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, it appears as if 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. 5.36a).
When observed with the EM, the hemidesmosome exhibits an intracellular 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 a desmoplakin-like family of proteins capable of anchoring intermediate filaments of the cytoskeleton. Three major proteins have been identified in the plaque:  Plectin (450 kilodaltons) functions as a cross-linker of the intermediate filaments that bind them to the hemidesmosomal attachment plaque. Recent studies indicate that plectin also interacts with microtubules, actin filaments, and myosin II. Thus, plectin cross-links and integrates all elements of the cytoskeleton.
 BP 230 (230 kilodaltons) attaches intermediate filaments to the intercellular attachment plaque. The absence of functional BP 230 causes bullous pemphigoid, a disease characterized clinically by blister formation. High levels of antibodies directed against components of the hemidesmosome, including antibodies against BP 230 and type XVII collagen, are detected in people with this disease. For this reason, BP 230 is called bullous pemphigoid antigen 1 (BPAG1), and the collagen XVII molecule is called bullous pemphigoid antigen 2 (BPAG2) or BP 180.
 Erbin (180 kilodaltons) mediates association of BP 230 with integrins.
FIGURE 5.36 • 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. 40,000. b. Diagram showing the molecular organization of a hemidesmosome. The intracellular attachment plaque is associated with transmembrane adhesion molecules, such as the family of integrins and transmembrane type XVII collagen, and contains plectin, BP 230, and erbin. 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 the extracellular matrix.