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• Cells in tissues can adhere directly to one another (cell–cell adhesion) through specialized integral membrane proteins called cell-adhesion molecules (CAMs) that often cluster into specialized cell junctions.
• Cells in animal tissues also adhere indirectly (cell–matrix adhesion) through the binding of adhesion receptors in the plasma membrane to components of the surrounding extracellular matrix (ECM).
• These two basic types of interactions not only allow cells to aggregate into distinct tissues but also provide a means for the bidirectional transfer of information between the exterior and the interior of cells.
• CAMs can be broadly distributed along the regions of plasma membranes that contact other cells or clustered in discrete patches or spots called cell junctions.
• The extracellular matrix (ECM) is a complex meshwork of proteins and polysaccharides that contributes to the structure and function of a tissue.
• Cell-adhesion molecules (CAMs) mediate direct cell–cell adhesions and cell-surface adhesion receptors mediate cell–matrix adhesions . These interactions bind cells into tissues and facilitate communication between cells and their environments.
• The cytosolic domains of CAMs and adhesion receptors bind multifunctional adapter proteins that mediate interaction with cytoskeletal fibers and intracellular signaling proteins.
• The major families of cell-surface adhesion molecules are the cadherins, selectins, Ig-superfamily CAMs, and integrins.
• Tight cell–cell adhesions entail both cis (lateral or intracellular) oligomerization of CAMs and trans (intercellular) interaction of like (homophilic) or different (heterophilic) CAMs.
• Certain cell-surface receptors, including some integrins, can bind components of the extracellular matrix (ECM), thereby indirectly adhering cells to each other through their interactions with the matrix.
• Three abundant ECM components are proteoglycans, a unique type of glycoprotein; collagens, proteins that often form fibers; and soluble multiadhesive matrix proteins (e.g., fibronectin).
• The relative volumes of cells versus matrix vary greatly among different animal tissues and organs.
• Some connective tissue, for instance, is mostly matrix, whereas many organs are composed of very densely packed cells with relatively little matrix.
• Although the extracellular matrix generally provides mechanical support to tissues, it serves several other functions as well.
• Different combinations of ECM components tailor the extracellular matrix for specific purposes: strength in a tendon, tooth, or bone; cushioning in cartilage; and adhesion in most tissues.
• In addition, the composition of the matrix, which can vary, depending on the anatomical site and physiological status of a tissue, can let a cell know where it is and what it should do (environmental cues).
• The matrix also serves as a reservoir for many extracellular signaling molecules that control cell growth and differentiation.
• In addition, the matrix provides a lattice through or on which cells can move, particularly in the early stages of tissue assembly.