Key Concepts of Section 20.1

• Cell-cell and cell–extracellular matrix (ECM) interactions are critical for assembling cells into tissues, controlling cell shape and function, and determining the developmental fate of cells and tissues. Diseases may result from abnormalities in the structures or expression of adhesion molecules.

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• Cell-adhesion molecules (CAMs) mediate direct cell-cell adhesions (homotypic and heterotypic), and adhesion receptors mediate cell-matrix adhesions (see Figure 20-1). These interactions bind cells into tissues and facilitate communication between cells and their environments.

• The cytosolic domains of CAMs and adhesion receptors bind adapter proteins that mediate interaction with cytoskeletal fibers and intracellular signaling proteins.

• The major families of CAMs are the cadherins, selectins, Ig-superfamily CAMs, and integrins (see Figure 20-2). Members of the integrin and Ig-CAM superfamilies can also function as adhesion receptors.

• Tight cell-cell adhesions entail both cis (lateral or intracellular) oligomerization of CAMs and trans (adhesive or intercellular) interactions of like (homophilic) or different (heterophilic) CAMs (see Figure 20-3). The combination of cis and trans interactions produces a Velcro-like adhesion between cells.

• The extracellular matrix (ECM) is a dynamic, complex meshwork of proteins and polysaccharides that contributes to the structure and function of a tissue (see Table 20-2). The major classes of ECM molecules are proteoglycans, collagens, and multi-adhesive matrix proteins, such as fibronectin and laminin.

• CAMs and adhesion receptors, together with their cytoplasmic adapter proteins, play major roles in “outside-in” and “inside-out” signaling, facilitating critically important communication between cells and their surroundings.

• The evolution of adhesion molecules with specialized structures and functions permits cells to assemble into diverse classes of tissues with varying functions.

• Mechanotransduction, the interconversion of a mechanical stimulus or force and biochemical processes, is mediated by CAMs, adhesion receptors, and mechanosensors. Mechanotransduction permits cells to respond to mechanical forces from their environments and to exert mechanical forces on their surroundings.