tions, and the basal lamina is attached to underlying con nective tissue by anchoring fibrils and fibrillin microfibrils.  Filtration. The movement of substances to and from the  Compartmentalization. Structurally, basal and external connective tissue is regulated in part by the basal lamina, largely through ionic charges and integral spaces. Filtration laminae separate or isolate the connective tissue from epis well characterized in the kidney, in which the plasma ithelia, nerve, and muscle tissues. Connective tissue— projections of lamina densa
FIGURE 5.34 • Schematic diagram and electron micrograph of the basal portion of epithelial cell. a. 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 integrins (yellow) are present in the lamina lucida and lamina densa and provide adhesion between the basal lamina and the intracellular attachment plaques of hemidesmosomes. b. This high-magnification electron micrograph of human skin shows the basal portion of human epithelial cells with underlying basal lamina. The electron-lucent space, the lamina lucida located just below the basal cell membrane, is occupied by anchoring filaments formed by the laminin-5 and type XVII collagen molecules. Anchoring filaments are responsible for attaching the basal cell membrane to the basal lamina. The looplike fibers originating from the basal lamina represent anchoring fibrils of type VII collagen that link the basal lamina with the reticular fibers (type III collagen) and with anchoring plaques located within the extracellular matrix. 200,000. (Courtesy of Douglas R. Keene.) filtrate must cross the compound basal laminae of capillar-processes of a cell use the basal lamina that remains after cell ies and adjacent epithelial cells to reach the urinary space loss, thus helping to maintain the original tissue architec within a renal corpuscle. ture. For example, when nerves are damaged, new neuro Tissue scaffolding. The basal lamina serves as a guide or muscular junctions from a growing axon will be established scaffold during regeneration. Newly formed cells or growing only if the external lamina remains intact after injury. The basal laminae also allow cells to migrate under physiologic Focal adhesions create a dynamic link between the actin conditions but act as barriers against tumor cell invasion.
 Regulation and signaling. Many molecules that reside in the basal lamina interact with cell surface receptors, influencing epithelial cell behavior during morphogenesis, fetal development, and wound healing by regulating cell shape, proliferation, differentiation, and motility as well as gene expression and apoptosis. For instance, the basal lamina of endothelial cells has recently been found to be involved in the regulation of tumor angiogenesis.
The organization of cells in epithelium depends on the support provided by the extracellular matrix on which the basal surface of each cell rests. Anchoring junctions maintain the morphologic integrity of the epithelium–connective tissue interface. The two major anchoring junctions are:  focal adhesions, which anchor actin filaments of the cytoskeleton into the basement membrane; and hemidesmosomes, which anchor the intermediate filaments of the cytoskeleton into the basement membrane.
In addition, transmembrane proteins located in the basal cell domain (mainly related to the integrin family of adhesion molecules) interact with the basal lamina.
cytoskeleton and extracellular matrix proteins.