extracellular matrix

Think of bricks held together not just by touching but by mortar filling every gap, giving the wall its strength and shape. In animal tissues, cells are the bricks, but a great deal of the body is the mortar — the material that cells secrete and live within, between and around themselves. This surrounding material is the extracellular matrix (ECM): the network of molecules outside cells that gives a tissue its structure, strength, and texture.

The extracellular matrix is a mesh of large molecules that cells build and release into the space around them. Its main ingredients are fibrous proteins that give strength and stretch — collagen (which resists pulling, like cables) and elastin (which lets tissue spring back, like rubber) — embedded in a water-rich gel of proteoglycans that resists squeezing, plus adhesive proteins such as fibronectin and laminin that glue cells to the mesh. The exact recipe varies enormously: in bone it is hardened with mineral, in tendon it is dense parallel collagen, in cartilage it is a springy gel, and in the loose tissue under skin it is a soft web.

The ECM is far more than passive packing material. It bears mechanical loads, so much of a tissue's strength is in the matrix rather than the cells themselves; it forms highways that crawling cells follow; and it carries chemical and mechanical signals that tell cells whether to divide, move, or even survive. A cell that loses its grip on the matrix often triggers its own death. So the matrix is best seen as an active partner that cells both build and constantly listen to — not merely the empty space between cells.