High-resolution 3-D imaging of a cell’s nucleus undergoing cell division is now possible, thanks to a combination of plunge-freezing and a new method of sample slicing. The findings open up a biological mystery because they indicate that some cells take one of the characteristic steps of mitosis significantly differently from others. Traditionally, two sets of chromosomes pair up at the center of the cell’s nucleus during mitosis. Then hollow rods of protein – microtubules composed of a cellular structure called the spindle apparatus – grab onto the chromosomes and essentially pull each set away from the center in opposite directions, ensuring that each cell receives a full copy of the genetic material. Typically, in the cells of fungi, plants and many animals, one or more microtubules attach to each chromosome before the spindle separates the sets of chromosomes from one another. However, when researchers at California Institute of Technology observed this step using their new technique, what they saw was not the usual cell division. Instead, they discovered a cell with fewer microtubules used than chromosomes. The group used electron cryotomography (ECT) to image biological samples. Unlike traditional electron microscopy, ECT involves plunge-freezing samples so quickly that they become trapped in a near-native state within a layer of transparent glasslike ice. High-resolution images can then be captured as the sample is rotated, usually one degree at a time. Beam-penetration issues have limited ECT to samples less than 500 nm thick, such as small bacteria and viruses. Observing eukaryotic cells Now, however, the Caltech group has extended the technique to observe eukaryotic cells, which typically are much bigger. The investigators located the smallest known eukaryote, Ostreococcus tauri, a cell with 20 chromosomes, and imaged it with ECT. Next, they set out to observe the eukaryote’s cell division, but even its tiny size exceeded the 500-nm limit when it underwent mitosis. They used a diamond knife to cryosect – cut a frozen sample into slices – enabling them to look at them through dividing cells in a near-native hydrated state. They made detailed observations of mitosis in O. tauri. Contrary to expectations, nowhere near 40 microtubules attached to the two sets of chromosomes during mitosis. Instead, they discovered only about 10 small, incomplete microtubles, suggesting that the chromosomes may link together to form a bundle that can then be segregated all at once by a smaller number of microtubules. The findings appeared in the Sept. 8 issue of Current Biology (doi: 10.1016/j.cub.2011.08.021). The researchers are now using the same method to try to image human cells.