GI – Duodenum and Small Bowel

The small intestine is part of the digestive tract that extends from the pylorus proximally to the ileo-caecal valve distally.  It is approximately 4-6 metres long in humans and highly convoluted in the abdomen. It is located intraperitoneally and is divided anatomically into:

• Duodenum
• Jejunum
• Ileum

The main functions of the small intestine are digestion, absorption of food and production of gastrointestinal hormones.

To aid in digestion and absorption:

1. The small intestine secretes enzymes and has mucous producing glands. The pancreas and liver also deliver their exocrine secretions into the duodenum.
2. The mucosa is highly folded.
   1. Large circular folds called plicae circulares are most numerous in the upper part of the small intestine.
   2. Smaller folds called villi, which are finger like mucosal projections, about 1 mm long
   3.  The lining columnar epithelial cells have fine projections on their apical surfaces called microvilli.

As you will recall from the GI – Stomach – Normal Histology CAL , the layers of the gastrointestinal tract are: Surface epithelium, lamina propria, muscularis mucosa (together these layers make up the mucosa), submucosa, muscularis propria and serosa. These are essentially the same throughout the GI tract.

Let’s now look at the different parts of the small bowel in more detail.

The duodenum (about 30 cm long) is the first section of the small intestine that connects the pyloric orifice with the jejunum. It can be divided into superior (D1), descending (D2), inferior (D3) and ascending (D4). Together these parts form a ‘C’ shape, which wraps around the head of the pancreas.

Normal duodenal mucosa is approximately 1 mm thick and is characterised by villi that project above the mucosal surface. In between the villi are crypts (Crypts of Lieberkuhn) that invaginate down into the mucosa. The villi should be approximately 3 times the height of the crypts. The epithelium is intestinal type, which means goblet cells are interspersed among the tall columnar absorptive cells known as enterocytes.

A close look shows us the epithelium lined by surface enterocytes and scattered goblet cells.

Within the lamina propria lymphocytes, plasma cells and eosinophils are normal inhabitants. It also contains vessels and lymphatics which absorbs the digestive products. A few intraepithelial lymphocytes may be seen, although these should be rare at the tips of the villi.

Notice the mix of inflammatory cells within the lamina propria.

One unique feature of the duodenum is the presence of Brunner’s glands. These are found in the submucosa and are mucus producing. This alkaline mucus helps to neutralise the acidic chyme produced by the stomach and produces a pH suitable for the digestive enzymes to work.

Figure – Brunner’s glands as highlighted by the black marker

The histology of the jejunum and ileum is essentially the same as the duodenum, but with a few subtle differences.

The jejunum is the middle of the three parts of the small intestine between the duodenum and ileum. The transition to the ileum is not sharply marked and only visible microscopically.

The jejunum is differentiated from the rest of the small intestine by the absence of Brunner’s glands (duodenum) and Peyer’s patches (ileum) however single lymphoid follicles are present.

The ileum is the last of the three parts of the small intestine. The ileum opens into the cecum via the ileocaecal valve; a functional sphincter formed by the circular muscle layers of both the ileum and cecum.

Compared to the jejunum, the parallel running circular folds in the mucosa (plicae circulares) are less prominent. In contrast it is rich in submucosal lymphoid aggregates or Peyer’s patches.

Peyer’s patches/lymphoid aggregates highlighted by the red circle.

At low power:

• First assess the height of the villi. These should be three times that of the crypt depth.
• Look at the epithelium – is it intestinal type?

At high power:

• Look at the epithelium, and the nuclei – are they normal?
• Look at the tips of the villi – are there intraepithelial lymphocytes? – there should not be many. Are there any neutrophils?
• Are there plasma cells in the lamina propria? There should be.
• Check for parasites and microorganisms, or abundant histiocytes in the lamina propria.

Duodenum –

• Villi and crypt architecture.
• Surface epithelium made up of tall absorptive enterocytes and goblet cells.
• Lamina propria contains a mix of inflammatory cells.
• Unique feature – Brunner’s glands.

Jejunum –

• Villi and crypt architecture.
• Surface epithelium made up of tall absorptive enterocytes and goblet cells.
• Lamina propria contains a mix of inflammatory cells.
• Unique feature – No Brunner’s glands or Peyer’s patches.

Ileum –

• Villi and crypt architecture.
• Surface epithelium made up of tall absorptive enterocytes and goblet cells.
• Lamina propria contains a mix of inflammatory cells.
• Unique feature – No Brunner’s glands, abundant Peyer’s patches.