Intercellular Adhesion & Other Junctions

Several membrane-associated structures contribute to adhesion and communication between cells. They are present in most tissues but are particularly numerous and prominent in epithelia and will be described here. Epithelial cells are extremely cohesive and relatively strong mechanical forces are necessary to separate them. Intercellular adhesion is especially marked in epithelial tissues that are subjected to traction and pressure (eg, in the skin).

The lateral membranes of epithelial cells exhibit several specialized intercellular junctions. Various junctions serve to function as:

- Seals to prevent the flow of materials between the cells (occluding junctions)

- Sites of adhesion (adhesive or anchoring junctions)

- Channels for communication between adjacent cells (gap junctions).

In several epithelia such junctions are present in a definite order from the apical to the basal ends of the cells.

Tight junctions, or zonulae occludens (singular, zonula occludens), are the most apical of the junctions. The Latin terminology gives important information about the geometry of the junction. "Zonula" indicates that the junctions form bands completely encircling each cell, and "occludens" refers to the membrane fusions that close off the space between the cells. In properly stained thin sections viewed in the TEM, the adjacent membranes appear tightly apposed or fused. The seal between the membranes is due primarily to direct interactions between the transmembrane protein claudin on each cell. After cryofracture, the replicas show these fusion sites as a band of branching strands around each cell. The number of these sealing strands or fusion sites is inversely correlated with the leakiness of the epithelium. Epithelia with one or very few fusion sites (eg, proximal renal tubule) are more permeable to water and solutes than are epithelia with numerous fusion sites (eg, the lining of the urinary bladder). Thus, the principal function of the tight junction is to form a seal that prevents the flow of materials

between epithelial cells (the paracellular pathway) in either direction. In this way, zonulae occludens in sheets of epithelial cells help form two functional compartments: an apical compartment that is composed of an organ cavity (such as the lumen of a secretory unit or the gut) and a basal compartment that begins at the junctions and encompasses the underlying tissue.

Дата добавления: 2015-10-30