Cryotomogram of cryosectioned mitotic Ostreococcus tauri 
(A) Tomographic slice of a frozen-hydrated "cryosection" of a cell. This ~150nm thick cryosection was cut using a cryogenically cooled diamond knife. The nucleus and two chloroplasts are outlined in blue and green, respectively. (B) Enlargement of the area boxed in (A). Cryosections are thinner than plunge-frozen cells, and therefore result in cryotomograms that have more contrast. In this example, the protofilaments in complete and incomplete microtubules (arrowheads) can clearly be resolved. Even thinner cryosections might produce images with sufficient contrast to resolve smaller macromolecules (see "purified structures" example below). (C) Model for the O. tauri spindle, based on cryotomograms like those in (A) and serial tomograms of plastic-embedded cells (more to come...). One of our favorite questions is: How does this tiny organism segregate it's 40 mitotic chromosomes? For more, see Gan et al.

A cryotomogram of the Ostreococcus tauri
(Left) Tomographic slice (2-D section computationlly extracted from the 3-D tomogram) from an O. tauri cell. The nucleus (N), chloroplast (C), Golgi body (G), and mitochondrion (M) are labeled. (Right) 3-D segmentation of this cell. This remarkable free-living cell has evolved to carry out all the conserved functions in a tiny and compact volume. For more, see Henderson et al.

Tomograms of purified structures
Here is an electron cryotomogram of a "sacculus" — the purified globally cross-linked peptidoglycan molecule from bacteria. Two sacculi are shown here, outlined with green and purple dotted lines. A granule (gr) and the stalk (St) are labeled. Colloidal gold particles (Au) were added to the sample to guide tilt-series alignment. 

This is an isosurface rendering of a subvolme of the sacculus(boxed above). The long green strands (green) are oriented perpendicular to the cell's long axis and parallel to the surface. The inset (blue box) shows a single glycan strand (ball-and-stick model). For more, see Gan et al.

The cryotomograms of sacculi demonstrate the resolution possible with current state-of-the-art cryo-ET technologies. Here, single glycan strands could be seen due to (1) the thinness of the ice (< 100 nm), (2) absence of macromolecules "above" and "below" the peptidoglycan, and (3) imaging on a 300 kV FEG cryo-TEM that was equipped with an energy filter and a lens-coupled CCD camera. By applying these and other lessons to the study of mitotic cells, we will start learning how key macromolecular complexes carry out mitosis in their native intracellular context.