This Fourth of July, many of you will spread out a blanket and enjoy an evening display of fireworks with their dramatic, colorful bursts. But here’s one pyrotechnic pattern that you’ve probably never seen. In this real-time video, researchers set off some fluorescent fireworks under their microscope lens while making an important basic discovery about how microtubules, the hollow filaments that act as the supportive skeleton of the cell, dynamically assemble during cell division.
The video starts with a few individual microtubule filaments (red) growing linearly at one end (green). Notice the green “comets” that quickly appear, followed by a red trail. Those are new microtubules branching off. This continuous branching is interesting because microtubules were generally thought to grow linearly in animal cells (although branching had been observed a few years earlier in fission yeast and plant cells). The researchers, led by Sabine Petry, now at Princeton University, Princeton, NJ, showed for the first time that not only do new microtubules branch during cell division, but they do so very rapidly, going from a few branches to hundreds in a matter of minutes .
Petry and her colleagues initially wanted to know how dividing cells assemble their mitotic spindles, the specialized structures made primarily of stacked microtubules that tether the chromosomes as one cell splits into two. But they had a problem: despite the tremendous progress in cell imaging, it was still not possible to tell microtubules apart from each other inside a dividing cell. It’s simply too crowded in there to make out exactly when and how the spindles are assembled.
To work their way around this problem, Petry and her colleagues turned to frog eggs. The cells are large and full of cytoplasm that can be extracted and viewed cell-free under a microscope. With the right biochemical prompts, microtubules in the cytoplasm solution begin forming and branching just as they would in an intact frog cell to do their job of forming a spindle.
When the team members placed the cell extract under a special microscope with a time-lapse camera, they were surprised to see that rather than growing in straight lines as expected, the microtubules started branching like crazy. They also soon realized that the branching made perfect sense. Branching allows cells to grow lots of microtubules quickly in the same direction that they will be needed to stabilize each of a frog’s 26 chromosomes within a spindle.
Until this work, branching microtubules had never been observed in a metazoan organism—and never during cell division in any organism. It surely also hadn’t been observed with bursts of color reminiscent of those you’ll see lighting up the night sky in the coming days. Happy Fourth of July!
 Branching microtubule nucleation in Xenopus egg extracts mediated by augmin and TPX2. Petry S, Groen AC, Ishihara K, Mitchison TJ, Vale RD. Cell. 2013 Feb 14;152(4):768-777.
Introduction to Xenopus, the frog model (Xenbase, University of Calgary, Alberta, Canada)
Sabine Petry (Princeton University, Princeton, NJ)
NIH Support: National Institute of General Medical Sciences