • Stomach
  • Part of the duodenum
  • Jejunum and ileum
  • Cecum and appendix
  • Transverse colon
  • Sigmoid colon
In general are organs that are "suspended" within the abdomen.
GI segments with Adventitia
  • Most of the duodenum
  • Pancreas
  • Ascending colon
  • Descending colon
  • Rectum
In general, organs that adhere to the abdominal wall.
Key differences in the tunics of the segments of the GI tract
Esophagus
  • Mucosa comprises stratified squamous epithelia, which protects against abrasions from swallowed foods; distal segments' mucosal tunics comprise simple columnar epithelia
  • Submucosa is rich in elastic fibers, to accommodate foods and liquids.
Stomach
  • Mucosa comprises mucosal cells and gastric pits
  • Muscularis externa layer comprises a third sublayer of muscle fibers, called the oblique layer, which wraps obliquely around the stomach to enhance mixing and churning of foods.
Small Intestine
  • Mucosa forms circular folds (aka, plicae circulares, aka, valves of Kerckring), which are covered with villi; this arrangement increases the surface area of the small intestine, and, therefore, the amount of nutrient absorption.
  • There are additional histological differences among the segments of the small intestine.
Large Intestine
  • Mucosa of the large intestine houses goblet cells, intestinal glands, and lymphatic structures.

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