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 disc-shaped structure consisting of very dense material called the desmosomal attachment plaque. This structure measures about 400  250  10 nm and anchors intermediate flaments (Fig. 5.21a). 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. 5.21b).
The intercellular space of the macula adherens is conspicuously wider (up to 30 nm) than that of the zonula adherens and is occupied by a dense medial band, the intermediate line. This line represents extracellular portions of transmembrane glycoproteins, the desmogleins and desmocollins, which are members of the cadherin family of Ca2-dependent cell adhesion molecules. In the presence of Ca2, extracellular portions of desmogleins and desmocollins bind adjacent identical molecules of neighboring cells (homotypic binding). X-ray crystallographic studies suggest that the extracellular binding domain of proteins from one cell interacts with two adjacent cadherin domains in an antiparallel orientation, thus forming a continuous cadherin zipper in the area of the desmosome (see Fig. 5.21b). The cytoplasmic portions of desmogleins and desmocollins are integral components of the desmosomal attachment plaque. They interact with the plakoglobins and desmoplakins that are involved in desmosome assembly and the anchoring of intermediate flaments.
The cells of different epithelia require different types of attachments.
In epithelia that serve as physiologic barriers, the junctional complex is particularly significant because it serves to create a long-term barrier, allowing the cells to compartmentalize and restrict the free passage of substances across the epithelium. Although it is the zonula occludens of the junctional complex that principally affects this function, it is the adhesive properties of the zonulae and maculae adherentes that guard against physical disruption of the barrier. In other epithelia, there is need for substantially stronger attachment between cells in several planes. In the stratified epithelial cells of the epidermis, for example, numerous maculae adherentes maintain adhesion between adjacent cells. In cardiac muscle, where there is a similar need for strong adhesion, a combination of the macula adherens and the fascia adherens serves this function.
Communicating junctions, also called gap junctions or nexuses, are the only known cellular structures that permit the direct passage of signaling molecules from one cell to another. They are present in a wide variety of tissues, including epithelia, smooth and cardiac muscle, and nerves. Gap junctions are important in tissues in which activity of adjacent cells must be coordinated, such as epithelia engaged in fluid and electrolyte transport, vascular and intestinal smooth muscle, and heart muscle. A gap junction consists of an accumulation of transmembrane channels or pores in a tightly packed array. It allows cells to exchange ions, regulatory molecules, and small metabolites through the pores. The number of pores in a gap junction can vary widely, as can the number of gap junctions between adjacent cells.