Basement membranes.

Basement membranes (BMs) are thin, dense sheets of specialized, self-assembled extracellular matrix that surround most animal tissues (Figure 1, top). The emergence of BMs coincided with the origin of multicellularity in animals, suggesting that they were essential for the formation of tissues. Their sheet-like structure derives from two independent polymeric networks - one of laminin and one of type IV collagen (Figure 1, bottom). These independent collagen and laminin networks are thought to be linked by several additional extracellular matrix proteins, including nidogen and perlecan (Figure 1, bottom). BMs are usually associated with cells and are anchored to cell surfaces through interactions with adhesion receptors and sulfated glycolipids (Figure 1, bottom). Various combinations of other proteins, glycoproteins, and proteoglycans - including fibulin, hemicentin, SPARC, agrin, and type XVIII collagen - are present in BMs, creating biochemically and biophysically distinct structures that serve a wide variety of functions. BMs have traditionally been viewed as static protein assemblies that provide structural support to tissues. However, recent studies have begun to uncover dynamic, active roles for BMs in many developmental processes. Here, we discuss established and emerging roles of BMs in development, tissue construction, and tissue homeostasis. We also explore how cells traverse BM barriers, the roles of BMs in human diseases, and future directions for the field.