A new imaging technique called spatial light interference microscopy (SLIM) soon may answer the much debated question as to whether cells grow at a constant rate or exponentially. An extremely sensitive method, SLIM can quantitatively measure mass with femtogram accuracy using two beams of light. It can gauge the growth of a single cell and even mass transport within the cell. It also can measure all cell types, including bacteria, mammalian cells, adherent and nonadherent cells, and populations, according to Mustafa Mir, a graduate student at the University of Illinois. The method was reported in the July 25 issue of Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1100506108). Artists’ rendition of a SLIM measurement of a pair of human osteosarcoma cells (real data). The colors on the image correspond to the dry mass density at each point. Courtesy of Quantitative Light Imaging Laboratory and the Imaging Technology Group Visualization Laboratory, Beckman Institute for Advanced Science and Technology. Combining phase-contrast microscopy and holography, SLIM does not require staining or any other special preparation. The noninvasive method uses white light and can be used with more traditional microscopy techniques, such as fluorescence, to monitor a cell as it grows. Thanks to the method’s sensitivity, researchers at the university were able to monitor cell growth through the phases of the cell cycle. They discovered that mammalian cells show clear exponential growth only during the G2 phase – after the DNA replicates and before the cell divides. This information has great implications not only for basic biology but also for diagnostics, tissue engineering and drug development. The researchers hope to apply their new findings on cell growth to various disease models. For example, they plan to use SLIM to see how growth varies between normal cells and cancer, and to determine the effects of treatments on growth rate. University of Illinois researchers have developed an imaging technique called spatial light interference microscopy (SLIM) that can quantitatively measure cell mass with light. Courtesy of Quantitative Light Imaging Laboratory. “Of all the current growth measurement techniques, SLIM is unique in its capabilities to study these interactions in typical culture conditions,” said Gabriel Popescu, a member of the Beckman Institute for Advanced Science and Technology at the university. “Since SLIM is designed as an add-on module to a commercial phase microscope, it requires minimal retraining to use and can readily be implemented in any biology laboratory. We expect that these capabilities will allow others to explore other fundamental questions in the field of cell growth.” Popescu has established SLIM as a shared resource on the university’s campus, hoping to harness its flexibility for basic and clinical research in a number of areas.