Figure 421

Basal lamina in the kidney glomerulus. Electron micrograph of a kidney minal) surface of the endothelial cell. xl2,000. Inset. Relationship at glomerular capillary, showing the basal lamina (BL) interposed between higher magnification. Note that the endothelial cells and epithelial cells the capillary endothelial cell (En) and the cytoplasmic processes (P) of ep- are separated by the shared basal lamina and that no collagen fibrils are ithelial cells (podocytes). The epithelial cell is located on the outer (ablu- present. N, nucleus of epithelial cell; L, lumen of capillary. x40,000.

rial also corresponds to a PAS-positive staining reaction, as described above (see Fig. 4.18). Although the term "basement membrane" is not ordinarily applied to the extracellular stainable material of these nonepithelial cells in light microscopy, the terms "basal lamina" or "external lamina" are typically used at the EM level.

Basal laminae have multiple functions

Various functions are now attributed to the basal lamina:

• Structural attachment. As noted, the basal lamina serves as an intermediary structure in the attachment of cells to the adjacent connective tissue.

• Compartmentalization. Structurally, basal and external laminae separate or isolate the connective tissue from ep-ithelia, nerve, and muscle tissues. Connective tissue—including all of its specialized tissues, such as bone and cartilage (with the exception of adipose tissue, in that its cells possess an external lamina)—can be viewed as a single, continuous compartment. In contrast, epithelia, muscles, and nerves are separated from adjacent connective tissue by intervening basal or external laminae. For any sub stance to move from one tissue to another (e.g., from one compartment to another), it must cross such a lamina.

• Filtration. The movement of substances to and from the connective tissue is regulated in part by the basal lamina, largely through ionic charges and integral spaces. Filtration is well characterized in the kidney, where the plasma filtrate must cross the compound basal laminae of capillaries and adjacent epithelial cells to reach the urinary space within a renal corpuscle.

• Polarity induction. Epithelial cells exhibit functionally different membrane properties as a result of surface exposure. Specific properties attributable to the basal membrane surface, as opposed to the apical and lateral membrane surfaces, are induced by the presence of the basal lamina. For example, epithelial cells grown in ordinary tissue culture flatten as they proliferate and grow in the culture. When grown on the surface of an artificial basal lamina in the culture medium, the same cells display their characteristic shape as well as normal polarity and function.

• Tissue scaffolding. The basal lamina serves as a guide or scaffold during regeneration. Newly formed cells or

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