Overview Collagen is a naturally occurring family of proteins over 25 types of collagen have been described Makes up the most abundant type of protein found in humans ~25% of the whole-body protein content is composed of some type of collagen found in multiple tissues like cartilage, tendon, bone, ligament, skin (see table below) multiple different forms of collagen exist, each with different biomechanical properties suited to the environment in which that form is expressed Collagen is made of elongated fibrils formed by fibroblast cells synthesis of collagen is unique in that it occurs both inside and outside the cell Biochemisty Composition collagen is composed of a triple helix of two alpha1 chains one alpha2 chain several common amino acid sequences are found in collagen including Glycine-X-Hydroxyproline Glycine-Proline-X Formation of collagen fibers alpha chains three alpha chains (two alpha1 and one alpha 2) are formed procollagen two alpha1 and one alpha2 combine to form procollagen tropocollagen procollagen is processed by extracellular protein modification into a tropocollagen molecule after being expressed from the golgi apparatus collagen fibril aldehyde formation on tropocollagen lysine and hydroxylysine allow for covalent bonding between tropocollagen molecules forming the collagen fibril collagen fiber multiple collagen fibrils aggregate to form a collagen fiber Types of Collagen Type of Collagen Primary Collagen Secondary types Bone I Ligament I Tendon I Meniscus I Disc - Annulus I Disc - Pulposus II Cartilage - Articular II V, VI, IX Cartilage - Basement Membrane IV Cartilage - Deep calcified layer I X Cartilage - Reparative(fibrocartilage) I II Skin I and III Blood vessels III Epithelial Basement Membrane VI and VIII Collagen lattice XI Type I Collagen accounts for more than 90% of the total collagen content in the body found in bone ligament tendon meniscus annulus of intervertebral disks skin healed cartilage scar tissue nerves related clinical conditions osteogenesis imperfecta Ehlers-Danlos syndrome Type II Collagen is found articular (hyaline) cartilage nucleus pulposus of intervertebral disks type II collagen has a very long half life Type III Collagen found in skin and blood vessels related clinical conditions Ehlers-Danlos syndrome Dupuytren's contracture Type IV Collagen found in basement membranes related clinical conditions renal diseases like Goodpasture's and Alport syndromes Type V, VI, IX Collagen occur in small amounts in articular cartilage type IX collagen gene deletion linked to development of OA in women and in knockout mice mutations in COL9A1, COL9A2, COL9A3 linked to multiple epiphyseal dysplasia (MED) however 70% of MED is associated with COMP fragmented ossific centers coxa vara hips genu valgum shortened, stunted metacarpals Type VII and VIII Collagen basement membrane (epithelial) Type X Collagen is found in the deep calcified layer of cartilage produced only by hypertrophic chondrocytes during enchondral ossification (growth plate, fracture callus, heterotopic ossification) associated with calcification of cartilage in the deep zone of articular cartilage increased in early arthritis related clinical conditions Schmid metaphyseal chondrodysplasia Type XI Collagen an adhesive with the function of holding the collagen lattice together in cartilage