Anchoring junctions provide lateral adhesions between epithelial cells, using proteins that link into the cytoskeleton of the adjacent cells. Two types of anchoring cell-to-cell junctions can be identified on the lateral cell surface:
• Zonula adherens (pi., zonulae adherentes), which interacts with the network of actin filaments inside the cell
• Macula adherens (pi., maculae adherentes) or desmo-some, which interacts with intermediate filaments
In addition, two other types of anchoring junctions can be found where epithelial cells rest on the connective tissue matrix. These focal adhesions (focal contacts) and hemidesmosomes are discussed in the section on the basal domain (see pages 109 to 111).
The zonula adherens provides lateral adhesion between epithelial cells
The integrity of epithelial surfaces depends in large part on the lateral adhesion of the cells with one another and their ability to resist separation. Although the zonula occludens involves a fusion of adjoining cell membranes, their resistance to mechanical stress is limited. Reinforcement of this region depends on a strong bonding site below the zonula occludens. Like the zonula occludens, this lateral adhesion device occurs in a continuous band or belt-like configuration around the cell; thus, the adhering junction is referred to as a zonula adherens. The zonula adherens is composed of the transmembrane adhesion molecule E-cadherin. On the cytoplasmic side, the tail of E-cadherin is bound to catenin (Fig. 4.11a). The resulting cadher'm-catenin complex binds to vinculin and a-actinin and is required for the interaction of cadherins with the actin filaments of cytoskeleton. The extracellular components of the E-cadherin molecules from adjacent cells are linked by Ca2+ ions or an additional extracellular link protein. Therefore, the morphologic and functional integrity of the zonula adherens is calcium dependent. Removal of Ca2+ leads to dissociation of E-cadherin molecules and disruption of the junction.
When examined with the TEM, the zonula adherens is characterized by a uniform 15- to 20-nm space between the opposing cell membranes (Fig. 4.11b). The intercellular space is of low electron density, appearing almost clear, but it is evidently occupied by extracellular components of adjacent E-cadherin molecules and Ca21 ions. Within the confines of the zonula adherens, a moderately electron-dense material called fuzzy plaque is found along the cytoplasmic side of the membrane of each cell. This material corresponds to the location of the cytoplasmic component of the E-cadherin-catenin complex and the associated proteins (a-actinin and vinculin) into which actin filaments attach. Evidence also suggests that the fuzzy plaque represents the stainable substance in light microscopy, the terminal bar. Associated with the electron-dense material is an array of 6-nm actin filaments that stretch across the apical cytoplasm of the epithelial cell, the terminal web.
The fascia adherens is a sheet-like junction that stabilizes nonepithelial tissues
Physical attachments that occur between cells in tissues other than epithelia are usually not prominent, but there is at least one notable exception. Cardiac muscle cells are arranged end to end, forming thread-like contractile units. The cells are attached to each other by a combination of typical desmosomes, or maculae adherentes, and broad adhesion plates that morphologically resemble the zonula adherens of epithelial cells. Because the attachment is not ring-like but rather has a broad face, it is called the fascia adherens (Fig. 4.12). At the molecular level, the structure of the fascia adherens is similar to that of the zonula adherens; it also contains the zonula occludens ZO-1 protein found in the tight junctions of epithelial cells.
membranes of adjacent cells membranes of adjacent cells
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