Figure 411

Zonula adherens, a. Diagram showing molecular organization of zonula adherens. Actin filaments of adjacent cells are attached to the E-cadherin-catenin complex by a-actinin and vinculin. The E cad-herin-catenin complex interacts with identical molecules embedded in the plasma membrane of the adjacent cell, interactions between transmembrane proteins are mediated by calcium ions. b. Electron micrograph of the zonula adherens from Figure 4.8a at higher magnification.

The plasma membranes are separated here by a relatively uniform intercellular space. This space appears clear, showing only a sparse amount of diffuse electron-dense substance, which represents extracellular domains of E-cadherin. The cytoplasmic side of the plasma membrane exhibits a moderately electron-dense material containing actin filaments, x 100,000.

The macula adherens (desmosome) provides localized spot-like adhesion between epithelial cells

The macula adherens is an anchoring cell-to-cell junction that provides a particularly strong attachment, as shown by microdissection studies. These junctions are localized on the lateral domain of the cell, much like a series of spot welds (see Fig. 4.8a). The macula adherens was originally described in epidermal cells and called a desmosome [Cr. desmo, bond + soma, body]. The name is often used interchangeably with macula adherens [L. macula, spot]. In epidermal cells, the macula adherens is the only attachment device present. In other epithelia, particularly those with cuboidal or columnar cells, the macula adherens is found in conjunction with a zonula adherens. The macula adherens occupies small, localized sites on the lateral cell surface, however; it is not a continuous structure around the cell, as is the zonula adherens. Thus, a section perpen dicular to the surface of a cell that cuts through the entire lateral surface will often not include a macula adherens. The section will always, however, include the zonula adherens.

Electron microscopy reveals that the macula adherens has a complex structure. On the cytoplasmic side of the plasma membrane of each of the adjoining cells is a disk-shaped structure consisting of very dense material called the desmosomal attachment plaque. This structure measures about 400 X 250 X 10 nm and anchors intermediate filaments (Fig. 4.13a). The filaments appear to loop through the attachment plaques and extend back out into the cytoplasm. They are thought to play a role in dissipating physical forces throughout the cell from the attachment site. At the molecular level, each attachment plaque is composed of several constitutive proteins, mainly desmoplakins and plakoglobins, which are capable of anchoring the intermediate filaments (Fig. 4.13b).

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