Small intestine
The small intestine is part of the digestive system. At first glance it looks like a tube of a couple of centimeters in diameter that is connected to the stomach. It is surrounded by two layers of smooth muscles that contract in a wavelike pattern. This peristaltic movement mixes the content of the small intestine and slowly moves it towards the large intestine. The pancreas secretes the digestive enzymes to the mixture. These enzymes break the food down into its molecular building blocks. These nutrients are absorbed by the cells of the intestinal lining and transported into the blood stream. That is exactly where pathogens want to get to, therefore the intestinal lining has to be sealed well and protected against intruders.
Cell junctions
The intestinal lining consists of a single columnar epithelium. The palisade-like cells are tightly connected with cell junctions. One type of these cell junctions is called desmosomes. They are protein complexes that connect the keratin fibers of two epithelial cells like a hook-and-loop fastener. Desmosomes help the epithelium to resist shearing forces.
The lateral (outer) side of the epithelium is sealed by a network of tight junctions. These sealing strands connect the plasma membrane of two neighboring cells forming a virtually impermeable barrier to fluid.
The tight junctions are supported by the adherens junctions, which are located a bit more basally (inside). Adherens junctions are anchor-like protein complexes similar to desmosomes, but they are connected with the actin cytoskeleton. Actin fibers also define the shape of microvilli; microscopic cell extensions on the lateral side of epithelial cells. The lateral plasma membrane is packed with digestive enzymes. The surface enlargement caused by the microvilli allows the cells to maximize the amount of enzymes lined up to process nutrients. In addition, the surface of the microvilli is covered with the glycocalix. These tree-like glycoprotein structures increase the active surface even more by anchoring digestive enzymes and binding substances for uptake. 