Schematic diagram of the basal portions of two epithelial cells. This diagram shows the cellular and extracellular components that provide attachment between epithelial cells and the underlying connective tissue. On the connective tissue side of the basal lamina, anchoring fibrils extend from the basal lamina to the collagen (reticular) fibrils of the connective tissue, providing structural attachment at this site. On the epithelial side, laminin (green), collagen XVII (red), and in-tegrins (yellow) are present in the lamina rara and lamina densa and provide adhesion between the basal lamina and the intracellular attachment plaques of hemidesmosomes.
other glycoprotein, has binding sites for all of the basal lamina components and probably serves to bind it to the subjacent connective tissue.
A layer of reticular fibers underlies the basal lamina
There is still lack of agreement as to the extent to which the basal lamina seen with the EM corresponds to the structure described as the basement membrane in the light microscope. Some investigators contend that the basement membrane includes not only the basal lamina but also a secondary layer of small-unit fibrils of type III collagen (reticular fibers) that forms the reticular lamina. The reticular lamina, as such, belongs to the connective tissue and is not a product of the epithelium. The reticular lamina was once regarded as the component that reacted with silver, whereas the polysaccharides of the basal lamina and the ground substance associated with the reticular fibers were thought to be the components stained with the PAS reaction. However, convincing arguments can be made for the basal lamina reacting with both PAS and silver in several sites. In normal kidney glomeruli, for example, no collagen (reticular) fibers are associated with the basal lamina of the epithelial cells (Fig. 4.21), although a positive reaction occurs with both PAS staining and silver impregnation. Also, in the spleen, where the basal lamina of the venous sinuses forms a unique pattern of ring-like bands rather than a thin, sheath-like layer around the vessel, exactly corresponding images are seen with the PAS and silver techniques as well as with the EM (Fig. 4.22).
Several structures are responsible for attachment of the basal lamina to the underlying connective tissue
On the opposite side of the basal lamina, the connective tissue side, several mechanisms provide attachment of the basal lamina to the underlying connective tissue:
• Anchoring fibrils (type VII collagen) (see Table 5.2) are usually found in close association with hemidesmosomes. They extend from the basal lamina matrix and appear to attach to connective tissue reticular fibrils (see Fig. 4.20).
• Fibrillin microfibrils are 10 to 12 nm in diameter and attach the lamina densa to elastic fibers. Fibrillin microfibrils are known to have elastic properties. A mutation in the fibrillin gene (FBN1) causes Marfan's syndrome and other related connective tissue disorders.
• Discrete projections of the lamina densa on its connective tissue side interact directly with the reticular lamina to form an additional binding site with type III collagen.
The multiple roles of the basal lamina and basement membrane are not yet fully elucidated. It is generally accepted that they facilitate attachment of epithelia to underlying connective tissue and that they influence the differentiation and proliferation of the epithelial cells that contact them.
Muscle cells, adipocytes, and peripheral nerve supporting cells exhibit an extracellular electron-dense material that resembles the basal lamina of epithelium. This mate-
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