Basement membrane: structure function relationships.

Basement membranes perform several functions including filtration and elastic support. In the glomerular capillary two regions of extracellular matrix are recognized: the laminae rarae and the lamina densa. Procollagen and noncollagen glycoproteins occupy the laminae rarae and lamina densa. Proteoglycans are present in the laminae rarae. The nature and spatial organization of the macromolecules within the basement membrane determine their filtration and elastic properties. The persistence of the procollagen molecules assures the low order of fibril organization and permits selective filtration, whereas the persistence of the noncollagen extension peptides in the procollagen and the presence of the glycoprotein confers elasticity. The anionic nature of basement membranes is determined primarily by the acidic amino acid content and to much lesser extent by sialic acid and proteoglycans.