Extracellular matrix
Extracellular matrix fibers (connective tissue fibers)
Collagen fibers, Reticular fibers, and Elastic fibers
Collagen
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).
Glycosaminoglycans (GAGs)
- 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
Proteoglycans
- Definition: proteins with numerous covalently linked GAG side chains
- Function
- Bind H2O → shock absorption and a supportive function (e.g., in cartilage → resistance to compression of articular cartilage)
- Formation of cell-cell junction or cell-matrix junction
- Further signaling and regulatory functions (e.g., by binding signaling molecules)
- Examples
- Aggrecan: large proteoglycan found in cartilage as an aggregate with hyaluronic acid → shock absorption withstands compression
- Decorin: small proteoglycan that binds collagen fibrils → regulates fibril assembly
Glycoproteins of the ECM
- 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
- Fibronectin