A new technique allows the mechanisms of a cell's mitotic spindle to be observed and measured, which may lead to a better understanding of cancers and birth defects arising from cell cycle disruptions. Researchers at the Harvard School of Engineering and Applied Sciences (SEAS) used a femtolaser pulse to slice into the mitotic spindle of a frog egg cell, making two cuts perpendicular to the growth plane. The mitotic spindle, which is composed of protein microtubules, forms during cell division. It is responsible for separating chromosomes into daughter cells. “The laser allowed us to make precise cuts and perform experiments that simply were not possible using previous techniques," said Eric Mazur, Balkanski Professor of Physics and Applied Physics at Harvard, who co-authored a study on the work. Previously, it was unknown how microtubules are organized in the spindles of animal cells. For a long time, it was believed that they span the length of the spindle between the two poles. However, Mazur and his colleagues demonstrated that they vary in length, with the shortest and oldest ones closest to the poles, and that they can begin to form throughout the spindle. Top: A series of fluorescent images of a spindle taken before the cut and at 5 seconds and 10 seconds after the cut. Scale bar, 10 µm. Bottom: A graphical representation of the cut microtubules. The cut generates new "plus" ends (red) and new "minus" ends (green). The newly generated minus ends remain stable, whereas the new plus ends depolymerize, which creates two depolymerization fronts of opposed polarity. (Image: Jan Brugués, Harvard SEAS) The researchers were able to take quantitative measurements on how the spindle reconstructs itself upon being cut. From this, they could work backward and determine the beginning and end points of each microtubule, developing a numerical model of the role of transport in the spindle. Further research, they hope, will one day give scientists a complete understanding of, and possibly even control over, the formation of the spindle. “Understanding the spindle means understanding cell division," said Jan Brugués, a postdoctoral fellow at SEAS. "With a better understanding of how the spindle is supposed to operate, we have more hope of tackling the range of conditions — from cancer to birth defects — that result from disruptions to the cell cycle or from improper chromosomal segregation.” Using laser ablation to slice through microtubules in the mitotic spindle, researchers have developed a clearer picture of how cell division occurs. (Image: Julie Eichhorn) Mazur and Brugués worked with Daniel Needleman, assistant professor of applied physics and molecular and cellular biology at Harvard, and Valeria Nuzzo, a former postdoctoral fellow in Mazur's lab at SEAS, to bring the tools of applied physics to bear on a biological question. The research was published in the April 27 issue of Cell. The research was funded by the National Science Foundation and by a fellowship from the Human Frontiers Science Program. For more information, visit: www.seas.harvard.edu