Cellto ExtracelluIar Matrix Junctions

The organization of cells in epithelium depends on the support provided by the extracellular matrix, on which the basal surface of each cell rests. Anchoring junctions maintain the morphologic integrity of the epithelium-connective tissue interface. The two major anchoring junctions are

• Focal adhesions, which anchor actin filaments of the cy-toskeleton into the basement membrane

• Hemidesmosomes, which anchor the intermediate filaments of cytoskeleton into the basement membrane

In addition, transmembrane proteins located in the basal cell domain (mainly related to the integrin family of adhesion molecules) interact with the basal lamina.

Focal adhesions create a dynamic link between the actin cytoskeleton and extracellular matrix proteins

Focal adhesions are responsible for attaching long bundles of actin filaments (stress fibers) into the basal lamina. They play a prominent role during dynamic changes that occur in epithelial cells, e.g., migration of epithelial cells in wound repair. These focal adhesions form dynamic attachments to the underlying connective tissue by linking actin filaments to extracellular matrix proteins (Fig. 4.23).

In general, focal adhesions consist of a cytoplasmic face to which actin filaments are bound, a transmembrane connecting region, and an extracellular face that binds to the proteins of the extracellular matrix. The main family of transmembrane proteins involved in focal adhesions are in-tegrins, which are concentrated in clusters within the areas

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