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Updated: Jun 17 2021


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  • 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
      1. alpha chains
        • three alpha chains (two alpha1 and one alpha 2) are formed
      2. procollagen
        • two alpha1 and one alpha2 combine to form procollagen
      3. tropocollagen
        • procollagen is processed by extracellular protein modification into a tropocollagen molecule after being expressed from the golgi apparatus
      4. collagen fibril
        • aldehyde formation on tropocollagen lysine and hydroxylysine allow for covalent bonding between tropocollagen molecules forming the collagen fibril
      5. 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
      • 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
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