The extracellular matrix (ECM) is a complex network of proteins, carbohydrates, and other macromolecules that provides structural and biochemical support to the cells within a tissue. The ECM is not only a passive scaffold but also plays an active role in regulating various cellular functions, such as adhesion, migration, proliferation, differentiation, and signaling.
The main components of the ECM include:
- Fibrous proteins: These proteins provide mechanical strength and resilience to the ECM. The major fibrous proteins in the ECM are collagen, elastin, and fibronectin. Collagens are the most abundant proteins in the ECM, providing tensile strength to tissues. Elastin imparts elasticity, allowing tissues to return to their original shape after being stretched. Fibronectin is a glycoprotein that facilitates cell adhesion to the ECM and plays a role in cell migration and wound healing.
- Proteoglycans: These are large molecules composed of a core protein to which long chains of carbohydrates called glycosaminoglycans (GAGs) are attached. Proteoglycans contribute to the ECM’s viscoelastic properties and can bind to various signaling molecules, regulating their availability and activity. Examples of proteoglycans include aggrecan, versican, and perlecan.
- Glycosaminoglycans (GAGs): GAGs are long, unbranched polysaccharides that are negatively charged, allowing them to attract water and form hydrated gels. This property makes GAGs excellent shock absorbers and lubricants in the ECM. Common GAGs include hyaluronic acid, chondroitin sulfate, and heparan sulfate.
- Matricellular proteins: These are a group of non-structural ECM proteins that modulate cell-matrix interactions and play roles in various biological processes, such as cell adhesion, migration, and signaling. Examples of matricellular proteins include thrombospondins, tenascins, and osteopontin.
The composition and organization of the ECM vary between different tissues and can change during development, tissue repair, and disease. The ECM is continuously remodeled by cells through the synthesis, deposition, and degradation of its components. Enzymes, such as matrix metalloproteinases (MMPs), are responsible for breaking down and remodeling the ECM in various physiological and pathological processes.
In summary, the extracellular matrix is a dynamic and complex structure that provides structural support, mediates cell-cell communication, and plays an essential role in various cellular functions and biological processes.