Tissues are not made up solely of cells. A substantial part of their volume is extracellular space, which is largely filled by an intricate network of macromolecules constituting the Extracellular Matrix. This matrix is composed of a variety of proteins and polysaccharides that are secreted locally and assembled into an organized meshwork in close association with the surface of the cell that produced them.Cells surrounded by spaces filled with extracellular matrix. The particular cells shown in this low-power electron micrograph are those in an embryonic chick limb bud. The cells have not yet acquired their specialized characteristics.
Whereas we have discussed cell junctions chiefly in the context of epithelial tissues, our account of the extracellular matrix concentrates on connective tissues . The extracellular matrix in connective tissue is frequently more plentiful than the cells it surrounds, and it determines the tissue's physical properties. Connective tissues form the framework of the vertebrate body, but the amounts found in different organs vary greatly—from cartilage and bone, in which they are the major component, to brain and spinal cord, in which they are only minor constituents.