Small Intestine

Structure

diagram of plicae circulares

Diagram of a plica circulares.

diagram of villi

This is a diagram which shows the villi of the small intestine, as indicated by the arrows in the diagram above, at higher magnification.

The main functions of the small intestine are digestion, absorption of food and production of gastrointestinal hormones. The small intestine is 4-6 metres long in humans.

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 (shown in the diagram to the right), most numerous in the upper part of the small intestine
    2. smaller folds called villi, which are finger like mucosal projections, about 1mm long.
    3. the lining columnar epithelial cells have fine projections on their apical surfaces called microvilli.

Together, these folds provide a huge surface area for absorption. Between the villi there are crypts, called crypts of Lieberkuhn, which extend down to the muscularis mucosae. These crypts are short glands.

The lamina propria which underlies the epithelium has a rich vascular and lymphatic network, which absorbs the digestive products, and there is a muscularis mucosae layer immediately at the base of the crypts. The lymphatic capillaries are called lacteals, and absorb lipids. The vascular capillaries are fenestrated to aid absorption.

The muscularis externa layer contains two layers of smooth muscle, an inner circular and outer longitudinal, for continuous peristaltic activity of the small intestine. There are around 200 or so lymphoid aggregations called Peyer's patches in the mucosa.

Look at this photograph of a section through the small intestine, and make sure you can identify the mucosa, submucosa, muscularis mucosae, muscularis externa, and villi.

Sometimes the villi are cut in cross-section, and sometimes longitudinally - and you can see this mixture of sections here.

You can also see part of the fold - plica circulares.

Lymphoid aggregations are commonly found in the sub-mucosa of the small intestine, an you can see one here. The larger aggregations of lymphoid tissue are known as Peyer's Patches.

These patches are more likely to be found in the ileum than in the duodenum.

Mucosa

On this magnified image of the mucosa of the small intestine, can you identify: villi, crypts of Lieberkuhn (L), muscularis mucosa, lamina propria and lymphoid aggregations?

Epithelium and Villi

TEM of an enterocyte

This picture shows a TEM of an enterocyte, showing the villi on the apical surface of the cell.

The epithelium of the villi is made up of tall columnar absorptive cells called enterocytes, and goblet cells, which secrete mucin, for lubrication of the intestinal contents, and protection of the epithelium.

photo of epithelium small intestine

This shows the epithelium of part of a villus at high magnification. You should be able to identify goblet cells, and enterocytes, and notice the 'brush border' on the apical surface of the enterocytes, which is due to the microvilli.

(Click here to compare the epithelia of the oesophagus, stomach, duodenum, small and large intestines)

Crypts

The crypts additionally contain

  • Paneth cells (at the base of the crypts) - they have a defensive function, and stain intensely eosinophilic, due to secretory granules of antimicrobial peptides called defensins, as well as lysozyme and phospholipase A. These cells last for several weeks.
  • Endocrine cells, (also eosinophilic) which produce secretin, somatostatin, enteroglucagon and serotonin. One type of endocrine cell for each type of hormone.
  • Stem cells, found at the base of the crypts, which divide continuously to replace enterocytes (every 2-3 days), goblet cells, paneth cells and neuroendocrine cells.
    Intraepithelial lymphocytes (mostly T-cells).

Duodenum

Look at this picture of a section through the human duodenum. Identify villi, crypts, muscularis mucosae, mucosa, muscularis externa and Brunner's glands.

The first part of the small intestine is the duodenum, and its structure is similar to that seen elsewhere in the small intestine, with some differences. The villi are broader, Peyers Patches are less common, and it has one unique feature: Brunner's glands, which are found in the sub-mucosa.

The duodenum is often mistaken for the small intestine, so take a moment to compare this section to that of the small intestine in the picture above. Make sure you can distinguish correctly between the two, and identify Brunner's glands correctly.

Both Brunner's glands, and the goblet cells in the duodenum secrete mucus. The mucus secreted by Brunner's glands is alkaline, and helps to neutralise the acid chyme produced by the stomach, to produce chyme with a pH suitable for the digestive enzymes of the small intestine.

Digestion:

The chyme is mixed with pancreatic enzymes, and molecules are absorbed by the enterocytes.
Proteins are denatured and chopped up by pepsin from gastric glands, and then further broken down by trypsin, chymotrypsin, elastase and carboxypeptidases in the lumen of the small intestine. Further enzymes in the plasma membrane of the enterocytes complete breakdown into amino acids, and each amino acid is actively transported into the enterocyte.
Carbohydrates are hydrolysed by amylases, and membrane bound enzymes convert sugars to monosaccharides which are absorbed by facilitated diffusion.
Lipids are converted into an coarse emulsion in the stomach, and into a fine emulsion in the duodenum by pancreatic lipases. Small lipid molecules are absorbed by the enterocytes.