The Small Intestine and Celiac Disease

by Robert Tallitsch, PhD | April 7, 2022

Video with Patient Case Example of Celiac Disease and Explanation of the Small Intestine!

Written by: Robert Tallitsch, PhD

Celiac disease (CD) is a chronic, autoimmune disorder experienced by 2 million people within the United States. Most of the pathological effects of CD occur within the small intestine. To understand CD and its effects on the small intestine, we first need to understand the anatomy of the small intestine of the digestive system.

Gross Anatomy of the Small Intestine

The small intestine consists of the duodenum, jejunum, and ileum.

Duodenum

The duodenum is the first and shortest (25 cm) part of the small intestine. It is also the widest and least mobile part of the small intestine. It is subdivided into four parts: superior, descending, horizontal, and ascending segments.

The superior segment is continuous with the pylorus of the stomach superiorly, and is at the level of L₁(first lumbar vertebra).
The descending segment descends along the right side of vertebrae L₁to L₃. The common bile duct and pancreatic duct empty into this segment of the duodenum.
The horizontal segment crosses from right to left within the abdominal cavity at the body of L₃.
The final ascending segment passes superiorly along the left side of the aorta, ascending to approximately L₂prior to emptying into the jejunum.

Jejunum and Ileum

The combined length of the jejunum and ileum ranges from 6 to 7 meters. The ileum ends at its junction with the cecum of the large intestine, which is termed the ileocecal valve. No clear demarcation line marks the junction between the jejunum and the ileum, but each segment has distinctive characteristics that are surgically important. Some of these include the color of each segment (jejunum being a deeper red compared to the ileum’s more pinkish color), the thickness of the segment’s walls (jejunal wall is thicker and heavier), the amount of fat within the supporting mesenteries (most found associated with the ileum), and the number of lymphoid nodules within the walls of the segments (most within the ileum).

The jejunum is found within the right and left upper quadrants (RUQ and LUQ) of the abdominal cavity, while the ileum is within the right and left lower quadrants (RLQ and LLQ). The greatest percentage of the jejunum is found within the LUQ, while the greatest percentage of the ileum is within the RLQ.

Histological Features of the Small Intestine

The walls of the small intestine, like most of the hollow organs of the digestive system, are composed of a series of layers (from superficial to deep): mucosa, muscularis mucosae, submucosa, muscularis externa, and serosa.

When one opens the small intestine by a longitudinal incision a series of mucosal folds running parallel to each other and passing in circular or oblique directions are seen on the inner surfaces of the small intestine. These mucosal folds, termed plicae circularis, increase the absorptive surface area of the small intestine. The surface area of the mucosa is increased further by a series of finger-like projections of the mucosa termed villi, which cover the entire surface of the small intestine.

In addition to the plicae and villi, the mucosa also possesses numerous depressions, termed intestinal crypts (crypts of Lieberkühn). These open between the villi and extend through the mucosa and into the muscularis mucosae.

The epithelium of the mucosa is a simple columnar epithelium with scattered goblet cells. Each of these columnar cells possess even smaller finger-like projections termed microvilli. These further increase the surface area of the small intestine.

The muscularis mucosae is composed of two thin layers of smooth muscle: an inner circularly arranged layer and an outer longitudinally arranged layer.

The submucosa is composed of dense, irregularly arranged connective tissue. The submucosa of some segments of the small intestine possess glands that open into the intestinal crypts of the mucosa.

The muscularis externa is composed of two thick layers of smooth muscle: an inner circularly arranged layer, and an outer, longitudinally arranged layer.

The serosa is the outermost layer of the small intestine. It is composed of a thin layer of irregularly arranged connective tissue.

Histological Variations Between Segments

Distinct histological variations are seen between the segments of the small intestine.

Large, prominent submucosal glands are seen only within the duodenum.
The number of goblet cells increases as you progress distally within the small intestine.
The shape of the villi vary from segment to segment.
The number of aggregated lymphoid nodules (Peyer’s Patches) are most numerous in the ileum.

Celiac Disease

Celiac disease (CD) is an autoimmune disease resulting in a permanent intolerance to certain peptides within gluten. Gluten is a protein naturally found within wheat, barley, and rye. While there is no known cure for CD, symptoms are minimized by a life-long gluten-free diet.

Certain molecules within the intestinal epithelium bind to peptides and present them to T cells of the immune system. If these peptides are pathogen-derived the T cells initiate an immune response. In CD the immune system is “misguided” and inappropriately responds to the gluten peptides, causing inflammation within the small intestine. This reduces the number and height of the intestinal villi, thereby reducing surface area and the ability to absorb nutrients. CD induces various symptoms, including chronic diarrhea, fatigue, anemia (typically with iron deficiency), loss of bone density, joint pain, and reduced splenic function.

Various tests are utilized to diagnose CD, including blood tests for iron and levels of antibodies to gluten. A biopsy of the small intestine may also be conducted.

The long-term prognosis for a patient with CD varies. With proper treatment and long-term medical follow-ups the prognosis is often excellent. Left untreated patients often suffer severe complications and, in rare instances, CD can be fatal.

Key Terms:

Gluten - A peptide found in grains, including wheat, barley, and rye.

Plica circularis - A series of mucosal folds running parallel to each other and passing in circular or oblique directions on the inner surfaces of the small intestine.

Duodenum - The first and shortest part of the small intestine.

Muscularis mucosae - The second layer of the small intestine, which is composed of two thin layers of smooth muscle: an inner circularly arranged layer and an outer longitudinally arranged layer.

Ascending segment of the duodenum - The final segment of the duodenum.

Ileocecal valve - The junction between the end of the ileum and the cecum of the large intestine.

Villi - Specializations of the mucosa of the small intestine. Villi increase the surface area available for absorption.

Intestinal crypts (crypts of Lieberkühn) - Numerous depressions which open between the villi and extend through the mucosa and into the muscularis mucosae.

Celiac disease - An autoimmune disease resulting in a permanent intolerance to certain peptides within gluten.

Questions:

List the layers of the small intestine, progressing from superficial to deep.
A: Mucosa, muscularis mucosae, submucosa, muscularis externa, serosa.
List the three segments of the small intestine, progressing from proximal to distal.
A: Duodenum, jejunum, and ileum
Which part of the small intestine is the least mobile?
A: Duodenum
List three specializations of the mucosa of the small intestine that serve to increase the surface area available for absorption.
A: Plicae circulares, villi, and microvilli
What marks the end of the ileum?
A: The junction between the ileum and cecum, termed the ileocecal valve.
True or False? The number of goblet cells within the mucosa of the small intestine decreases from proximal to distal in the small intestine.
A: False. The number of goblet cells within the mucosa of the small intestine increases from proximal to distal within the small intestine.
What are the depressions in the mucosa that open between the villi and extend into the muscularis mucosae?
A: Intestinal crypts (crypts of Lieberkühn)
True or False? Celiac disease is an autoimmune disease that can be cured by eliminating gluten from one’s diet.
A: False. Celiac disease is an autoimmune disease but it has no known cure. Eliminating gluten from one’s diet reduces or eliminates the symptoms of the disease, but the disease cannot be cured.
True or False? The inflammation caused by CD reduces the number and height of the villi within the small intestine, thereby reducing surface area and the intestine’s ability to absorb nutrients.
A: True

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