at very low concentrations. The hydrophilic nature of GAGs causes a swelling pressure, or turgor, which enables the ECM to withstand compression forces. The cartilage matrix lining the knee joint, for example, can support pressures of hundreds of atmospheres because of its high GAG content. Hyaluronic acid (HA), a chief component of the ECM, is a large GAG that attracts water and is found in increased amounts in damaged or growing tissues. HA stimulates cytokine production by macrophages, thereby promoting angiogenesis. Proteoglycans Proteoglycans retain water and form a gel-like substance through which ions, hormones and nutrients can move freely. Given the great abundance and structural diversity of proteoglycan molecules, it would be surprising if their function was limited to providing lubrication around and between cells. Perlecan is a proteoglycan present in the basement membrane, where it forms a gel of varying pore size and charge density. The epidermal cells of the skin derive their metabolic nutrients from the dermis via the diffusion of molecules through the basement membrane, so proteoglycans may play a key role in selectively filtering molecules that pass through the basement membrane beneath the epidermal cells. Proteoglycans also appear to have an important role in regulating signalling between cells. For example, the heparin sulphate chains of proteoglycans bind to several different growth factors, including fibroblast growth factors (FGFs), helping them to bind to their specific cell-surface receptors [3] , [4] . Proteoglycans can also bind chemokine molecules on the surface of endothelial cells, prolonging the inflammatory response.