Extracellular Matrix in Animals

The extracellular matrix, or ECM, helps animal tissues stay organized and keeps cells in the right place. It is made of structural molecules that support tissue shape and give certain tissues their special properties.

The ECM usually contains a gel-like ground substance, fibrous parts, and many molecules with different jobs. One major group is glycosaminoglycans, or GAGs, which are polysaccharides. GAGs fill much of the space outside cells and can take up a large volume compared with their...

Glycosaminoglycans, or GAGs, are long, linear polymers that help build and protect animal tissues. They are also called mucopolysaccharides. Their repeating units are made of an amino sugar, such as N-acetylglucosamine or N-acetylgalactosamine, plus a uronic acid, usually glucuronic acid or iduronic acid.

GAGs are found in the extracellular matrix of vertebrates, invertebrates, and bacteria. Their polar nature lets them attract water. This makes them useful as lubricants and as shock absorbers...

Collagen fibers give connective tissues strength and help them resist stretching. They are one of three main fiber types secreted by fibroblasts, along with elastic fibers and reticular fibers. Collagen fibers are made from fibrous protein subunits linked into long, straight fibers.

These fibers are flexible, but they also have great tensile strength. Tensile strength is the ability to resist pulling forces. Collagen fibers help ligaments and tendons stay resilient and strong. They also hold...

FACIT collagens are fibril-associated collagens found in the extracellular matrix. FACIT stands for fibril-associated collagens with interrupted triple helices. These collagens help connect collagen fibrils to each other and to other extracellular matrix proteins.

Type II collagen fibrils in cartilage provide a clear example. They have covalently bound type IX fibril-associated collagens at regular intervals. Other FACIT collagens include types XII, XIV, XVI, XIX, XX, XXI, and XXII.

These...

Elastic tissues contain elastin, a protein mixed with smaller amounts of other proteins and glycoproteins. Elastin helps tissue return to its original shape after it is stretched or compressed. This property is important in body parts that must bend, stretch, and recoil.

Elastic fibers are found in elastic tissues in the skin and in the elastic ligaments of the vertebral column. Ligaments and tendons are both made of dense regular connective tissue, but ligaments do not always have all of...

Fibronectin is an adhesive glycoprotein in the extracellular matrix of embryogenic and adult tissue. It helps regulate cell motility and attachment. In many tissues, fibronectin gives cells a place to bind and move along.

A fibronectin molecule is made of two identical polypeptide chains. These chains are joined at the C-terminal by a pair of disulfide bonds. Fibronectin also binds proteoglycans and collagen. It then links to integrin proteins, which interact with transmembrane proteins in the...

Proteoglycans and matrix glycoproteins help build the extracellular matrix. The extracellular matrix is the material outside cells that gives tissues support and structure. Proteoglycans are heavily glycosylated proteins, and they are commonly found woven through collagen fibers.

Hyaline cartilage shows how proteoglycans work. This is the most common type of cartilage in the body. It contains short, scattered collagen fibers with large amounts of proteoglycans. The proteoglycans carry long...

Matrix metalloproteases (MMPs) are enzymes that break down proteins and glycoproteins in the extracellular matrix, or ECM. The ECM is the dense network around cells. By cutting ECM molecules, MMPs help cells move and grow through tissue.

MMPs are important during embryonic development and morphogenesis, the process that shapes body form. A classic example comes from a tadpole’s tail during metamorphosis. The first MMP activity discovered was a collagenase, an enzyme that cuts collagen. This...

The basal lamina is a thin extracellular layer that sits beneath cells and separates them from other tissues. It acts as a support layer for epithelium and helps anchor it to the tissue below. Its structure includes three layers: lamina lucida, lamina densa, and lamina reticularis.

The basal lamina is made of glycoproteins and collagen. Key proteins in this framework include laminins, perlecan, entactin, and type IV collagen. Cell membrane proteins such as integrins and dystroglycan bind to...

Laminins are large extracellular matrix proteins that help build the basal lamina. Each laminin molecule has three chains: alpha, beta, and gamma. These chains are encoded by five alpha genes, four beta genes, and three gamma genes.

In humans, the alpha chains are LAMA1, LAMA2, LAMA3, LAMA4, and LAMA5. The beta chains are LAMB1, LAMB2, LAMB3, and LAMB4. The gamma chains are LAMC1, LAMC2, and LAMC3. Laminin types are named based on which three chains they contain.

Laminins join with one...

Type IV collagen is a network-forming collagen found in basement membranes. It acts as a barrier between epithelial and endothelial cells. It also helps form the backbone of the basement membrane with laminin, entactin, proteoglycans, and fibronectin.

Beyond support, type IV collagen also helps with signaling functions linked to normal body processes and disease. A type IV collagen molecule contains six alpha chains. These chains can form at least three heterotrimeric triple-helical...