Molecular Biology of the Cell - Volume 2 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1993

Cell adhesion, cell junctions, and the extracellular matrix

Most Cells in Multicellular animals cooperate in organized assemblies called Tissues, which in turn combine in various ways to form larger functional units known as Organs (Fig. 14-1). Cells within tissues typically interact with a complex network of macromolecules that make up the Extracellular matrix. This matrix helps maintain multicellular structures and provides an organized framework within which cells can migrate and communicate with one another. In some cases, cells attach to the matrix at specialized regions of Cell/30.html">The Plasma Membrane, while adjacent cells in direct contact are frequently linked by specialized cell junctions.

It is helpful to divide animal tissues into two major groups, depending on the distinct roles that the matrix and cell junctions play. In Connective Tissues (Section 14.2), there is an extensive extracellular matrix in which cells are sparsely distributed (Fig. 14-1). The matrix is rich in fibrous polymers, especially Collagen, and consequently bears most of the mechanical stress to which the tissue is subjected, rather than the cells themselves. Cells are anchored to matrix components to which they can transmit mechanical forces, whereas cell-to-cell attachments are relatively insignificant. By contrast, in Epithelial Tissues, cells are tightly packed into continuous sheets (called epithelia); there is very little extracellular matrix, and cells account for the bulk of the tissue volume. Here, the cells themselves rather than the matrix bear most of the mechanical load through sturdy intracellular protein filaments (cytoskeletal components) that span the Cytoplasm of each epithelial cell in various directions. These filaments anchor to the inner surface of the plasma membrane, forming specialized junctions at these sites either with neighboring cells or with the underlying extracellular matrix.

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Fig. 14-1. As illustrated in this schematic cross-section, the intestinal wall is composed of epithelial, connective, and Muscle Tissues. Each tissue represents an organized system of cells held together by cell junctions and/or an extracellular matrix.

What are the spatial relationships between epithelial and connective tissues in the body? Epithelial cell sheets line all Body Cavities and free surfaces, and thanks to specialized Intercellular junctions, these sheets can act as barriers restricting the Movement of Water, solutes, and cells from one body compartment to another. As shown in Fig. 14-1, epithelia almost invariably rest on a supporting layer of Connective Tissue, which can anchor them to other tissues (such as muscle) that lack a distinct epithelial or connective tissue Organization.

In this chapter, we will first examine the Structure and organization of cell-cell and cell-matrix junctions, and then explore how cells recognize one another during the assembly of tissues and organs.

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