Still more complexity in mammalian basement membranes

At the epithelial/mesenchymal interface of most tissues lies the basement m embrane (BM). These thin sheets of highly specialized extracellular matrix vary in composition in a tissue-specific manner, and during development and repair. For about two decades it has been apparent that all BMs contain la minins, entactin-1/nidogen-1, Type IV collagen, and proteoglycans. However, within the past few years this complexity has increased as new components are described. The entactin/nidogen (E/N) family has expanded with the rece nt description of a new isoform, E/N-2/osteonidogen. Agrin and Type XVIII c ollagen have been reclassified as heparan sulfate proteoglycans (HSPGs), ex panding the repertoire of HSPGs in the BM. The laminin family has become mo re diverse as new alpha-chains have been characterized, increasing the numb er of laminin isoforms. Interactions between BM entactin components are now appreciated to be regulated through multiple, mostly domain-specific mecha nisms. Understanding the functions of individual BM components and their as sembly into macromolecular complexes is a considerable challenge that may i ncrease as further BM and cell surface ligands are discovered for these pro teins.