GI Tunics AP Biology > the Digestive and Urinary Systems > the Digestive and Urinary Systems

GI Tunics AP Biology > The Digestive and Urinary Systems > The Digestive and Urinary Systems

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.

1 / 3 • 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.