The junctional mechanisms are shown in epithelial cells, while the nonjunctional mechanisms are shown in nonepithelial cells. A junctional adhesion is operationally defined as one that can be seen as a specialized region of contact by conventional or freeze-fracture electron microscopy. A nonjunctional adhesion shows no such obvious specialized structure. Note that the integrins and cadherins are involved in both nonjunctional and junctional cell-cell (cadherins) and cell-matrix (integrins) contacts. Cadherins generally mediate homophilic interactions, whereas integrins mediate heterophilic interactions (see Figure 19-26). The cadherins, integrins, and selectins act as transmembrane adhesion molecules and depend on extracellular divalent cations to function; for this reason, most cell-cell and cell-matrix contacts are divalent-cation-dependent. On blood cells, selectins and integrins can also act as heterophilic cell-cell adhesion molecules: the selectins bind to carbohydrate, while the cell-binding integrins bind to members of the Ig superfamily. The integrins and integral membrane proteoglycans that mediate nonjunctional adhesion to the extracellular matrix are discussed later. (Insert courtesy of Daniel S. Friend.)