The extracellular matrix in animals consists of a variety of proteins
secreted locally by cells
and forms an organised meshwork around them. Variations in the relative amounts of these macromolecules
and the ways in which they are arranged creates a wide diversity of forms to suit the particular tissues
. For example, bone
are formed from a calcified form of the extracellular matrix while the transparent matrix of the cornea
, the filter of the kidney glomerulus
and the ropelike organisation of tendons
are all formed from different types of extracellular matrix.
Gycosylaminoglycan chains are particularly important componants of the extracellular matrix. These are unbranched polysaccharide chains composed of repeating disaccharide units of which one of the two sugars is always an amino sugar and is usually sulfated. Due to their high (negative) charge density, stiffness and being hydrophilic these molecules occupy huge volumes relative to their mass, forming gels even at low concentrations. Their negative charge attracts cations such as sodium ions which in turn pull in water through osmosis creating turgor enabling the extracellular matrix to resist high compressive forces. All glycosylaminoglycans are attached to a core protein to form proteoglycans with the exception of hyaluronan, the simplest of the glycosylaminoglycans used as a space filler during embryonic development and a lubricant in joint fluid.
Important proteins in the extracellular matrix include collagen, elastin, fibronectin, laminin and integrins. Collagen is made up of three helical protein chains wrapped around each other to form a triple helix. These helices can associate with each other to form networks or rope-like fibrils which can also aggregate into fibres. Anchoring fibrils are especially common in skin and collagen is the single most abundant protein in animals. Elastin, as its name suggests is a hydrophobic protein with elastic properties, particularly important in the arteries. Fibronectin is a glycoprotein which contributes to the organisation of macromolecules and cells with the extracellular matrix. Laminins are large, flexible proteins which can self-assemble into felt-like sheets and are particularly important in the basal lamina. Integrins are transmembrane protein dimers that bind to other protein componants of the extracellular matrix and tie the extracellular matrix to the cell's cytoskeleton.
Although the extracellular matrix was once thought to serve as an inert scaffold to stabilise the physical structure of tissues it is now known to exert effects on cells regulating development, migration, proliferation, shape and funciton. Proteoglycans are thought to be important in cell signalling, binding growth factors and other types of secreted proteins to regulate their action, for example by concentrating a protein for more effective presentation to cell-surface receptors. In addition, some proteoglycans attached to the plasma membrane can act as co-receptors binding either to the extracellular matrix or signalling molecules in conjunction with conventional cell-surface receptors. Integrins, in addition to attaching the cell to its surroundings, mediate signalling between the cell and the extracellular matrix. Signals from within the cell can enhance or disrupt binding of the integrins to the extracellular matrix and signals can also be propagated from the integrins into the cell and can affect the arrangement of the cell's cytoskeleton. These signals are dependant on the nature of the extracellular componants bound by the integrin dimer.
The controlled degradation of the extracellular matrix by proteases (protein digesting enzymes) is necessarry for tissue turnover and is particularly rapid during the involution of the uterus following childbirth or the resorption of a tadpole's tail during metamorphosis. On a more localised scale degradation of the extracellular matrix aids the migration of cells, for example when white blood cells need to enter a tissue from the bloodstream in response to infection or injury a path must be cleared through the extracellular matrix barrier. Cancer cells migrate at higher rates then the tissues from which they originate and as such protease inhibitors are currently subject to much research as potential treatments to prevent the spreading of cancers.
The information presented here is available from the incredibly useful Molecular Biology of the Cell 4th edition, by Alberts et al., Garland Sciences, USA, (2002) and is suplemented by level 3 biochemistry lectures provided by Dr John Walker of the University of Leeds in the UK.