Connecting cells together

• Actin filaments


  • 5-9 nm in diameter
  • Can be bundled together into thicker filaments which are much stronger than individual filaments
• Microtubules


  • Larger than actin filaments
  • Outer diameter of 25 nm
  • Hollow cylinders
  • Much more rigid than actin filaments
• Intermediate filaments


  • More diverse than microtubules or actin filaments
  • Provide mechanical strength to the cell
  • Used to connect cells together in tissues
  • Form the nuclear lamina
• The cytoskeleton of a cell is extremely dynamic and is constantly re-organising
• Actin filaments extend and contract constantly, pushing out the plasma membrane as the cell senses its environment
• As cells move the entire actin network can be remodelled extremely rapidly
• Microtubules are also dynamic and remodel to allow the cell to send organelles or vesicles to new parts of the cell or during cell division when the mitotic spindle forms
• The dynamic nature of acting filaments and the rest of the cytoskeleton allows the cell to very rapidly change shape or its direction of movement in response to signals from the environment
• Tight junctions


  • Cells are joined very tightly
  • Produce a virtually impermeable barrier between the cells
  • Tight junction proteins link to the actin cytoskeleton
• Cadherin molecules


  • Cell surface molecules that allow similar cells to recognise each other and form connections
  • Different types of cadherin molecule join different types of cells together
• Desmosomes


  • Connect cells through their intermediate filaments
  • The gap between cells is much wider than tight junctions (about 30 nm)
• The extracellular matrix (ECM) is the scaffolding that surrounds cells
• Cells attach to the ECM and it provides structure and both mechanical and biochemical support for the tissues
• Cells will adapt to the ECM – stem cells placed onto different ECMs will differentiate into different types of cells depending on the nature of the ECM
• The ECM can also form basement membrane – thin sheets of ECM at the base of tissues such as skin
• Components of the ECM


  • Collagen
  • Proteoglycans
  • Fibronectin
• Collagen is the most abundant protein in the body
• About one quarter of all of the protein in your body is collagen
• Collagen is a major structural protein, forming molecular cables that strengthen the tendons and bones
• Teeth are made by adding mineral crystals to collagen
• Collagen provides structure to our bodies, protecting and supporting the softer tissues and connecting them with the skeleton
• Proteoglycans


  • Proteins that are heavily glycosylated
  • Contain very long chains of sugar molecules branching off of the main protein chain
  • Can function to lubricate the ECM
  • Can modify the activity and stability of other proteins in the ECM
• Integrins


  Cell surface molecules that act as matrix receptors
• Integrins connect the cell to the ECM and transmit signals into the cell allowing the cell to sense and respond to its environment
• Hemidesmosomes


  • Connect to the ECM through adhesion complexes (integrins linked to actin filaments)
  • Attach to basement membranes through hemidesmosomes
  • Provide additional mechanical strength to the cells and tissues