The extracellular matrix (ECM) is a network of molecules filling the spaces between cells. This non-cellular component provides physical scaffolding for all tissues and organs and is composed of proteins, glycoproteins, and polysaccharides. The ECM’s composition gives tissues their physical characteristics and influences how cells communicate and behave.
Key Protein Fibers: Collagen and Elastin
A major part of the extracellular matrix consists of fibrous proteins providing structural support. The most abundant of these is collagen, which makes up about 25% of the total protein in mammals. Collagen’s primary function is to provide tensile strength, resisting forces that would pull the tissue apart. This strength comes from its triple helix structure, where three polypeptide chains wind into a rope-like formation.
Interwoven with the strong collagen fibers are those made of elastin. This protein provides elasticity and resilience, allowing tissues to stretch and then recoil to their original shape. Elastin is a component in tissues that undergo repeated physical deformation, such as the lungs, the walls of large blood vessels, and the skin. Elastic fibers are composed of molecules that are cross-linked together, forming a durable network that works with collagen to maintain tissue integrity.
Adhesive Proteins: Fibronectin and Laminin
The ECM also contains adhesive glycoproteins that act as a molecular glue, connecting cells to the matrix. One of these proteins is fibronectin, which is involved in cell adhesion, migration, and growth. Fibronectin acts as a bridge, with binding sites for cell surface receptors (integrins) and ECM components like collagen. This connection allows cells to anchor and move through the matrix, a process used in wound healing.
Another adhesive protein is laminin, a major component of specialized ECM structures called basement membranes. Basement membranes are thin, sheet-like layers that support epithelial and endothelial cells, such as those lining the skin and blood vessels. Laminins are large, cross-shaped proteins that bind to integrins and other cell surface receptors, as well as to other ECM molecules. This network of interactions provides a stable scaffold for tissue organization and guides cell differentiation.
The Hydrated Gel: Proteoglycans and Glycosaminoglycans
The fibrous and adhesive proteins of the ECM are embedded within a hydrated, gel-like substance. This “ground substance” is primarily composed of proteoglycans and glycosaminoglycans (GAGs), which are responsible for resisting compressive forces. GAGs are long polysaccharide chains whose negative charge attracts positive ions. This in turn draws a massive amount of water into the matrix, forming a swollen gel.
Proteoglycans are formed when one or more GAG chains are attached to a central core protein. These molecules are diverse in size and structure; for example, aggrecan in cartilage has over 100 GAG chains, while decorin has only one. This variety allows proteoglycans to resist compression, fill space, and regulate the movement of molecules and cells. By trapping growth factors, they can also influence cell signaling.
ECM Composition Across Different Tissues
The specific composition and organization of the ECM vary dramatically to meet distinct functional demands, which dictates the physical properties of each tissue. For instance, tendons connect muscle to bone and must withstand immense pulling forces. Consequently, their ECM is dominated by densely packed, parallel arrays of type I collagen fibers, providing high tensile strength.
In contrast, cartilage in joints must be resilient and resist compression, so its ECM is rich in proteoglycans and type II collagen. The basement membrane is a specialized layer supporting epithelial cells, with a unique composition of type IV collagen and laminin. Bone is another specialized tissue where the collagen framework is mineralized with calcium phosphate for exceptional rigidity.
How Cells Build and Reshape the ECM
The extracellular matrix is not a static scaffold but a dynamic environment that is constantly assembled and remodeled. This work is carried out by the cells residing within the tissue. For example, cells called fibroblasts are the primary producers of the ECM in most connective tissues, synthesizing and secreting proteins like collagen and fibronectin. Chondrocytes build the cartilage matrix, and osteoblasts create the bone matrix.
These same cells also control the breakdown and reshaping of the matrix. They secrete enzymes, such as matrix metalloproteinases (MMPs), which can degrade ECM components like collagen. This continuous cycle of synthesis and degradation allows tissues to develop, grow, and repair themselves. The balance between building and breaking down the ECM is regulated to maintain tissue homeostasis and adapt to changing needs.