Paneth cells
Characteristic features of the small intestine include:
• Intestinal villi. These are finger-like projections into the lumen (consisting of surface epithelium
and underlying lamina propria).
*** The epithelium lining the lumen consists of a simple columnar epithelium with goblet cells.
The apical surface of the absorptive epithelial cells has a “brush border” (resulting from an orderly arrangement of closely-packed microvilli, which may number several hundred per absorptive
cell). The main function of the microvilli is to increase the surface area available for absorption.
*** The lamina propria of the small intestine is formed from loose connective tissue. This contains
blood vessels, nerves, and large lymphatic vessels (site of absorption of lipids).
• Intestinal glands. These are simple tubular glands that open to the intestinal lumen between the
base of the villi. The intestinal glands are sometimes called the crypts of Lieberkühn. Paneth cells
are specialized secretory epithelial cells located at the bases of intestinal crypts (crypts of
Lieberkühn). They are most common in the ileum. Their function is still not fully understood, but it
is known that they secrete lysozyme, which has anti-bacterial properties and helps maintain the gastrointestinal barrier.
• Valves of Kerckring. The lining of the small intestine has permanent folds known as valves of
Kerckring or plicae circulares. These are most prominent in the jejunum. These folds, seen macroscopically in transverse sections, consist of mucosa and submucosa.
Three types of epithelial cells line the microvilli of the “brush border”:
1. Goblet cells: secrete mucus, abundant in ileum.
2. Absorptive cells: participate in absorption, simple columnar cells.
3. Enteroendocrine cells: secrete enterogastrones (secretin and cholecystokinin) into the bloodstream. Abundant in the duodenum.
Remember:
1. Brunner’s glands (also called duodenal glands or submucosal glands) are small, branched,
coiled, tubular glands situated deeply in the submucosa of the duodenum. These glands secrete an
alkaline mucus to protect the walls from the intestinal juice.
2. The ileum contains aggregates of mesenteric lymph nodes called Peyer’s patches, which intercept and destroy bacteria before they are absorbed by the digestive tract.