Key Concepts of Section 20.2

• Epithelial cells have distinct apical, basal, and lateral surfaces. Microvilli projecting from the apical surfaces of many epithelial cells considerably expand the cells’ surface areas.

• Three major classes of cell junctions—anchoring junctions, tight junctions, and gap junctions—assemble epithelial cells into sheets and mediate communication between them (see Figures 20-1 and 20-11). Anchoring junctions can be further subdivided into adherens junctions, focal contacts, desmosomes, and hemidesmosomes.

• Adherens junctions and desmosomes are cadherin-containing anchoring junctions that bind the membranes of adjacent cells, giving strength and rigidity to the entire tissue.

• Cadherins are cell-adhesion molecules (CAMs) responsible for Ca²⁺-dependent interactions among cells in epithelial and other tissues. They promote strong cell-cell adhesion by mediating both lateral intracellular (cis) and adhesive intercellular (trans) interactions.

• Adapter proteins that bind to the cytosolic domain of cadherins, other CAMs, and adhesion receptors mediate the association of cytoskeletal and signaling molecules with the plasma membrane (see Figures 20-8 and 20-14). Strong cell-cell adhesion depends on the linkage of the interacting CAMs to the cytoskeleton.

• Hemidesmosomes are integrin-containing anchoring junctions that attach cells to elements of the underlying extracellular matrix.

• Integrins are a large family of αβ heterodimeric cell-surface proteins that mediate both cell-cell and cell-matrix adhesions and inside-out and outside-in signaling in numerous tissues.

• Tight junctions block the diffusion of proteins and some lipids in the plane of the plasma membrane, contributing to the polarity of epithelial cells. They also limit and regulate the extracellular (paracellular) flow of water and solutes from one side of the epithelium to the other (see Figure 20-20). Two key integral membrane proteins found in tight junctions are occludin and claudin.

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• Gap junctions are constructed of multiple copies of connexin proteins, which are assembled into a transmembrane channel that connects the cytosols of two adjacent cells (see Figure 20-21). Small molecules and ions can pass through gap junctions, permitting metabolic and electrical coupling of adjacent cells.