7) The Extracellular Matrix

• The ECM is a network of macromolecules that surround cells to provide mechanical strength and facilitate cell communication.
• ECM function in connective tissues:

  • Connective tissues contain the most ECM as their main function is to provide structural support and mechanical strength
  • Different ECM compositions make different types of connective tissue
• Types of Connective Tissue:

  1. General connective tissue (tissues)
  2. Specialised connective tissue (bones, tendons, cartilage)
• Summary of ECM Structure:

  • Fibrous proteins (collagen/elastin): Provide ’tensile strength‘ and ‘elastic properties’
  • Glycosaminoglycans (GAGs): Provide ‘compressive strength’ (shock absorbance) and ‘lubrication’
  • Glycoproteins (Fibronectin): Mediates ‘Cell-ECM’ communication and ‘ECM organisation’
• Collagen Structure and Function:

  Collagen is a protein that cells synthesise and deposit into the ECM

  1. procollagen (inital form of collagen) is secreted outside the cell
  2. enzymes (procollagen-proteinases) cleave terminal extentions
  3. Collagen can polymerise into Peptide chains -> Triple helices -> Fibrils -> Fibres with the assistance of fibroblasts

  • There are various types of collagen, allowing them to have tissue-specific functions

  
• Elastin Summary:

  • Fibrous Protein
  • Structure: Elastin molecule network connected by multiple random cross-links
  • Function: Form ’Elastin fibres’ which allow tissues to ‘recoil’ after being stretched

  
• Proteoglycans/Glycosaminoglycans (GAGs) Summary:

  • Structure: Linear hydrophilic polysaccharides linked to a core protein (Forming a Proteoglycan)
  • Hydrophilic due to negatively charged disaccharide units
  • Main Function: Hydrophilic nature allows them to absorb water to generate swelling pressure and resist compression in the ECM
  • Addition functions: Bind to GFs, Block or encourage cell migration, control substance exchange in the ECM

  
• Glycoprotein summary:

  • Structure: Contain multiple binding sites for ECM proteins and cell surface receptors
  • Function: Organise the ECM, Cell-ECM attachment, Cell migration

  
• Fibronectin Summary:

  • Glycoprotein
  • Structure: Multiple domains containing binding sites for other ECM proteins. Can be found in a soluble form, or in fibrils.
  • Function: Acts as a bridge between cells and ECM.
• The basal lamina (Basment membrane) summary:

  • Specialised ECM in muscle and epithelial cells
  • Structure: Synthesised by cells on both sides to form a very intertwined network of proteins
  • Degraded by proteases to allow cells to proliferate and migrate while still being attached to the ECM
  • Function: Provides structural support and separates different cell types