Layers of the Alimentary Canal

Mucosa

The mucosa, composed of simple epithelium cells, is the innermost layer of the gastrointestinal (GI) tract. It is the absorptive and secretory layer of the GI tract.

Learning Objectives

Describe the structure and function of the mucosa of the GI tract

Key Takeaways

Key Points

  • The mucosa is the innermost layer of the GI tract. It is made up of three layers: the epithelium, lamina propria, and muscularis mucosae.
  • The mucosa surrounds the lumen, or open space within the digestive tube. This layer comes in direct contact with digested food (chyme).
  • The epithelium of the mucosa is particularly specialized, depending on the portion of the digestive system.

Key Terms

  • lumen: The inside space of a tubular structure, such as an artery or intestine.

Layers of GI Tissue

The GI tract is composed of four layers. Each layer has different tissues and functions. From the inside out they are called: mucosa, submucosa, muscularis, and serosa.

The mucosa is the innermost layer, and functions in absorption and secretion. It is composed of epithelium cells and a thin connective tissue.

The mucosa contains specialized goblet cells that secrete sticky mucus throughout the GI tract. On the mucosa layer, small finger-like projections called villi and microvilli help to increase surface area for nutrient absorption.

This is a drawing that shows the layers of GI tissue: the mucosa, submucosa, muscularis, and serosa. Note the mucosa, located at the innermost layer.

Layers of GI tissue: Note the mucosa, located at the innermost layer.

Layers of Tissue Within the Mucosa

Since the mucosa is the innermost layer within the GI tract, it surrounds an open space known as the lumen. Food, mucus, and digestive juices pass through the lumen, and the mucosa comes in direct contact with digested food (chyme).

This drawing of a cross section shows the mucosa in relation to the interior space, or lumen—it directly surrounds the lumen.

General structure of the gut wall: This cross section shows the mucosa in relation to the interior space, or lumen.

The mucosa is made up of three layers:

  1. The epithelium is the innermost layer and it is responsible for most digestive, absorptive, and secretory processes.
  2. The lamina propria is a layer of connective tissue that is unusually cellular compared to most connective tissue.
  3. The muscularis mucosae is a thin layer of smooth muscle and its function is still under debate.

The mucosae (singular: mucosa) are highly specialized in each organ of the gastrointestinal tract in order to deal with different digestive tract conditions. The most variation is seen in the epithelium tissue layer of the mucosa.

  • In the esophagus, the epithelium is stratified, squamous, and non-keratinizing, for protective purposes.
  • In the stomach. the epithelium is simple columnar, and is organized into gastric pits and glands to deal with secretion.
  • In the small intestine, the epithelium (particularly the ileum) is specialized for absorption, with villi and microvilli increasing surface area.

Submucosa

The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa.

Learning Objectives

Describe the submucosa of the GI tract

Key Takeaways

Key Points

  • The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa.
  • In the gastrointestinal tract, the submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle (fibers that run circularly within a layer of longitudinal muscle).
  • Tiny parasympathetic ganglia are scattered around to form the submucosal plexus (or Meissner’s plexus) where preganglionic parasympathetic neurons create synapses with postganglionic nerve fibers that supply the muscularis mucosae.

Key Terms

  • parasympathetic ganglia: The autonomic ganglia of the parasympathetic nervous system; they lie near or within (respectively) the organs they innervate.
  • lymphatic: The system that carries a clear fluid called lymph that is formed from interstitial fluid collected through the capillaries.
  • nerve: A bundle of neurons with their connective tissue sheaths, blood vessels, and lymphatics.

The Layers of the GI tract

The GI tract is composed of four layers. Each layer has different tissues and functions. From the inside out they are called:

  1. Mucosa
  2. Submucosa
  3. Muscularis
  4. Serosa

The Submucosa

The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa.

The submucosa also has glands and nerve plexuses. The submucosa lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer, the muscularis externa.

The stomach is illustrated, with a closeup view of the layers of stomach lining: the mucosa, submucosa, muscularis, and serosa.

Layers of stomach lining: Stomach. The serosa is labeled at far right, and is colored yellow.

The Muscularis

The muscularis in the stomach differs from that of other GI organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia—layers of connective tissue that are continuous with the omenta.

The general structure of the gut wall is illustrated.

General structure of the gut wall: The general structure of the gut wall is illustrated.

The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that branch into the mucosa and muscularis externa. It contains Meissner’s plexus, an enteric nervous plexus, situated on the inner surface of the muscularis externa.

In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa. It also joins the mucosa to the bulk of underlying smooth muscle (fibers running circularly within layer of longitudinal muscle).

Blood vessels, lymphatic vessels, and nerves (all supplying the mucosa) will run through here. Tiny parasympathetic ganglia are scattered around forming the submucosal plexus (or Meissner’s plexus) where preganglionic parasympathetic neurons create synapses with the postganglionic nerve fibers that supply the muscularis mucosae.

Muscularis

The muscularis is responsible for the segmental contractions and peristaltic movements in the gastrointestinal (GI) tract.

Learning Objectives

Identify the function of muscularis in the GI tract

Key Takeaways

Key Points

  • The muscularis, or muscularis externa, consists of an inner circular muscular layer and a longitudinal outer muscular layer. The coordinated contractions of these layers is called peristalsis, which propels the food through the GI tract.
  • Between the two muscle layers is the myenteric or Auerbach’s plexus, which controls peristalsis.
  • In the colon, the muscularis externa is much thicker because the feces are large and heavy, requiring more force to push along.
  • The stomach has a third layer of muscularis externa: the inner oblique layer. This helps churn the chyme in the stomach.
  • Peristaltic activity  in the muscularis externa is regulated by the enteric nervous system and the autonomic nervous system.

Key Terms

  • muscularis externa: A region of muscle in many organs in the vertebrate body, adjacent to the submucosa membrane. It is responsible for gut movement such as peristalsis.
  • oblique layer: This layer is responsible for creating the motion that churns and physically breaks down the food.
  • tiniae coli: These are three, separate longitudinal ribbons of smooth muscle on the outside of the ascending, transverse, descending, and sigmoid colons.

The gastrointestinal (GI) tract is composed of four layers of tissue, known as tunics. Each layer has different structures and functions. From the inside out they are called the mucosa, submucosa, muscularis externa, and serosa.

Structure of the Muscularis Externa

This is a drawing of the the muscularis mucosa of the submucosa. The muscularis mucosa is adjacent to the submucosa, and should not be confused with the muscularis externa.

Muscularis mucosa of the submucosa: The muscularis mucosa is adjacent to the submucosa, and should not be confused with the muscularis externa.

The muscularis externa is responsible for segmental contractions and peristaltic movement in the GI tract. These muscles cause food to move and churn together with digestive enzymes down the GI tract. The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. It should not be confused with a thin layer of muscle known as the muscularis mucosa, which lies within the submucosa, a layer of tissue adjacent to the muscularis externa. The muscularis mucosa is made up of smooth muscle, and is most prominent in the stomach.

Within the muscularis externa, the circular muscle layer prevents food from traveling backward, while the longitudinal layer shortens the tract. The layers are not truly longitudinal or circular, rather the layers of muscle are helical with different pitches. The inner circular is helical with a steep pitch and the outer longitudinal is helical with a much shallower pitch.

The coordinated contractions of these layers is called peristalsis. Between the two muscle layers is the myenteric or Auerbach’s plexus, which controls peristalsis. Peristaltic activity is regulated by these nerve  cells, and the rate of peristalsis can be modulated by the rest of the autonomic nervous system.

The thickness of muscularis externa varies in each part of the tract. In the colon, for example, the muscularis externa is much thicker because the feces are large and heavy, and require more force to push along. The outer longitudinal layer of the colon thins out into three discontinuous longitudinal bands known as tiniae coli (bands of the colon). This is one of the three features helping to distinguish between the large and small intestine.

This is a drawing of the general structure of the gut wall—the muscularis externa is labeled circular muscle and longitudinal muscle here.

General Structure of the gut wall: General structure of the gut wall—the muscularis externa is labeled circular muscle and longitudinal muscle here.

Occasionally in the large intestine (two to three times a day), there will be mass contraction of certain segments, moving a lot of feces along. This is generally when one gets the urge to defecate.

The pylorus of the stomach has a thickened portion of the inner circular layer: the pyloric sphincter. Alone among the GI tract, the stomach has a third layer of muscularis externa. This is the inner oblique layer, and helps churn the chyme in the stomach.

Serosa

Serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movements.

Learning Objectives

Diagram the structure of the serosa

Key Takeaways

Key Points

  • The serous layer provides a partition between the internal organs and the abdominal cavity.
  • Cells of the serous layer secrete a serous fluid that provides lubrication to reduce friction.
  • The connective tissue layer provides blood vessels and nerves.
  • The three serous cavities within the human body are the pericardial cavity (surrounding the heart ), the pleural cavity (surrounding the lungs), and peritoneal cavity (surrounding most organs of the abdomen).
  • The serous membrane covers the heart; it has an inner layer (the parietal pericardium ) and an outer layer (the visceral pericardium).
  • The serosa of the uterus is called the perimetrium.

Key Terms

  • pleural cavity: The body cavity that surrounds the lungs and is enclosed by the pleura.
  • coelom: A fluid-filled cavity within the body of an animal. The digestive system is suspended within the cavity that is lined by a tissue called the peritoneum.
  • serosa: A membrane that lines an internal cavity to protect the contents and secretes serum.

The Serous Membrane

In anatomy, the serous membrane (or serosa) is a smooth membrane that consists of a thin connective tissue layer and a thin layer of cells that secrete serous fluid. Serous membranes line and enclose several body cavities, known as serous cavities, where they secrete a lubricating fluid to reduce friction from muscle movements.

Serosa is not to be confused with adventitia, a connective tissue layer that binds together structures rather than reduces friction between them.

This is a drawing of the layers of the stomach lining.The mucosa, submucosa, muscle layers, and the serosa—the outermost layer—are highlighted.

Layers of stomach lining: The serosa is labeled at far right, and is colored yellow.

Each serous membrane is composed of a secretory epithelial layer and a connective tissue layer underneath. The epithelial layer, known as mesothelium, consists of a single layer of avascular flat nucleated cells (simple squamous epithelium) that produce the lubricating serous fluid. This fluid has a consistency similar to thin mucus.

These cells are bound tightly to the underlying connective tissue. The connective tissue layer provides the blood vessels and nerves for the overlying secretory cells, and also serves as the binding layer that allows the whole serous membrane to adhere to organs and other structures.

For the heart, the surrounding serous membranes include: the outer, inner, parietal pericardium, and visceral pericardium (epicardium). Other parts of the body may also have specific names for these structures. For example, the serosa of the uterus is called the perimetrium.

The pericardial cavity (surrounding the heart), pleural cavity (surrounding the lungs) and peritoneal cavity (surrounding most organs of the abdomen) are the three serous cavities within the human body. While serous membranes have a lubricative role to play in all three cavities, in the pleural cavity it has a greater role to play in the function of breathing.

The serous cavities are formed from the intraembryonic coelom and are basically an empty space within the body surrounded by a serous membrane. Early in embryonic life, visceral organs develop adjacent to a cavity and invaginate into the bag-like coelom.

Therefore each organ becomes surrounded by a serous membrane—they do not lie within the serous cavity. The layer in contact with the organ is known as the visceral layer, while the parietal layer is in contact with the body wall.