electron microscopes

• • To prepare samples for use with electron microscopes they are treated with a solution of heavy metals (like lead) -
  • this process is the equivalent of staining samples that are to be viewed with a light microscope
  • The metal ions act to scatter the electrons that are fired at the sample and give contrast between different structures.
• • The images produced by electron microscopy are always black and white
  • colour can be added to images after they've been made to make them easier to interpret
• diagram

  A) electron gun

  B) anode

  C) specimen

  D) condenser lens

  E) objective aperture lens

  F) intermediate lens

  G) projector lens

  H) fluorescent screen

  
• electron gun

  contains tungsten filament and shoots the electrons
• anode

  oppositely charged to the electrons to attract them
• condenser lens

  condenses the electrons down to the specimen
• objective aperture lens

  magnifies the image of the specimen
• intermediate lens 


  forces the image down to the projector lens
• projector lens

  projects the image onto the fluorescent screen
• fluorescent lens

  where the image of the specimen is produced
• the objective, intermediate and projector lens magnify and direct the electrons to the fluorescent screen
• a vacuum has to be maintained in an electron microscope to stop any particles in the air to affect the electrons as the electrons could get displaced
• Tissue sample preparation

  1. Place in glutaraldehyde

  2. Wash in buffer

  3. Immerse in osmium tetroxide

  4. Fixation
• Fixation

  Helps preserve the cell membranes of the cells
• Dehydration

  Place tissue in a series of alcohols to remove water
• Embedding


  Place tissue in resin so very thin sections can eventually be cut
• Sectioning

  Cut thin sections of the resin using an ultramicrotome with a glass or diamond knife
• Staining

  Stain thin sections with heavy salts such as lead salts and uranyl salts
• Staining

  Creates electron dense areas where electrons cannot pass through to the screen, allowing contrast between different parts of the cell
• Thin sections

  • Placed on copper grids, not glass slides as electrons will be absorbed by glass
  • Electrons can pass through the holes of the copper grid