Direct imaging of endogenous biometal distributions within millimetre-scale organisms at micrometre resolution – x-ray fluorescence tomography

Abstract

First-row transition metals are required for all forms of life on earth. The high reactivity of these elements means that an array of mechanisms has evolved to regulate key processes governing their transport and binding action. Tracking metals within biological tissue is non-trivial; tagging approaches suffer from lack of specificity, and can fail to find strongly-bound species; in addition, tags can interfere with normal biochemistry. Electron microscopy provides stupendous resolution, but probes miniscule volumes due to the short penetration of electrons. With μM sensitivity, X-ray Fluorescence Microscopy (XFM) can probe endogenous metal concentrations at resolutions at the μm length scale. Elemental maps are quantitative. With penetration depth and depth of field well matched at around 0.5 mm, the method can be up-scaled to 3-D visualisations via tomography. Here we report on our application of X-ray fluorescence tomography of Zn, Cu, Fe, and Mn in C. elegans and discuss recent progress in developing self-absorption corrections that will enable accurate mapping of light elements.