Abstract

Distributions of chemical elements within unstained sectioned cells can provide important information about various classes of macromolecules. For example, phosphorus in nucleic acid enables determination of the packing density of DNA within the chromatin of a cell’s nucleus; variations of calcium levels in mitochondrial and endoplasmic reticulum compartments can help understand the mechanisms of calcium regulation and explain normal physiological as well as pathophysiological processes within cells. Mapping of calcium/phosphorus distributions at ~10 nm spatial resolution in biological specimens is typically achieved using energy-dispersive x-ray spectroscopy or electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Here, we discuss a complementary approach based on energy-filtered transmission electron microscopy (EFTEM) to map chemical elements in thin unstained sectioned cells. With EFTEM , it is possible to obtain 2D elemental distributions from large cellular regions and combine it with conventional electron tomography to generate 3D elemental maps. Using this technique we determine the 2D calcium distribution inside mitochondria as well as the 3D phosphorus distribution within cell nuclei. Although the detection limits for a given pixel are not as favorable as for STEM /EELS, it is possible to improve the sensitivity of this method by averaging large number of pixels.