In order to maintain tissue organization, many animal cells are surrounded by structural molecules that make up the extracellular matrix (ECM). Together, the molecules in the ECM maintain the structural integrity of tissue as well as the remarkable specific properties of certain tissues.
The extracellular matrix (ECM) is commonly composed of ground substance, a gel-like fluid, fibrous components, and many structurally and functionally diverse molecules. These molecules include polysaccharides called glycosaminoglycans (GAGs). GAGs occupy most of the extracellular space and often take up a large volume relative to their mass. This results in a matrix that can withstand tremendous forces of compression. Most GAGs are linked to proteins—creating proteoglycans. These molecules retain sodium ions based on their positive charge and therefore attract water, which keeps the ECM hydrated.
The ECM also contains rigid fibers such as collagens—the primary protein component of the ECM. Collagens are the most abundant proteins in animals, making up 25% of protein by mass. A large diversity of collagens with structural similarities provide tensile strength to many tissues.
Notably, tissue like skin, blood vessels, and lungs need to be both strong and stretchy to perform their physiological role. A protein called elastin gives particular fibers the ability to stretch and retract. Fibronectin is a glycoprotein important in cell adhesion, as it directly attaches to proteins that span the membrane of cells, specifically integrins, linking the membrane to the ECM. Integrin also interacts with collagen, which may elicit intracellular responses.
The makeup and relative proportion of each of these molecules are determined by the location, physiological function, and neighboring cell types of the tissue in which the cells reside. This specific molecular makeup of the ECM is referred to as the local microenvironment. Cells in a particular tissue secrete molecules that determine the surrounding ECM. For example, intestinal cells synthesize, modify, and secrete the molecules necessary for the matrix that surrounds them, while osteoblasts generate the molecules of the rigid ECM of human bone. This diversity in ECM composition in different tissues creates particular properties according to their unique role and function.
The interaction between cells and the local ECM has been shown to have an intracellular impact as well. For example, forces on transmembrane integrin molecules can result in activation of the intracellular actomyosin network. This may promote cell migration, division, and other cellular responses. Some of these responses include changes in gene expression and cell signaling cascades. Likewise, integrin can communicate intracellular information to the outside of the cell. Additionally, ECM is known to bind signaling molecules, which can be released upon ECM degradation.
Animal cells need to have the capacity to degrade and remodel the ECM. This is particularly true in cases of tissue repair and growth. Consequently, cells typically possess the enzymes necessary to break down the ECM. These enzymes include matrix metalloproteases (MMPs) which work with other enzymes to degrade proteins such as collagen and fibronectin. ECM degradation and remodeling are important in healthy tissue growth, including blood vessel branching. On the downside, ECM remodeling also contributes to the metastasis of cancerous cells as they spread through the body.
