Extracellular matrix (ECM), in its broadest definition, refers to all the molecules that are inunobilized in the spaces between cells. The most extensively studiedECMs are those found in connective tissues, such as bone, cartilage, tendons, ligaments, fascia, and the stromal components of organs. Connective tissues are typically cell-poor, i.e., the ECM comprises a major fraction of the tissue mass. Accordingly, connective tissue ECMs are comparatively easy to isolate in large quantities, pennitting direct biochemical analysis.