FIGURE 5.31 • Molecular components of the basal lamina. To produce a basal lamina, each epithelial cell must first synthesize and secrete its molecular components. The assembly of the basal lamina occurs outside the cell at its basal domain. The calcium-dependent polymerization of laminin molecules that occurs at the basal cell surface initiates basal lamina formation. Laminin polymers are next anchored to the cell surface by integrin receptors. At the same time, the type IV collagen suprastructure is assembled (see Fig. 5.30) in close proximity to laminin polymers. These two structures are connected by entactin or nidogen bridges and are additionally secured by other proteins (i.e., perlecan). The primary scaffold of type IV collagen connected to laminin polymers provides the site for other basal lamina molecules to interact and form the fully functional basal lamina.
LEnPEnBLPPNLL En P En BL P P N L FIGURE 5.32 • Basal lamina in the kidney glomerulus. Electron micrograph of a kidney glomerular capillary showing the basal lamina (BL) interposed between the capillary endothelial cell (En) and the cytoplasmic processes (P; podocytes) of epithelial cells. The epithelial cell is located on the outer (abluminal) surface of the endothelial cell. 12,000. Inset. Relationship at higher magnification. Note that the endothelial cells and epithelial cells are separated by the shared basal lamina and that no collagen fibrils are present. N, nucleus of epithelial cell; L, lumen of capillary. 40,000.
FIGURE 5.33 • Demonstration of basement membrane material in splenic vessels. a. Photomicrograph of a silver preparation revealing two longitudinally sectioned venous sinuses in the spleen. These blood vessels are surrounded by a modified basement membrane, which takes the form of a ringlike structure, much like the hoops of a barrel, rather than a continuous layer or lamina. The rings are blackened by the silver and appear as bands where the walls of the vessel have been tangentially sectioned (arrows). To the right, the cut has penetrated deeper into the vessel and shows the lumen (L). Here the cut edges of the rings are seen on both sides of the vessel. In the lower vessel, the cut rings have been sectioned in a virtually perpendicular plane, and the rings appear as a series of dots. 400. b. Electron micrograph of the wall of a venous sinus, showing a longitudinally sectioned endothelial cell (EnC). The nucleus (N) of the cell is protruding into the lumen (L). The basal lamina material (asterisks) has the same homogeneous appearance as seen by electron microscopy in other sites except that it is aggregated into ringlike structures rather than into a flat layer or lamina. Moreover, its location and plane of section correspond to the silver-reactive, dotlike material in the panel above. 25,000.
all basal laminae appear similar, their molecular composition including all of its specialized tissues, such as bone and car-and functions are unique to each tissue. The following are tilage (with the exception of adipose tissue, in that its cells various functions now attributed to the basal lamina. possess an external lamina)—can be viewed as a single,  Structural attachment. As noted, the basal lamina serves continuous compartment. In contrast, epithelia, muscles, and nerves are separated from adjacent connective tissue as an intermediary structure in the attachment of cells to by intervening basal or external laminae. For any substance the adjacent connective tissue. Epithelial cells are anchored to move from one tissue to another (e.g., from one com- into the basal lamina by cell-to–extracellular matrix juncpartment to another), it must cross such a lamina.