Biochemistry of basement membranes.

Basement membranes are thin (20 to 300 nm) extracellular matrices with a ubiquitous occurrence in the body. They consist mainly of collagenous and noncollagenous glycoproteins, are formed early during embryonic development, and in mature tissues they compartmentalize various types of cells and tissue structures. The close apposition to cells is the most remarkable feature of basement membranes, which can either surround cells completely (muscle, fat, nerve axons) or separate them from underlying stroma in a polarized fashion (epithelium, endothelium). These cell contacts are mediated by cellular receptors and specific matrix components and have profound effects on polarization, differentiation, and proliferation of cells and on the control of their migratory behavior. In addition, basement membranes represent barriers regulating filtration of macromolecules and penetration by cells. The supramolecular organization of basement membranes is still insufficiently known. In transmission electron microscopy cross-sections show a two-layered morphology that includes an electron dense (lamina densa) and an electron lucent (lamina rara) zone, the latter being closer to the cells. Other specialized basement membranes such as those in renal glomeruli appear more complex and contain two laminae rarae. Here, presumably epithelial and invading endothelial cells each produce their own basement membrane, which then fuse together to form the filtration units of the glomerulus. Further structural and functional study of authentic basement membranes has been limited because these membranes comprise only a small fraction of tissues and most of their components are notoriously insoluble. Rodent tumors that produce large amounts of basement membrane material and the use of recombinant DNA technology in the past decade have paved the way for a more precise biochemical characterization of several basement membrane proteins. These components include collagen type IV, the cell-binding protein laminin, several proteoglycans, and other proteins. In the following pages we will briefly review the biochemical properties of the basement membranes and discuss their possible functions and contributions to supramolecular structures. For more extensive discussions, including certain biologic and pathologic aspects of basement membrane function, we refer the reader to several recent reviews.