Figure 424

Molecular structure of hemidesmosome. a. Electron micrograph of the basal aspect of a gingival epithelial cell. Below the nucleus (N), intermediate filaments are seen converging on the intracellular attachment plaques (arrows) of the hemidesmosome. Below the plasma membrane are the basal lamina (BL) and collagen (reticular) fibrils (most of which are cut in cross section) of the connective tissue, x40,000. b. Diagram showing the molecular organization of a hemidesmosome. The intracellular attachment plaque is associated with transmembrane adhesion molecules, such as the family of inte-grins and transmembrane type XVII collagen. Note that the intermediate filaments seem to originate or terminate in the intracellular attachment plaque. Extracellular portions of integrins bind to laminin 5 and type IV collagen. With the help of anchoring fibrils (type VII collagen) laminin and integrin, the attachment plaque is secured to the reticular fibers (type III collagen) of extracellular matrix.

actin filament a-actinin vinculin talin where the junctions can be detected. Integrins are capable of transducing signals to the interior of the cell, where they affect cell migration, differentiation, and growth. On the cytoplasmic face, integrins interact with actin-binding proteins (a-actinin, vinculin, talin, paxillin) as well as many regulatory proteins, such as focal adhesion kinase or tyrosine kinase. On the extracellular side, integrins bind to extracellular matrix glycoproteins, usually laminin and fibronectin.

A variant of the anchoring junction similar to the desmosome is found in certain epithelia subject to abrasion and mechanical shearing forces that would tend to separate the epithelium from the underlying connective tissue. Typically, it occurs in the cornea, the skin, and the mucosa of the oral cavity, esophagus, and vagina. In these locations, only half the desmosome is present, hence the name hemidesmosome. Hemidesmosomes are found on the basal cell surface, where they provide increased adhesion to the basal lamina (Fig. 4.24a). When observed with the EM, the hemidesmosome exhibits an attachment plaque on the cytoplasmic side of the basal plasma membrane. The protein composition of this structure is similar to that of the desmosomal plaque, as it contains desmoplakin-like pro-

intracellular attachment plaque integrin laminin 5

collagen III collagen IV

integrin

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