A tight junction, also known as a zona occludens, is one of a group of junctions that join cells to one another and to the tissues around them, helping to give structural stability and strength. Tight junctions also enable substances to be carried across epithelial cells efficiently by segregating transport proteins to different regions of the cell. The junctions are situated at the upper part, or apex, of the cell and create seals that prevent movement between the base and apex in either direction. This means that the apex of each cell effectively forms a compartment separate from the base. Tight junctions also stop substances passing from one epithelial cell to the next.
Ridges from adjoining cells meet and join firmly together to form tight junctions. They allow certain molecules to pass through but otherwise completely close off the space between the cells. Proteins are prevented from moving within the membrane, helping to concentrate them in specific areas where they carry substances between cells. This provides a way of allocating different functions to separate parts of the cell.
For example, the epithelial cells lining the intestine allow nutrients from the gut contents to pass through their apical surfaces. Nutrients then move through their basal and lateral surfaces to reach the extracellular fluid and pass into blood vessels. Two different sets of transport proteins are needed for this process, one at the apex and one at the base and sides of the cell, and tight junctions ensure that they stay in their respective areas. Molecules are also prevented from returning to the gut through the spaces between cells because tight junctions seal them. Sometimes, epithelial cells are able to adjust tight junctions to allow extra water and substances through, for example when concentrations in the gut are raised after a meal.
Tight junctions are made up of a network of sealing strands that hold plasma membranes together. They are mostly formed from a group of proteins called claudins. Other types of junctions include anchoring junctions, which connect cell cytoskeletons together and are made from cadherin or integrin proteins. Cytoskeletons are networks of filaments that give cells shape. Finally, gap junctions directly allow passage of molecules between cells.
Zonulin is a protein that regulates the permeability of tight junctions in the intestines and is thought to play a role in autoimmune diseases such as celiac disease and diabetes. In celiac disease, consumption of gluten leads to high levels of zonulin making the gut more permeable than usual. Gluten then enters the blood, triggering an autoimmune response where antibodies target the gut leading to symptoms such as abdominal pain and diarrhea. Treatment generally involves avoiding gluten in the diet but drugs created to block the action of zonulin could prove useful in the future.