FIGURE 5.19 • Zonula adherens. a. Molecular organization of zonula adherens. Actin filaments of adjacent cells are attached to the E-cadherin–catenin complex by -actinin and vinculin. The E-cadherin–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 5.14a 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. 100,000.
The fascia adherens is a sheetlike 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 threadlike 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 ringlike but rather has a broad face, it is called the fascia adherens (Fig. 5.20). 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.
The macula adherens (desmosome) provides a localized spotlike junction between epithelial cells.
The macula adherens [L. macula, spot] represents a major anchoring cell-to-cell junction that provides a particularly strong attachment, as shown by microdissection studies.
The macula adherens was originally described in epidermal cells and was called a desmosome [Gr. desmo, bond  soma, body]. These junctions are localized on the lateral domain of the cell, much like a series of spot welds (see Fig. 5.14a), and they mediate direct cell-to-cell contact by providing anchoring sites for intermediate filaments. Increasing evidence suggests that the macula adherens, in addition to its structural function, participates in tissue morphogenesis and differentiation.
In simple epithelium formed by cuboidal or columnar cells, the macula adherens is found in conjunction with occluding (zonula occludens) and adhering (zonula adherens) junctions. Because the macula adherens occupies small, localized sites on the lateral cell surface, it is not a continuous structure around the cell, as is the zonula adherens. Thus, a section perpendicular 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.
In the area of the macula adherens, desmogleins and desmocollins provide the linkage between the plasma membranes of adjacent cells.
FIGURE 5.20 • Fascia adherens. Electron micrograph showing the end-to-end apposition of two cardiac muscle cells. The intercellular space appears as a clear undulating area. On the cytoplasmic side of the plasma membrane of each cell, there is a dense material similar to that seen in a zonula adherens containing actin filaments. Because the attachment site here involves a portion of the end face of the two cells, it is called a fascia adherens. 38,000.