In a transmission electron microscope (TEM), a beam of electrons is passed through a very thin specimen. Areas that absorb or scatter many electrons appear dark on the image; areas that let electrons through appear light.Preparing the specimen
• The tissue is fixed with chemicals to preserve its structure, then dehydrated.
• It is embedded in resin and cut into extremely thin sections (often less than 100 nm thick) using an ultramicrotome.
• It is stained with salts of heavy metals such as lead or uranium (or osmium). These scatter electrons, so membranes and other structures that take up the stain appear dark and give contrast.
Strengths
The TEM has the highest resolution of any microscope used in biology, able to separate points less than 1 nm apart (about 0.1 nm under ideal conditions), and it shows the internal structure of cells and organelles, such as cristae in mitochondria, thylakoids in chloroplasts and ribosomes on rough ER.Weaknesses
• The image is only two-dimensional (a flat slice), so the 3D shape must be worked out from many sections.
• Sections must be very thin, and preparation can produce artefacts.
• Specimens are dead, and images are black and white.For example, a TEM section through a pancreas cell shows the double nuclear envelope with pores and stacks of rough ER, detail that a light microscope could never reveal.
• The tissue is fixed with chemicals to preserve its structure, then dehydrated.
• It is embedded in resin and cut into extremely thin sections (often less than 100 nm thick) using an ultramicrotome.
• It is stained with salts of heavy metals such as lead or uranium (or osmium). These scatter electrons, so membranes and other structures that take up the stain appear dark and give contrast.
StrengthsThe TEM has the highest resolution of any microscope used in biology, able to separate points less than 1 nm apart (about 0.1 nm under ideal conditions), and it shows the internal structure of cells and organelles, such as cristae in mitochondria, thylakoids in chloroplasts and ribosomes on rough ER.Weaknesses
• The image is only two-dimensional (a flat slice), so the 3D shape must be worked out from many sections.
• Sections must be very thin, and preparation can produce artefacts.
• Specimens are dead, and images are black and white.For example, a TEM section through a pancreas cell shows the double nuclear envelope with pores and stacks of rough ER, detail that a light microscope could never reveal.