What Is Collagen?
Collagen is a protein that helps strengthen the structure of tissues such as bones, tendons, cartilage, ligaments, vertebral disks, skin, and blood vessels. These tissues all contain collagen, but they have different proportions of different kinds of collagen (as well as various other constituents) and their structural characteristics vary.
The collagen in tendons and ligaments is arranged in bundles of parallel fibers, giving tendons and ligaments a rope-like structure. Some of the fibers in tendons and ligaments also run transverse to the parallel bundles, forming cross-links that add strength to the structure. The collagen in cartilage is arranged in a mesh with a large amount of gel-like substance between the collagen fibers, making the structure of cartilage more like a sponge. The characteristics of collagen-containing tissues also vary with position within the structure; for example, tendons and ligaments are different at the point of insertion to the bone than they are in the middle of the tendon or ligament.
Researchers have identified 19 kinds of collagen and given them names with Roman numerals. The main collagens found in connective tissue are Types I, II, and III. Tendons, ligaments, skin, and bone have mostly Type I collagen, and cartilage has mostly Type II collagen.
Tendons and ligaments also contain proteoglycans, elastin, and fibroblast cells. The collagen, elastin, and proteoglycans form the extracellular matrix. The fibroblast cells are embedded in the matrix and synthesize and secrete the matrix collagen, elastin, and proteoglycans.
The proteoglycans are protein/polysaccharide complexes that trap water and affect the viscoelastic properties of the tissue, helping the tissue resist compressive forces. Proteoglycans consist of a protein core with attached glycosaminoglycans (GAGs). Cartilage contains a high percent of a mixture of proteoglycans and water that provides a gel-like cushioning for joints. Tendons contain less proteoglycans and water than cartilage. The proteoglycan/water component of tendon, ligament, and cartilage is called the "ground substance."
The elastin fibers, which can stretch and return to their original form, are interwoven with the collagen fibers to add elasticity and prevent tearing. The elastin fibers form a network throughout the tissue, but they only represent 1-2% of the dry weight of tendon. Collagen represents 65-80% of the dry weight of tendon and is by far the most abundant component of tendon.
When new tendon tissue is being formed, the fibroblasts are actively creating new collagen. When the tissue is mature, the fibroblasts become less active and are called fibrocytes. The fibrocytes don't actively create new tissue unless they are called on to repair damage or do remodeling of the old tissue. Fibroblasts tend to look thicker, rounder, and larger than fibrocytes, which tend to look thinner and more linear. Fibrocytes found in tendons are called tenocytes. (Likewise, fibrocytes found in cartilage are called chondrocytes and fibrocytes found in bone are called osteocytes.)
A typical collagen molecule consists of three subunits called alpha chains. For example, each molecule of Type I collagen has two alpha1 chains and one alpha2 chain. Each molecule of Type III collagen has three alpha1 chains. Since it is composed of three alpha chains, the collagen molecule is called a tripeptide. The alpha chains are composed of combinations of amino acids, which are the basic building blocks of proteins. The most abundant amino acids in collagen are glycine, proline, and lysine.
Collagen Structure: Collagen is a protein formed from three polypeptide chains, also called amino acids, wound together into a triple helix. In tendons, collagen triple helices bundle together into fibrils, which themselves bundle together into strong fibers. The whole tendon structure is covered by a loose sheath called the epitenon that contains the vascular, nerve, and lymphatic supplies.
The Making of Collagen: Type I, II, and III collagens are made in several steps. First, the fibroblast cell joins three alpha chains to make procollagen according to the instructions in the genes. Then, the procollagen is released from the cell membrane. The fibroblast cells secrete enzymes that remove extra sequences at the ends of the procollagen to make tropocollagen. Then the tropocollagen assembles into collagen fibrils, which then assemble into collagen fibers.
How Genes Control The Making of Collagen: Researchers have identified at least 30 collagen genes, and most of them encode procollagens. For example, the colIA1 gene encodes the alpha1 chain for Type I collagen, known as alpha1(I), and the colIA2 gene encodes the alpha2 chain for Type I collagen, known as alpha2(I). Defects in the collagen genes can cause the collagen to be constructed incorrectly (with abnormal quantity or quality), leading to weak tissue and various collagen diseases.