Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 23

The tight junction was first resolved in the electron microscope as tightly associated regions between the cell membranes of adjacent cells. Tight junctions are also known as “occluding junctions” or “zonula occludens.” They prevent the flow of water and molecules between cells, thus restricting them from exiting the extracellular environment to penetrate the intercellular space. This is called “paracellular movement.”

Tight junctions serve another critical role because they also prevent the movement of membrane components between the top (apical) and bottom (basal) sides of the cell. Thus they serve to restrict protein movements that would otherwise be possible because of the fluidity of the cell membrane. Restricting where certain proteins are localized is a critical event in establishing the polarity of cells. In the example used here, the tight junctions clearly define where the microvilli can form (i.e., apical) versus the cell’s interaction with and adhesion to the basement membrane (i.e., basal).

Figure 3.5. An ultrastructural side view of a tight junction.

The image above reveals the way the tight junction looks at the ultrastructural level (Figure 3.5). A model of a tight junction structure is shown below (Figure 3.6). Tightly aligned rows of tight junction proteins serve to stitch the membrane together effectively sealing the association between adjacent cells. This serves to block the movement of materials through the intercellular space. While we will only cover a couple of the major players in the structure and function of tight junctions, it’s important to understand that the protein complexes that stitch up the membrane are quite complex and form a protein scaffold which can interact with various components in the cytoplasm especially the cytoskeleton.

Figure 3.6. A diagrammatic representation of a tight junction.