The Extracellular Matrix

The extracellular matrix, or ECM, is a network of fibrous proteins like collagen and elastin and a ground substance that surrounds cells and fills the spaces between them.

The ground substance is a hydrated gel made up of glycosaminoglycans, proteoglycans, glycoproteins, and interstitial fluid, which is mainly water with dissolved ions and gases.

Local cells, especially fibroblasts, secrete most ECM components. Macrophages and other cells also help remodel the matrix.

The ECM contains glycosaminoglycans, or GAGs, which bind water. When attached to core proteins, GAGs form proteoglycans.

In cartilage, hyaluronic acid forms large proteoglycan aggregates. Proteoglycans that contain chondroitin sulfate bind to hyaluronic acid and help the tissue resist compression. Loss of these proteoglycans in cartilage can contribute to osteoarthritis.

Collagens provide rigidity and are especially important in tendons, which join muscles to bones.

Fibronectin acts like a molecular glue and helps cells attach to collagen and GAGs. Cells bind to fibronectin through integrins, which connect the ECM to the cell interior and support signaling.

During migration, cells use these attachments to pull themselves forward across the ECM, which also acts as a support network that guides their movement and behavior.