27.3:
Collagens are the Major Structural Proteins of ECM
Collagens are one of the most commonly found proteins in mammals. They are known for their high tensile strength and provide excellent resistance to pulling forces. Collagens are basically fibrous proteins exclusively found in the extracellular matrix.
They are primarily produced by fibroblasts and in smaller quantities by smooth muscle cells and epithelial cells.
Collagen molecules are trimers consisting of three polypeptide chains, called alpha chains. These chains are rich in glycine, proline and hydroxyproline, necessary for structural stability and triple-stranded helix formation.
In the case of fibrillar collagen, these trimers or protocollagens first assemble into collagen fibrils. Then, these fibrils assemble into collagen fibers, forming tendons that attach muscle to bone, rendering a stable framework for tissues.
Conversely, non-fibrillar collagens, such as type IV collagen, do not form fibrils. They are an important component of basal lamina, which separates cells from other cells and from connective tissue.
27.3:
Collagens are the Major Structural Proteins of ECM
Three main types of fibers are secreted by fibroblasts: collagen fibers, elastic fibers, and reticular fibers. Collagen fiber is made from fibrous protein subunits linked together to form a long, straight fiber. Collagen fibers, while flexible, have great tensile strength, resist stretching, and give ligaments and tendons their characteristic resilience and strength. These fibers hold connective tissues together, even during the body's movement.
Connective tissue proper includes loose connective tissue and dense connective tissue. Dense connective tissue contains more collagen fibers than does loose connective tissue. As a consequence, it displays greater resistance to stretching. Dense regular elastic tissue contains elastin fibers and collagen fibers, allowing the ligament to return to its original length after stretching. The skin's dermis is an example of dense irregular connective tissue rich in collagen fibers. Hyaline cartilage, the most common type of cartilage in the body, consists of short and dispersed collagen fibers and contains large amounts of proteoglycans. Fibrocartilage is tough because it has thick bundles of collagen fibers dispersed through its matrix. Menisci in the knee joint and the intervertebral discs are examples of fibrocartilage. Elastic cartilage contains elastic fibers as well as collagen and proteoglycans.
This text is adapted from Openstax, Anatomy and physiology 2e, Section 4.3: Connective tissue supports and protects.