In contrast to the desmosome, whose transmembrane proteins belong to the cadherin family of calcium-dependent molecules, the majority of transmembrane proteins found in the hemidesmosome belong to the integrin class of cell matrix receptors. These include:  46 integrin, a heterodimer molecule containing two polypeptide chains. Its extracellular domain enters the basal lamina and interacts with type IV collagen supra structure containing laminins (laminin-5), entactin/nidogen or the perlecan. On the extracellular surface of the hemidesmosome, laminin-5 molecules form threadlike anchoring flaments that extend from the integrin molecules to the structure of the basement membrane (Fig. 5.36b). Interaction between laminin-5 and 64 integrin is essential for hemidesmosome formation and for the maintenance of epithelial adhesion. Mutation of the genes encoding laminin-5 chains results in junctional epidermolysis bullosa, another hereditary blistering skin disease.
 Type XVII collagen (BPAG2, BP 180), a transmembrane molecule (180 kilodaltons), that regulates expression and function of laminin-5. In experimental models, type XVII collagen inhibits migration of endothelial cells during angiogenesis and regulates keratinocyte migration in the skin (see Fig. 5.36b).
 CD151 (32 kilodaltons), a glycoprotein that participates in the clustering of integrin receptors to facilitate cellto–extracellular matrix interactions.
Despite their similarity in names, the terms anchoring filaments and anchoring fibrils do not describe the same structure. Anchoring flaments are formed mainly by laminin-5 and type XVII collagen molecules. They attach the basal cell membrane of epithelial cells into the underlying basal lamina. Anchoring fbrils are formed by type VII collagen and attach the basal lamina to the underlying reticular fibers (see page 140).
Morphologic Modifications of the Basal Cell Surface
Many cells that transport fluid have infoldings at the basal cell surface. They significantly increase the surface area of the basal cell domain, allowing for more transport proteins and channels to be present. These basal surface modifications are prominent in cells that participate in active transport of molecules (e.g., in proximal and distal tubules of the kidney; Fig. 5.37) and in certain ducts of the salivary glands. Mitochondria are typically concentrated at this basal site to provide the energy requirements for active transport. The mitochondria are usually oriented vertically within the folds. The orientation of the mitochondria, combined with the basal membrane infoldings, results in a striated appearance along the basal aspect of the cell when observed in the light microscope. Because of this phenomenon, the salivary gland ducts that possess these cells are referred to as striated ducts.
FIGURE 5.37 • Basal infoldings. Electron micrograph of the basal portion of a kidney tubule cell showing the infolding of the plasma membrane. Note the aligned mitochondria. The infoldings of adjoining cells result in the interdigitations of cytoplasm between the two cells. 25,000.
Typically, glands are classified into two major groups according to how their products are released (Table 5.5):  Exocrine glands secrete their products onto a surface directly or through epithelial ducts or tubes that are connected to a surface. Ducts may convey the secreted material in an unaltered form or may modify the  secretion by concentrating it or adding or reabsorbing constituent substances.
 Endocrine glands lack a duct system. They secrete their products into the connective tissue, from which they enter the bloodstream to reach their target cells. The products of endocrine glands are called hormones.