Extracellular matrix fibers (connective tissue fibers)

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Collagen fibers, Reticular fibers, and Elastic fibers

Collagen

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Structure

• A collagen molecule is a protein with a repeating amino acid sequence (Gly-X-Y)_n.
  • The first amino acid of this triplet is glycine (collagen is comprised of ⅓ glycine).
  • Position X: most commonly proline (also common: lysine, hydroxylysine)
  • Position Y: most commonly hydroxyproline (also common: hydroxylysine, lysine)

Types

And scar tissue (late stages of wound healing, e.g. post-MI) See Osteogenesis imperfecta

Tip

• Compare with ligaments, which are made of elastin.
• Ligaments attach bone to bone, and tendons attach muscle to bone.
• Connect presence of elastin with Marfan syndrome
  • FBN1 can disrupt the normal regulation of TGF-β → Tall stature
  • Skin → Skin hyperextensibility
  • Large arteries → aortic dilation, aneurysms, or dissection
  • Elastic ligaments → Joint hypermobility
  • Lung → Increased risk of spontaneous pneumothorax

Deficient in vascular type of Ehlers-Danlos syndrome (threE D).

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See Goodpasture syndrome

Glycosaminoglycans (GAGs)

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• Definition: a family of unbranched polysaccharide chains of repeating disaccharide units with multiple negative charges that constitute a large volume fraction of the ECM
• Structure: polymer of repeating disaccharide units
• Four main groups
  • Hyaluronic acid
  • Chondroitin sulfate and dermatan sulfate
  • Heparan sulfate
  • Keratan sulfate
• Function
  • Bind H2O in connective tissue due to its negative charges: act as a cushion
  • Component of proteoglycans

Glycoproteins of the ECM

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• Definition: proteins with short carbohydrate side chains that contribute to the organization of the extracellular matrix by offering specific binding sites for cells and other matrix molecules
• Examples
  • Fibronectin
    • Glycoprotein important for cell-matrix interactions
    • Function: binds to collagen and integrins and plays an important role in embryogenesis (regulates cell migration) and hemostasis (cross-linking of fibrin molecules with thrombocytes and fibroblasts)
  • Laminin
    • Major component and organizer of the basal lamina (besides type IV collagen)
    • Function: binds to collagens, integrins, and proteoglycans