The basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules referred to as the basement membrane (Figure 1), a semipermeable filter for sub stances reaching epithelial cells from below. Glycoproteins and other components in this structure can often be stained and visualized with the light microscope (Figure 2).

Fig1. Epithelia and adjacent connective tissue.

Fig2. Basement membranes.

With the transmission electron microscope (TEM) two parts of the basement membrane may be resolved. Nearest the epithelial cells is the basal lamina, a thin, electron-dense, sheet-like layer of fine fibrils, and beneath this layer is a more diffuse and fibrous reticular lamina (Figure 3a). The terms “basement membrane” and “basal lamina” are sometimes used interchangeably, but “basal lamina” usually denotes the fine extracellular layer seen ultrastructurally and “basement mem brane” the entire structure beneath the epithelial cells visible with the light microscope.

Fig3. Basal and reticular laminae of basement membranes.

The macromolecules of the basal lamina are secreted from the basal sides of the epithelial cells and form a sheet-like array. ECM components are described more fully in Chapter 5, but those of the basal lamina characteristically include the following:

■ Type IV collagen: Monomers of type IV collagen self assemble into a two-dimensional network of evenly spaced subunits resembling the mesh of a window screen.

■ Laminin: These are large glycoproteins that attach to transmembrane integrin proteins in the basal cell mem brane and project through the mesh formed by the type IV collagen.

■ Nidogen and perlecan: Respectively a short, rod-like protein and a proteoglycan, both of these cross-link laminins to the type IV collagen network, helping to provide the basal lamina’s three-dimensional structure, to bind the epithelium to that structure, and to determine its porosity and the size of molecules able to filter through it.

Basal laminae often called external laminae but with similar composition also exist as thin sleeves surrounding muscle cells, nerves (Figure 3b), and fat-storing cells, where they serve as semipermeable barriers regulating macromolecular exchange between the enclosed cells and connective tissue.

The more diffuse meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue (Figure 3).

Besides acting as filters, functions of basement mem branes include helping to provide structural support for epithelial cells and attach epithelia to underlying connective tissue. Basal lamina components help organize integrins and other proteins in the plasma membrane of epithelial cells, maintaining cell polarity and helping to localize endocytosis, signal transduction, and other activities. Basement membrane proteins also mediate many cell-to-cell interactions involving epithelia and mark routes for certain cell migrations along epithelia. Finally, the basement membrane also serves as a scaffold that allows rapid epithelial repair and regeneration.

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