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For the last 60 years, electron microscopy has been a key imaging method to investigate biological ultrastructure such as the brain. In more recent years, novel volume electron microscopy techniques have opened the door for nanometer-scale visualization of cells and tissues in three dimensions.
Electron microscopy was first developed in the 1930s and soon became a powerful means of biological imaging, employed to investigate macromolecules and organization of cell structures at organelle level. It utilizes a beam of electrons to create an image of a specimen. Compared to light microscopy, the technique is capable of higher magnifications and has greater resolving power, meaning it can image smaller objects in finer detail.
An electron microscope contains electromagnetic and/or electrostatic lenses which control the path of electrons; the beam of electrons passes through the center of a solenoid (a coil of wire around the outside of a tube). An electric current travels through the solenoid which generates an electromagnetic field; the electron beam is sensitive to the electromagnetic field, and can be controlled by varying the current through the solenoid. The faster the electrons travel the shorter their wavelength, and since the wavelength of light used to form an image is directly related to resolution, reducing the wavelength increases the resolution of an image.
However, electrons can only penetrate thin surfaces. In order to visualize, for example, a whole cell, the cell would have to be manually sliced and imaged section by section using transmission electron microscopy. The process is slow, prone to error and sections can be easily lost or damaged, not to mention it requires extreme manual dexterity.
The technique was extended in the early 1980s when scanning electron microscopy was combined with a miniature ultramicrotome, and volume electron microscopy (VEM) was born. Volume electron microscopy suddenly became readily accessible and increasingly automated; the ultramicrotome thinly sliced the sample with a diamond or glass knife, before the sample surface was scanned with a focused beam of electrons to produce an image. VEM has broadened the scope of where electron microscopy can be applied beyond the brain, and has enabled scientists to study complex biological structures such as organelles or neuronal networks in unprecedented detail in three dimensions. Volume electron microscopy can capture of thousands of serial images with little human interaction, quickly, easily and reliably.
There are several methods that a scientist might choose depending on factors such as size of target volume, required resolution, if sections need to be retained and the time available for acquisition:
- Serial block-face electron microscopy (SBEM) utilizes a diamond knife to iteratively remove a slice from a block-face before imaging. Once cut away, the slice is lost.
- Focused ion beam SEM (FIB SEM) employs a beam of gallium ions instead of a knife to slice a thin layer from a sample. Again, when the slice has been imaged, it is lost.
- Automated tape-collecting ultramicrotome SEM (ATUM-SEM) collects serial thin sections of a sample onto tape. The sample is then manually transferred onto wafers for imaging. Samples here can be retained for reimaging when necessary.
Automated volume electron microscopy has filled an imaging gap by allowing for readily available ultrastructural three-dimensional datasets. It has significantly improved the acquisition of biological tissues in three dimensions, but there is still further to go, especially in neuroscience, if larger samples are to be images. The introduction of integrated light and electron microscopy has the potential to revolutionize volume electron microscopy – its prospects are challenging, but bright and exciting.
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