Extracellular Matrix

The extracellular matrix (ECM) is the network of proteins and polysaccharides that fills the spaces between cells in animal tissues. It supports cells physically, and it also sends them signals that guide growth, movement and specialization.

The main structural protein is collagen, the most abundant protein in mammals, which forms strong fibers, while elastin gives tissues the ability to stretch and recoil. Glycoproteins such as fibronectin and laminin link cells to the matrix. Proteoglycans, which have long sugar chains such as hyaluronic acid attached to a protein core, hold water and cushion tissues, and many of the sugars arise by glycosylation.

Integrins, receptor proteins that span the cell membrane, bind matrix proteins outside and connect to the cytoskeleton inside. Through them cells sense the stiffness and composition of their surroundings, and forces and signals pass in both directions. A specialized sheet called the basement membrane lies under epithelia.

Enzymes such as matrix metalloproteases continuously remodel the matrix during development and wound healing. Excess deposition causes fibrosis, defects in collagen genes cause disorders such as Ehlers-Danlos syndrome, and cancer cells break through the matrix to invade other tissues. Plants and fungi have a different, related structure in the cell wall, with cellulose as a main part.

• Cell Membrane — the surface where cells meet the matrix
• Cytoskeleton — the internal scaffold linked through integrins
• Glycosylation — the sugar modification common in matrix molecules