Overview

• Duodenum: The shortest and most proximal segment, extending from the pylorus to the duodenojejunal flexure. It receives bile and pancreatic secretions and plays an important role in chemical digestion.
• Jejunum: The middle segment, specialized for nutrient absorption. It has no sharp anatomic boundary with the ileum.
• Ileum: The most distal segment, ending at the ileocecal valve. Particularly important for the absorption of bile salts and vitamin B12.

• Functions:

• Digestion of chyme by pancreatic enzymes and bile, and absorption of nutrients.
• Contribution to immune defence through gut-associated lymphoid tissue, especially Peyer patches in the ileum.

• The absorptive surface area of the small intestine is amplified by three structural levels: plicae circulares, villi, and apical microvilli forming the brush border.
• Microscopically, the wall is composed of 4 layers from the inside outward: mucosa, submucosa, muscularis propria, and serosa.

Mucosa

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide box (PEIR-VM)

Ileal segment with prominent folds "plicae circulares" projecting into the lumen, each lined by numerous villi.

i

Slide :
Ileum, 334

Contribution :
© UAB Histology slide box (PEIR-VM)

As in the rest of the digestive tract, the wall of the small intestine is composed of four layers: mucosa, submucosa, muscularis propria, and serosa.

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide box (PEIR-VM)

Intestinal folds are formed by the mucosa and submucosa, while villi represent projections of the mucosa alone.

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide box (PEIR-VM)

The intestinal lumen may contain vegetable debris and other luminal material.

i

Slide :
Ileum, 334

Contribution :
© UAB Histology slide box (PEIR-VM)

Longitudinal section of ileal villi showing numerous goblet cells with clear cytoplasm.

i

Slide :
Ileum, 334

Contribution :
© UAB Histology slide box (PEIR-VM)

Transverse section of villi showing a single-layered epithelium surrounding a central lamina propria core.

• 1. Epithelial layer:
• The intestinal mucosa is lined by a simple columnar epithelium composed predominantly of enterocytes and goblet cells, extending from the villi to the crypts:

• Enterocytes: Tall columnar cells with basal nucleus and apical brush border formed by densely packed microvilli, which increase the absorptive surface area.
• Goblet cells: Mucus-secreting epithelial cells scattered among enterocytes, progressively increasing in number from the duodenum to the ileum. They contribute to luminal lubrication and mucosal protection.
• Enteroendocrine cells: Specialized hormone-producing cells scattered within the intestinal glands and surface epithelium, involved in regulating digestion, secretion, and gut motility.
• Paneth cells: typically located at the base of the crypts and contain prominent eosinophilic cytoplasmic granules, contributing to innate mucosal defense.
• Microfold cells “M cells”: Overlie lymphoid follicles, particularly over Peyer patches in the ileum, and participate in antigen sampling by transporting luminal material to the underlying immune cells.
  

• The villi are lined mainly by enterocytes with scattered goblet cells, whereas the crypts contain stem cells, proliferating precursor cells, Paneth cells, enteroendocrine cells, goblet cells, and enterocytes.
• Scattered intraepithelial lymphocytes, predominantly T cells, are also present within the epithelial lining.

• Delicate basement membrane separates epithelium from lamina propria.

i

Slide :
Ileum, 333

Contribution :
© UAB Histology slide box (PEIR-VM)

The intestinal mucosa is lined by a simple columnar epithelium composed predominantly of enterocytes and goblet cells, extending continuously from the villi to the crypts.

i

Slide :
Ileum, 333

Contribution :
© UAB Histology slide box (PEIR-VM)

At higher magnification, enterocytes show elongated nuclei, eosinophilic cytoplasm, and a characteristic apical brush border, goblet cells are interspersed between them.

i

Slide :
Ileum, 334

Contribution :
© UAB Histology slide box (PEIR-VM)

Goblet cells are mucus-secreting epithelial cells characterized by a large apical mucin vacuole that displaces the nucleus toward the base.

i

Slide :
Jejunum, 142

Contribution :
© UAB Histology slide box (PEIR-VM)

PAS staining highlights numerous goblet cells within the intestinal epithelium.

i

Slide :
Ileum, 303

Contribution :
© UAB Histology slide box (PEIR-VM)

The intestinal crypts are lined by a simple columnar epithelium composed mainly of enterocytes and goblet cells, with Paneth cells typically located at the base and showing characteristic eosinophilic cytoplasmic granules.

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide  box (PEIR-VM)

Frequent mitotic figures are present near the base of the crypts, reflecting continuous epithelial renewal.

• 2. Lamina propria:

• Forms the connective tissue core of the villi and extends around the intestinal crypts within the mucosa.
• Contains a mixed resident inflammatory cell population composed of lymphocytes, plasma cells, eosinophils, and macrophages; in the ileum, lymphoid aggregates may form Peyer patches, located in the lamina propria and often extending into the submucosa.
• Within the villi, it contains capillaries, delicate smooth muscle fibers, and a central blind-ending lymphatic channel known as a lacteal.

• 3. Muscularis mucosa:

• The muscularis mucosae is a thin layer of smooth muscle separating the mucosa from the submucosa.
• Fine smooth muscle fibers extend from the muscularis mucosae into the villi and contribute to villous motility.

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide box (PEIR-VM)

Prominent ileal lymphoid aggregates within the lamina propria form Peyer patches, often with pale germinal centers.

i

Slide :
Ileum, 168

Contribution :
© UAB Histology slide box (PEIR-VM)

The epithelium overlying or adjacent to lymphoid aggregates may show increased intraepithelial lymphocytes, a normal finding in this setting.

i

Slide :
Small intestine, H&E, 029-1

Contribution :
© University of Michigan Virtual Microscopy

Lymphoid aggregates within the lamina propria may overlie and interrupt the muscularis mucosae, making this layer appear discontinuous and difficult toidentify.

i

Slide :
Ileum, 303
‍
Contribution :
© UAB Histology slide box (PEIR-VM)

Delicate smooth muscle fibers arising from the muscularis mucosae extend into the villi.

i

Slide :
Ileum, 303
‍
Contribution :
© UAB Histology slide box (PEIR-VM)

Delicate smooth muscle bundles extending from the muscularis mucosae are observed in the villous cores, where they contribute to villous motility.

i

Slide :
Ileum, 168
‍
Contribution :
© UAB Histology slide box (PEIR-VM)

Lymphatic channels may be seen within the villi, particularly in the jejunum where lipid absorption is most active, these central lymphatics are known as lacteals.

i

Slide :
Ileum, 302
‍
Contribution :
© UAB Histology slide box (PEIR-VM)

The submucosa lies between the muscularis mucosae internally and the muscularis propria externally, and consists of dense connective tissue containing blood vessels, lymphatics, and nerve bundles.

i

Slide :
Ileum, 171
‍
Contribution :
© UAB Histology slide box (PEIR-VM)

The submucosa may contain a variable amount of adipose tissue, which is often more conspicuous in the ileum.

i

Slide :
‍Gastroduodenal junction, H&E, 161

‍Contribution :
‍© University of Michigan Virtual Microscopy

The duodenal submucosa contains Brunner glands, which drain via ducts opening into the intestinal crypts and release their secretions into the lumen.

i

Slide :
Duodenum, 100

Contribution :
© Dr Smiths Virtual Histology Slides

Brunner glands appear as compound tubuloacinar mucous glands with clear cytoplasm, located in the submucosa; their ducts traverse the muscularis mucosae and open into the intestinal crypts.

Muscularis propria

• Consists of smooth muscle organized into an inner circular layer and an outer longitudinal layer, with the myenteric plexus (Auerbach plexus) located between them.
• Auerbach plexus primarily regulates peristaltic activity.