Extracellular matrix or ECM is the non-cellular component of the tissue. It is most abundant in connective tissues. ECM consists of protein fibers and ground substances secreted by cells that self-assemble to form a mesh to hold the cells together. Based on its composition, ECM imparts unique properties to different connective tissues. For example, mineralized collagen fibrils make bones rigid, elastic fibers allow the arteries to be flexible, and the ECM's high water content in plasma provides fluidity to blood. ECM helps regulate cell survival, shape, polarity, and migration. It also undergoes slow, constant turnover to maintain tissue health. A specialized form of ECM called the basement membrane, is a thin layer in contact with the basal surface of epithelial cells. It interacts with cell-surface receptors for signaling and provides support and nutrients to the cells. Another specialized form of ECM, the interstitial matrix, is a fluid-filled layer under the basement membrane. It supports cells, preserves tissue integrity, and helps nourish and hydrate the tissue.