Cryo-electron tomography offers a means of obtaining the 3-D ultrastructure of specimens that have not been chemically fixed or stained. The technique is still under development, but it has already been applied to several biological specimens including prokaryotic cells, centrioles, sperm axonemes, mitochondria, spindle-pole bodies, lipid vesicles, isolated skeletal-muscle triad junctions, chromatin fragments, actin bundles, and macromolecules.
Electron tomography itself is a nearly mature technology. It is most often used with conventional plastic sections 100- 1000nm thick. The specimen is imaged in the TEM, using a tilt stage to record a series of projection images over a large angular range. Using image-processing techniques, the tilt images are back-projected to form a reconstructed volume of the specimen. The reconstruction is a 3-D array of pixels or volume elements (voxels), and can be sliced in any direction to form a series of 2-D images.