How do we see what goes on inside the living cell? An entire factory of biochemical components works together to energize, build, repair, and ultimately reproduce a minimal living unit. Many of these components can be isolated or reconstructed in the lab, outside the cell, but their coordination in space and time requires a study in place. A wide variety of microscopes have been invented for this purpose, beginning with the light microscope that we encounter in school years. In terms of pure resolving power, the champion is the electron microscope. The high vacuum of the microscope is not a natural environment for cells and tissues, however. The first challenge is to preserve the specimen in a state as close as possible to the native condition. This is the realm of cryo-microscopy. The biological sample, still embedded in water, is frozen instantly into a glass without forming crystals of ice. The second challenge is that the concentrated electron beam used for imaging is an ionizing radiation that can cause severe damage to the delicate organic specimen. Optimizing the information gained before the damage accumulates, the useful contrast per exposure, is the name of the game. The CryoSTEM project explores and develops a new approach to cryo-electron microscopy: phase contrast by scanning transmission EM. Traditional STEM creates an image based on scattering of electrons, as charged particles. Phase contrast is based on interference of the electrons, as waves. Using sophisticated electron detectors, on one hand, and advancing image processing, on the other, we learn to exploit the wave properties of the electrons, as in a hologram from light. Moreover, since the components of a cell are much smaller than the volume they fill, we image in three dimensions by tomography. The approach is similar to medical CT at the hospital, but at the nanometer scale of biological molecules. Phase contrast methods by CryoSTEM offer a new horizon in the range of studies accessible by electron microscopy.
