Electron micrograph of two adjoining epithelial cells with their basal basal lamina (BL) appears as a thin layer that follows the contours of lamina. The micrograph shows only the basal portions of the two cells the basal domain of the overlying cell. Below the basal lamina are nu-and parts of their nuclei (N). The intercellular space is partially ob- merous cross-sectioned collagen (reticular) fibrils, x30,000. scured by lateral interdigitations between the two cells (arrows). The ithelium and the adjacent connective tissue (Fig. 4.19) called the basal lamina or, sometimes, lamina densa. This layer exhibits a network of fine, 3- to 4-nm filaments when observed at high resolution. Between the basal lamina and the cell is a relatively clear or electron-lucent area, the lamina lucida (also about 40 nm wide). This area contains extracellular portions of cell adhesion molecules, mainly fi-bronectin receptors. These receptors are members of the large family of transmembrane proteins called integrins. Integrins are linked to the cytoskeleton intracellularly and have extracellular portions that bind to the principal glycoproteins of the extracellular matrix (collagen, laminin, fibronectin).
The basal lamina includes at least four groups of molecules
Analyses of basal laminae derived from epithelia in many locations (kidney glomeruli, lung, cornea, lens of the eye) indicate that it consists of collagens, proteoglycans, laminin, and the glycoproteins entactin and fibronectin.
• Collagen. Several collagen species are present in the basal lamina. The major component, type IV collagen, represents one of approximately 19 types of collagen currently characterized in the body (see Table 5.2). Unlike most of the other collagens in the body, which are products of fibroblasts and related cells, type IV collagen and the other collagens of the basal lamina are products of the epithelial cells and other cell types that possess a basal or external lamina. Type IV collagen consists of short filaments that are thought to provide structural integrity to the basal lamina. It has more hydroxyproline and hydroxylysine than other collagens as well as a larger number of carbohydrate side chains. Type VII collagen forms anchoring fibrils that link the basal lamina to the underlying reticular lamina (described below). These molecular associations are reinforced by fibronectin and additional glycoproteins. Still incompletely characterized, these glycoproteins (some of which may be additional minor collagen types) interconnect or cross-link the other molecules to impart even greater stability in the interactions between the basal lamina and its associated cells and structures.
• Proteoglycans. Much of the volume of the basal lamina is probably due to the proteoglycans (heparan sulfate and chondroitin sulfate proteoglycans). Owing to their highly anionic character, these molecules are extensively hydrated. Because of their high negative charge density, sulfated proteoglycans are believed to play an important role in the regulation of the passage of ions across the basal lamina.
• Laminin. This cross-shaped glycoprotein molecule bridges the lamina lucida to link the basal lamina to the integrins of the basal domain of the overlying epithelial cells (Fig. 4.20). Laminin, heparan sulfate, and chondroitin sulfate proteoglycans are products of these cells; when secreted along with type IV collagen, they self-assemble into basal lamina sheets.
• Entactin and fibronectin. The remaining two substances are less well characterized. Entactin is a small, sulfated glycoprotein whose specific location and function within the basal lamina are not yet elucidated. It has binding sites for collagen type IV. It also binds to laminin, forming a stable entactin-laminin complex. Fibronectin, an-
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