12.3: Structural Organization of the Intestinal Wall and Peritoneum

The digestive tract is organized into four layers that vary by region depending on the specific function of that region.

Master this section and you'll be able to:

• Describe the four main tissue layers of the digestive tract and how each contributes to digestive function.
• Compare how the mucosa, submucosa, muscularis, and serosa/adventitia vary among the different digestive organs shown.

Histology of the Digestive Tract Organs

Throughout its length, the digestive tract is built from four main tissue layers — mucosa, submucosa, muscularis, and serosa (or adventitia) — each arranged slightly differently to support the specific functions of different regions. The figure below also highlights the mesentery, which contains the blood vessels and nerves that run between the two serosal layers, and will be discussed a little later.

Figure \(\PageIndex{1}\):  Layers of the Digestive Tract. The digestive tract is made up of four distinctive layers: the most inner layer is the mucosa, formed by the epithelium, lamina propria, and muscularis mucosae. Outside of it lies the supportive submucosa, followed by the muscularis with its inner circular and outer longitudinal smooth-muscle layers that drive mixing and propulsion. The outermost serosa connects to the mesentery, a double-layered fold of peritoneum that holds the digestive organs in place and provides a pathway for vessels and nerves to reach them. Mucosa

Mucosa

The mucosa is often called a mucous membrane because producing mucus is one of the hallmarks of the gut epithelium. This inner lining includes an epithelial layer that directly contacts ingested food, a layer of areolar connective tissue, and a thin sheet of smooth muscle known as the muscularis mucosa. This muscularis mucosa should not be confused with the thicker muscularis layer located farther outward in the digestive tract wall.

Epithelium:

In the oral cavity, pharynx, esophagus, and anal canal, the epithelial lining is mostly non-keratinized stratified squamous epithelium, which provides protection against abrasion.

In the stomach and intestines, it shifts to simple columnar epithelium specialized for secretion and absorption. Because this epithelium is in direct contact with the lumen, its cells constantly encounter food, chemicals, and microbes. Goblet cells scattered among the epithelial cells release mucus and fluid into the lumen, while enteroendocrine cells secrete hormones into the surrounding interstitial spaces to influence digestive activity. These epithelial cells have a short lifespan — from only a few days in the mouth to about a week in the intestines — allowing the mucosa to renew itself quickly despite continual wear.

Lamina propria:

The lamina propria is a layer of areolar connective tissue containing an extensive network of blood and lymphatic vessels that carry absorbed nutrients away from the digestive tract. It also plays a key immune role by housing clusters of lymphocytes known as mucosa-associated lymphoid tissue (MALT). These immune structures are especially well developed in the distal ileum, where they form Peyer’s patches. Because the gut is constantly exposed to food-borne microbes and foreign materials, this immune defense is crucial for maintaining digestive tract health.

Muscularis mucosa:

The muscularis mucosa is a thin sheet of smooth muscle that keeps the mucosal surface in gentle motion. In the stomach and small intestine, this movement creates small folds that greatly increase the surface area available for digestion and absorption.

Figure \(\PageIndex{2}\): Comparing the Mucosa of the Small Intestine and Esophagus. Both images show the mucosa, the innermost layer of the digestive tract, but each organ’s mucosa is adapted to a different job. The small intestine (left) has simple columnar epithelium with tall villi that increase surface area for nutrient absorption. In contrast, the esophagus (right) has a protective non-keratinized stratified squamous epithelium suited for resisting abrasion from swallowed food. Both epithelia are backed by backed by a lamina propria and muscularis mucosa. Image credit: "Mucosa of the GI Tract" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, incorporating illustration by Goran tek-en with edits made by Collectivedogs via Wikimedia Commons and slides provided by the Regents of the University of Michigan Medical School © 2022.)

Submucosa

As its name implies, the submucosa lies immediately beneath the mucosa. This broad layer of connective tissue varies in thickness along the digestive tract. Positioned between the muscularis mucosa and the muscularis, it contains blood and lymphatic vessels that transport absorbed nutrients, along with scattered submucosal glands that release digestive secretions. The submucosa also houses a dense branching network of nerves called the submucosal plexus, which helps regulate local secretions and blood flow.
 

Muscularis

The muscularis (or muscularis externa) is the third major layer of the digestive tract. It is composed of two layers of smooth muscle: an inner circular layer that wraps around the tube, and an outer longitudinal layer that runs along its length. The coordinated contractions of these layers help mechanically break down food, mix it with digestive secretions, and propel it forward through the canal.

In the most proximal and distal regions of the tract, including the oral cavity, pharynx, the upper portion of the esophagus, and the external anal sphincter, the muscularis consists of skeletal muscle. This arrangement allows voluntary control over swallowing and defecation.

Several organs modify the basic two-layer pattern. The stomach adds a third, deep oblique layer that strengthens its ability to churn and mix chyme. The large intestine retains two layers; however, its outer longitudinal layer is gathered into three narrow bands called the teniae coli, which create the characteristic pouch-like appearance of the colon.

Figure \(\PageIndex{3}\): Comparing the Submucosa and Muscularis of the Small Intestine and Stomach. These paired histology images highlight how the same two digestive tract layers — the submucosa and muscularis — look different depending on the organ. The small intestine’s submucosa supports tall villi and contains scattered glands, while its muscularis shows the typical two smooth-muscle layers used for peristalsis. In contrast, the stomach has a thicker, more gland-rich submucosa and a much more robust muscularis, which includes an additional oblique layer that helps churn and mix food during mechanical digestion.
 

Serosa or Adventitia

The serosa is the outermost layer of the digestive tract in regions that lie within the abdominal cavity. It consists of a thin sheet of visceral peritoneum resting on loose connective tissue. Because it is derived from the peritoneum, the serosa provides a smooth, lubricated surface that allows abdominal organs to slide past one another as they move.

In contrast, the oral cavity, pharynx, and esophagus lie outside the abdominal cavity and are not surrounded by the peritoneum. These structures therefore lack a serosa. Instead, they are wrapped in a tougher outer covering called the adventitia, a dense layer of collagen fibers that anchors them in place near the ventral surface of the vertebral column.

The Peritoneum

The peritoneum is a serous membrane that lines the abdominal cavity (parietal peritoneum) and covers most abdominal organs (visceral peritoneum). It is made of a thin sheet of simple squamous epithelial cells that produces a smooth, slippery lining, very much alike other serous membranes, including the pleura and pericardium. A small amount of lubricating fluid reduces friction between organs as they move. The peritoneum also protects and supports the abdominal organs and serves as a passageway for blood vessels, lymphatic vessels, and nerves that supply them.

The abdominal cavity is the large internal space enclosed by the diaphragm, abdominal wall, vertebrae, and pelvic floor. Within this cavity is the intraperitoneal space, which is wrapped in peritoneum. The large, open portion of the peritoneal cavity that you enter as soon as the parietal peritoneum is pierced is called the greater sac (see image below). It fills most of the space between the parietal peritoneum and the visceral peritoneum covering the abdominal organs. Almost all intraperitoneal organs — such as the stomach, liver, and small intestine — lie within or project into this space.

Not all organs inside the abdominal cavity are surrounded by peritoneum though. For example, the kidneys are located in the abdominal cavity but lie behind the peritoneum, making them retroperitoneal rather than intraperitoneal.

Figure \(\PageIndex{4}\): Peritoneal Cavity in Cross-Section. This cross-section of the abdomen shows how the parietal peritoneum lines the inner body wall and how the visceral peritoneum covers the abdominal organs. The space between these layers forms the peritoneal cavity. The mesentery, a double layer of peritoneum, anchors the small intestine to the posterior abdominal wall and carries its blood vessels, nerves, and lymphatics.  

Although they ultimately form one continuous sheet, there are two layers of peritoneum and potential space between those layers.

• The outer layer, called the parietal peritoneum, is attached to the abdominal wall.
• The inner layer, the visceral peritoneum, is wrapped around the internal organs that are located inside the intraperitoneal cavity.
• The potential space between these two layers is the peritoneal cavity. It is filled with a small amount of slippery serous fluid that allows the two layers to slide freely over each other.

The term mesentery is often used to refer to a double layer of visceral peritoneum. There are generally blood vessels, nerves, and other structures between these layers. The space between the two layers is technically outside of the peritoneal sac, and thus not in the peritoneal cavity.

Figure \(\PageIndex{5}\): Relationship of Abdominal Organs to the Peritoneum. The peritoneum folds around the abdominal organs to create two main spaces: the intraperitoneal space and the retroperitoneal space. The parietal peritoneum lines the inner surface of the abdominal wall, while the visceral peritoneum covers selected organs. Organs such as the stomach and much of the intestines are wrapped in visceral peritoneum and suspend within the peritoneal cavity, which includes the large central region called the greater sac (shown in dark blue in the drawing). In contrast, structures such as the kidneys, pancreas, and portions of the duodenum lie behind the peritoneal lining in a region known as the retroperitoneal space.