A new imaging tool could enable researchers to get three-dimensional images of single living cells without resorting to the time-honored procedure of staining their inner structures with chemicals.
“We can image the cell as it is,” says Wonshik Choi of the Massachusetts Institute of Technology (MIT). The new device could be retrofitted to existing microscopes and could track dynamic processes such as cell reproduction or microbial invasions, Choi adds.
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Most cells are colorless, translucent, and barely visible even under a microscope. For more than a century, researchers have stained living cells with dyes to increase the contrast between parts such as the nucleus and the cytoplasm.
In recent years, researchers have learned to image live cells by mapping how the cells’ materials slow the speed of light to different extents. Light’s lower speed in water than in air is what makes a pencil look broken when it’s dipped halfway into water.
In the MIT device, a laser beam passes through a sample into a microscope and then on to a digital camera. The camera’s detector records tiny shifts in the light waves with respect to a reference beam from the same laser. Those shifts indicate how the laser light slowed as it crossed different cell parts.
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A system of tilting mirrors and lenses deflects the beam, allowing it to scan the sample across a 120-degree range of viewpoints. A computer algorithm—similar to the ones that produce computerized tomography scans from multiple X-ray views—then reconstructs a 3-D image of the sample. Differences in the speed of light as it passes through various parts of the cell can be displayed as different colors.
The researchers produced distinct images of nucleoli—structures contained in cellular nuclei—and the cytoplasm of cervical cancer cells, they report in an upcoming Nature Methods. The team also scanned and highlighted the internal structures of live microscopic roundworms.
The researchers estimate that their equipment can already resolve details as small as half a micron, and that they can probably bring the resolution down by two-thirds. Other imaging devices, such as electron microscopes, have much higher resolution, but they can’t image living cells.
Last year, Christian Depeursinge and his colleagues at the École Polytechnique Fédérale in Lausanne, Switzerland, took similar 3-D scans of cells by rotating the sample while holding it inside a pipette. That 360-degree scan can yield a more detailed 3-D image than the MIT device provides, Depeursinge says. However, the Swiss team’s technique required suspending cells in glycerin.
“The main advantage of [the MIT] approach seems to be speed,” says Stanford University’s Thomas Baer. That could enable researchers to image viruses, bacteria, or other microbes as they invade cells, and “lead to a better understanding of how these processes occur,” Baer says.
The MIT researchers say that their device can take as many as 10 frames per second, making it possible to film such processes as they unfold—although real-time imaging is not yet possible, since computers take up to 30 minutes to process each frame. “One obvious thing” would be to make a movie of a cell as it divides, says MIT team leader Michael Feld.