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Cell Junctions and the Extracellular Matrix
Of all the social interactions between cells in a multicellular organism, the most fundamental are those that hold the cells together. Cells may be linked by direct interactions, or they may be held together within the extracellular matrix, a complex network of proteins and polysaccharide chains that the cells secrete. By one means or another, cells must cohere if they are to form an organized multicellular structure that can withstand and respond to the various external forces that try to pull it apart.
The mechanisms of cohesion govern the architecture of the body—its shape, its strength, and the arrangement of its different cell types. The making and breaking of the attachments between cells and the modeling of the extracellular matrix govern the way cells move within the organism, guiding them as the body grows, develops, and repairs itself. Attachments to other cells and to extracellular matrix control the orientation and behavior of the cell’s cytoskeleton, thereby allowing cells to sense and respond to changes in the mechanical features of their environment. Thus, the apparatus of cell junctions and the extracellular matrix is critical for every aspect of the organization, function, and dynamics of multicellular structures. Defects in this apparatus underlie an enormous variety of diseases.
The key features of cell junctions and the extracellular matrix are best illustrated by considering two broad categories of tissues that are found in all animals (Figure 19–1). Connective tissues, such as bone or tendon, are formed from an extracellular matrix produced by cells that are distributed sparsely in the matrix. It is the matrix—rather than the cells—that bears most of the mechanical stress to which the tissue is subjected. Direct attachments between one cell and another are relatively rare, but the cells have important attachments to the matrix. These cell–matrix junctions link the cytoskeleton to the matrix, allowing the cells to move through the matrix and monitor changes in its mechanical properties.
In epithelial tissues, such as the lining of the gut or the epidermal covering of the skin, cells are tightly bound together into sheets called epithelia. The extracellular matrix is less pronounced, consisting mainly of a thin mat called the basal lamina (or basement membrane) underlying the sheet. Within the epithelium, cells are attached to each other directly by cell–cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cells, from mechanical stresses are transmitted from cell to cell by cytoskeletal flaments anchored to cell–matrix and cell–cell adhesion sites extracellular matrix directly bears mechanical stresses of tension and compression adhesion site to adhesion site. The cytoskeleton of epithelial cells is also linked to the basal lamina through cell–matrix junctions.