“Localizing Molecules in Cellular CT Scans”
Carolyn Larabell, Professor and Vice Chair, Department of Anatomy
University of California, San Francisco
Much like medical CT scans reveal anatomical structures in the body, soft x-ray tomography (SXT) visualizes and quantifies the organization of sub-cellular structures within a cell. In SXT, the specimen is illuminated with x-ray photons from within a region of the spectrum known as the ‘water window’ (284 – 543eV). ‘Water window’ x-ray photons are absorbed an order of magnitude more strongly by carbon- and nitrogen-containing organic material than by water. Consequently, variation in biomolecule composition and concentration gives rise to quantitative, high-contrast images of intact, fully hydrated cells without the need to use contrast-enhancing agents. Cells imaged by SXT are, therefore, highly representative of the cell in its native, functional state. Attenuation of soft x- rays, as they pass through the specimen, adheres to the Beer-Lambert Law. Attenuation is, therefore, a function of chemical composition and concentration of organic material, yielding unique quantitative Linear Absorption Coefficient (LAC) measurements for cellular components. To localize molecules without perturbing cell structures, we use correlated high numerical aperture cryogenic fluorescence tomography (CFT). This multi-modal approach – imaging the same cell using both CFT and SXT – allows localization of labeled molecules directly in the context of a high-resolution 3-D tomographic reconstruction of the cell.