bio sphere
Scanning electron microscopy gets drenched
© 2004 AMERICAN CHEMICAL SOCIETY
(a)
(b) COURTESY OF VERED BEHAR, QUANTOMIX LTD.
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ince the 1960s, electron microscopy has provided us with stunning images of cells. However, a nagging caveat remains. In order for the electron microscope to operate, water must be completely removed so it doesn’t boil in the instrument’s vacuum. However, because water is an essential component of biological samples, its removal may cause cellular structures to distort and produce artifacts. Not only does the cell have to be dehydrated, it also has to be fixed and stained by a host of chemicals. These conditions are hardly “physiological”. To maintain water in its rightful place, Elisha Moses, Stephan Thiberge, and colleagues at the Weizmann Institute of Science and Quantomix (both in Israel) have developed a novel wet scanning electron microscopy (SEM) technique. With the help of an extremely thin polymer membrane, they have created a special sample chamber that preserves physiological conditions for cells and tissues during SEM (Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 3346–3351). The images obtained by wet SEM provide both whole-cell morphology and smaller details within the cells. Standard SEM only provides information about the sample surface. The new sample chamber has a 145nm-thick polyimide membrane that acts as a transparent window for the incident electron beam. The membrane also provides a support for cells and tissues and doesn’t collapse under the differential pressures between the fluid inside the chamber and the vacuum. Moses and his colleagues first used the chamber to image samples without standard SEM stains. The stains, which consist of electron-dense materials such as gold or uranyl acetate, are usually needed to increase contrast and resolution. Biological samples tend to be rich in carbon, oxygen,
Fluid SEM images of (a) stained and (b) unstained rat nerves.
and hydrogen, which usually don’t scatter electrons strongly enough on their own to obtain sufficient resolution. The investigators discovered they could resolve samples in the fluid SEM chamber at a resolution far better than what was considered theoretically possible without electron-dense stains. Milk-fat droplets
