Cartilage and bone extracellular matrix.

The extracellular matrix (ECM) is a complex of self assembled macromolecules. It is composed predominantly of collagens, non-collagenous glycoproteins, hyaluronan and proteoglycans. ECM is not only a scaffold for the cells; it serves also as a reservoir for growth factors and cytokines and modulates the cell activation status and turnover. ECM should be considered a dynamic network of molecules secreted by cells that in turn regulate cell behavior by modulating their proliferation and differentiation. The ECM provides structural strength to tissues, maintaining a complex architecture around the cells and the shape of organs. Various cell types secrete different matrix molecules and the nature and the amount of these molecules change during developmental age. Cartilage ECM is composed mainly of two components defining its mechano-physical properties: the collagenous network, responsible for the tensile strength of the cartilage matrix, and the proteoglycans (mainly aggrecan), responsible for the osmotic swelling and the elastic properties of the cartilage tissue. The conversion of cartilage into bone requires several processes that directly involve the different ECM components. Homeostasis of cartilage and bone is maintained by complex mechanisms controlling turnover and remodeling of ECM. In bone, as well as in cartilage, the ECM resident cells produce local factors, inflammatory mediators, and matrix-degrading enzymes. Turnover and degradation of normal and pathological matrices are dependent on the responses of the local cell to auto and paracrine anabolic and catabolic pathway.