 Anchoring junctions provide mechanical stability to epithelial cells by linking the cytoskeleton of one cell to the cytoskeleton of an adjacent cell. These junctions are important in creating and maintaining the structural unity of the epithelium. Anchoring junctions interact with both actin and intermediate flaments and can be found not only on the lateral cell surface but also on the basal domain of the epithelial cell. Through their signal transduction capability, anchoring junctions also play important roles in cell-to-cell recognition, morphogenesis, and differentiation.
 Communicating junctions allow direct communication between adjacent cells by diffusion of small (1,200 daltons) molecules (e.g., ions, amino acids, sugars, nucleotides, second messengers, metabolites). This type of intercellular communication permits the coordinated cellular activity that is important for maintaining organ homeostasis.
The zonula occludens (pl., zonulae occludentes) represents the most apical component in the junctional complex between epithelial cells.
The zonula occludens is created by localized sealing of the plasma membrane of adjacent cells.
Examination of the zonula occludens or tight junction with the transmission electron microscope (TEM) reveals a narrow region in which the plasma membranes of adjoining cells come in close contact to seal off the intercellular space (Fig. 5.15a). At high resolution, the zonula occludens appears not as a continuous seal but as a series of focal fusions between the cells. These focal fusions are created by transmembrane proteins of adjoining cells that join in the intercellular space (Fig. 5.15b). The arrangement of these proteins in forming the seal is best visualized by the freeze fracture technique (Fig. 5.15c). When the plasma membrane is fractured at the site of the zonula occludens, the junctional proteins are observed on the P-face of the membrane, where they appear as ridgelike structures. The opposing surface of the fractured membrane, the E-face, reveals complementary grooves resulting from detachment of the protein particles from the opposing surface. The ridges and grooves are arranged as a network of anastomosing particle strands, thus creating a functional seal within the intercellular space. The number of strands as well as the degree of anastomosing varies in different cells.
FIGURE 5.15 • Zonula occludens. a. Electron micrograph of the zonula occludens showing the close approximation of the outer lamellae of adjoining plasma membranes. The extracellular domains of proteins involved in the formation of this junction (occludins) appear as single, electron-dense lines (arrows). 100,000. b. Diagram showing the organization and pattern of distribution of the transmembrane protein occludin within the occluding junction. Compare the linear pattern of grooves with the ridges detected in the freeze fracture preparation on the right side. c. Freeze fracture preparation of zonula occludens shown here reveals an anastomosing network of ridges (arrows) located on the fracture membrane surface near the apical part of the cell [note microvilli (Mv) present at the cell surface]. This is the P-face of the membrane. (The E-face of the fractured membrane would show a complementary pattern of grooves.) The ridges represent linear arrays of transmembrane proteins (most likely occludins) involved in the formation of the zonula occludens. The membrane of the opposing cell contains a similar network of proteins, which is in register with the first cell. The actual sites of protein interaction between the cells form the anastomosing network. 100,000. (Reprinted with permission from Hull BE, Staehelin LA. Functional significance of the variations in the geometrical organization of tight junction networks. J Cell Biol 1976;68:688–704.)
FIGURE 5.16 • Molecular structure of zonula occludens.