Cryo-scanning transmission electron tomography of vitrified cells
by Michael Elbaum, Sharon Wolf and Lothar Houben
Cryo-electron tomography of fully hydrated, vitrified biological specimens has emerged as a vital tool for biological research. For cellular studies, the conventional imaging modality of transmission electron microscopy places stringent constraints on sample thickness because of its dependence on phase coherence for contrast generation.
Prof. Michael Elbaum and Dr. Sharon Wolf of the Weizmann Institute of Science together with Dr. Lothar Houben of the ER-C have investigated a new way of imaging biological samples in 3D using an electron microscope. The team suggests an alternative method for cryo-tomography, to scan with a narrow focused beam, called STEM, for “scanning transmission electron microscopy”. The method is flourishing in research of materials (such as semiconductors), and is also gaining ground in tomography of plastic-embedded, metal-stained biological specimens. However, conventional wisdom says there’s no point in using STEM with cryogenic biological samples without metal stains. With only weak electron scattering they should produce low contrast and poor quality images. The new insight is that this “weak” scattering is actually simply directed to especially small angles, where previously researchers were not bothering to detect signals.
By arranging the detectors in an unconventional manner, the team was able to capture the STEM signal and use it to make an image. They compare the STEM approach for the imaging of whole bacteria and human tissue culture cells with the conventional method that relies on phase contrast in defocussed TEM images, finding favourable contrast and detail in the reconstructions from STEM signals. Particularly at high sample tilts that are required for tomographic reconstructions from tilt series, the STEM signals contain more informative data than energy-filtered TEM phase contrast images, resulting in improved depth resolution. The favourable effect is based on the physics of the scattering process. The STEM detection is less sensitive to inelastic scattering than phase contrast imaging, and for organic tissue this inelastic scattering is dominant over elastic scattering. As a side effect careful control over dose delivery in STEM permits relatively high cumulative exposures before the onset of beam damage. The increase in acceptable specimen thickness and electron dose broadens the applicability of electron cryo-tomography.
Michael Elbaum, Sharon Wolf and Lothar Houben: Cryo-scanning transmission electron tomography of vitrified cells, Nature Methods XXX (2014) YYY.