Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

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PART
III IV V
Cells in Their Social Context
Cell Junctions and the
Extracellular Matrix
chapter
19
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
In This Chapter
CELL–CELL JUNCTIONS
THE EXTRACELLULAR MATRIX
OF ANIMALS
CELL–MATRIX JUNCTIONS
THE PLANT CELL WALL