The viscous watery fluid that surrounds cells in animal tissues. Secreted by the cells themselves, it is the medium through which they receive materials (e.g. nutrients, hormones) from elsewhere in the body and via which they communicate with other cells. The ECM is the environment in which cells migrate during tissue development and it contains constituents that bind cells together to maintain tissue integrity. The bulk of the matrix consists of proteoglycans, which associate with water molecules. Other key constituents are collagens—insoluble fibrous proteins that form various bundles, chains, and other structural components. Also present are multiadhesive proteins, which bind to other matrix components and to cell adhesion molecules in plasma membranes. The ECM is especially prominent in connective tissues, such as bone, cartilage, and adipose tissue, in which it is sometimes called ground substance. The development of artificial ECM is an important step in creating three-dimensional scaffolds for tissue engineering and may also have therapeutic potential as a vehicle for drug delivery and cancer therapy. See also basement membrane.

http://jcs.biologists.org/content/123/24/4195 The extracellular matrix at a glance from the Journal of Cell Science