A new imaging method that can measure cell mass using two beams of light — called spatial light interference microscopy (SLIM) — offers new insights into whether cells grow at a constant rate or exponentially. SLIM is extremely sensitive, quantitatively measuring mass with femtogram accuracy. By comparison, a micron-size droplet of water measures 1000 fg. The technique can measure the growth of a single cell, and even mass transport within the cell. Led by Gabriel Popescu, a University of Illinois research team described its work with SLIM in a recent edition of Proceedings of the National Academy of Sciences. “A significant advantage over existing methods is that we can measure all types of cells — bacteria, mammalian cells, adherent cells, nonadherent cells, single cells and populations” said Mustafa Mir, a co-author of the paper. “And all this while maintaining the sensitivity and the quantitative information that we get.” Illinois researchers have developed a novel imaging technique, dubbed spatial light interference microscopy, that can quantitatively measure cell mass. (Image: Quantitative Light Imaging Laboratory) Unlike most other cell-imaging techniques, SLIM — a combination of phase-contrast microscopy and holography — does not need staining or any other special preparation. Because SLIM is completely noninvasive, the researchers can study the natural functions of cells as they occur. The technique uses white light and can be combined with more traditional microscopy techniques, such as fluorescence, to monitor cells as they grow. “We were able to combine more traditional methods with our method because this is just an add-on module to a commercial microscope,” Mir said. “Biologists can use all their old tricks and just add our module on top.” Because of SLIM’s sensitivity, the researchers could monitor cells’ growth through different phases of the cell cycle. They found that mammalian cells show clear exponential growth during the G2 phase of the cell cycle only, after the DNA replicates and before the cell divides. This information not only has great implications for basic biology, but also for diagnostics, drug development and tissue engineering. The researchers hope to apply their new knowledge of cell growth to different disease models. For example, they plan to use SLIM to see how growth varies between normal cells and cancer cells, and the effects of treatments on the growth rate. Popescu is establishing SLIM as a shared resource on the Illinois campus, hoping to harness its flexibility for basic and clinical research in a number of areas. “It could be used in many applications in both life sciences and materials science,” Popescu said. “The interferometric information can translate to the topography of silicon wafers or semiconductors.” For more information, visit: www.illinois.edu