Tight Junctions and the Intestinal Barrier

The primary function of the gastrointestinal tract is to digest and absorb nutrients. To accomplish this, it must maintain a barrier between the luminal environment, technically a space outside the body, and the internal environment of the body; and it must selectively absorb and secrete nutrients, solutes, and water across the barrier. Separation of tissue spaces throughout the gastrointestinal tract is accomplished by continuous sheets of polarized columnar epithelial cells. An exception exists in the upper two-thirds of the esophagus which is covered by a nonkeratinizing squamous epithelium. Epithelial barriers are selective and capable of excluding potentially noxious luminal contents, such as gastric acid, colonic bacteria, and bacterial antigens, while at the same time capable of directional absorption and secretion of large volumes of solutes and water. Material can pass from one side of the epithelium to the other side along one of the two routes, either through the cell membranes or the space between them, referred to as the transcellular and paracellular pathways, respectively. The connection between individual epithelial cells is created by a series of intercellular junctions, the tight junction (TJ) being the most important for defining the characteristics of the paracellular barrier and its selectivity.