Still more complexity in mammalian basement membranes

At the epithelial/mesenchymal interface of most tissues lies the basement membrane (BM). These thin sheets of highly specialized extracellular matrix vary in compo-sition in a tissue-specific manner, and during development and repair. For about two de-cades it has been apparent that all BMs contain laminins, entactin-1/nidogen-1, Type IV col-lagen, 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 recent description of a new isoform, E/N-2/osteonidogen. Agrin and Type XVIII collagen have been reclassified as heparan sulfate proteoglycans (HSPGs), expanding the repertoire of HSPGs in the BM. The laminin family has become more diverse as new a-chains have been characterized, increasing the number of laminin isoforms. Interactions between BM components are now appreciated to be regulated through multiple, mostly domain-spe-cific mechanisms. Understanding the functions of individual BM components and their as-sembly into macromolecular complexes is a considerable challenge that may increase as further BM and cell surface ligands are discovered for these proteins