Electron Microscopy and the study of the Cell.

There are 2 types of electron microscope, the scanning electron microscope and transmission electron microscope.  Transmission electron microscope create a 2D image of the cross-section of a sample, this microscope type has the highest resolving power.  Scanning electron microscopes give a 3D effect and show surface detail.

When looking at a specimen under an electron microscope the specimen must thin enough for electrons to pass through, about as thin as the film around a soap bubble.  There are two ways of preparing a sample for electron microscopy, negative staining and freezing.  Negative staining involves using an element such as uranium or gold to stain a sample.  An electron micrograph is an electron density map.  Elements such as Carbon, Hydrogen and Oxygen, which make up the largest portion of most cells, are not electron dense (do not contain many electrons), so they create a faint electron micrograph.  Staining the sample with elements which are electron dense means that there is a large contrast and the sample can be seen more clearly.  When staining a sample it is embedded in a resin which becomes hard, when hard it is sliced into very thin pieces with a glass knife known as a microtome, it is then treated with a solution of a heavy metal salt like uranyl acetate, and then dehydrated.  The metal ions then surround the exterior of the specimen; this can cause distortions with the shape of the sample.

To try and prevent this distortion the sample can be frozen with liquid ethane or liquid nitrogen; because the sample is not dehydrated it remains the same.  The frozen sample is ‘snapped’ into small slices.  Accurate observations are difficult to make with this method as there are no metal ions present, although the low temperature decreases damage done by the electrons and allows for a higher magnification. This method is known as the freeze fracture technique.
Perhaps the most obvious advantage of electron microscopy when compared to optical microscopy is the clarity and the depth of field possible to investigate, the wavelength of electrons allows magnification of up to 500,000x which is huge when compared with the magnification of just 2000x, that of a light microscope.  You might ask, why use light microscopes at all?  Well, as an electron microscope requires special confinement for its use, as it is affected by magnetic fields and having large components it is expensive to set up.  The expense does not stop there, the expertise and further equipment required to prepare specimens means that it is also hugely expensive to run.  The optical light microscope being small, portable, and with little understanding of light microscopy required to use it means it is readily available to even the most novice of scientists.  The light microscope allows living cells to be viewed and filmed whereas the preparation for electron microscopy ensures that nothing can survive.  Natural colours can be viewed with light microscopes and samples are rarely distorted, false colouring techniques are used on electron micrographs.

I think these differences mean there is a demand for both microscopes, although the electron microscope has revolutionised cell biology there will always be a need for the light microscope.