Better 3D Imaging of Living Cells

The insides of living cells can be seen in their natural state in greater detail than ever before using a new technique developed by researchers in Japan. This advance should help reveal the complex and fragile bio­logical inter­actions of medical mysteries, like how stem cells develop or how to deliver drugs more effectively. “Our system is based on a simple concept, which is one of its advan­tages,” said Takuro Ideguchi from the University of Tokyo Research Institute for Photon Science and Tech­nology.

Illustration of the new imaging method – biochemical quantitative phase imaging – with midinfrared photothermal effect. (Source: s-graphics.co.jp / CC BY-NC-ND)

The new method also has the advan­tages of not needing to kill the cells, damage them with intense light, or arti­ficially attach fluorescent tags to specific molecules. The technique combines two pre-existing micro­scopy tools and uses them simul­taneously. The combi­nation of these tools can be thought of simply as like a coloring book. “We gather the black-and-white outline of the cell and we virtually color in the details about where different types of molecules are located,” said Ideguchi.

Quantitative phase micro­scopy gathers information about the black-and-white outline of the cell using pulses of light and measuring the shift in the light waves after they pass through a sample. This information is used to reconstruct a 3D image of the major structures inside the cell. Molecular vibra­tional imaging provides the virtual color using pulses of mid­infrared light to add energy to specific types of molecules. That extra energy causes the molecules to vibrate, which heats up their local sur­roundings. Researchers can choose to raise the tempera­ture of specific types of chemical bonds by using different wave­lengths of midinfrared light.

Researchers take a quanti­tative phase micro­scopy image of the cell with the mid­infrared light turned off and an image with it turned on. The difference between those two images then reveals both the outline of major structures inside the cell and the exact locations of the type of molecule that was targeted by the infrared light. Researchers refer to their new combined imaging method as bio­chemical quanti­tative phase imaging with midinfrared photo­thermal effect.

“We were impressed when we first observed the molecular vibrational signature charac­teristic of proteins, and we were further excited when this protein-specific signal appeared in the same location as the nucleolus, an intracellular structure where high amounts of proteins would be expected,” said Ideguchi. Ideguchi’s team hopes their technique might allow researchers to determine the distribution of funda­mental types of molecules inside single cells. The quanti­tative phase micro­scopy outline of major structures could be virtually colored in using different wave­lengths of light to speci­fically target proteins, lipids (fats) or nucleic acids (DNA, RNA).

Currently, capturing one complete image can take 50 seconds or longer. Researchers are confident that they can speed up the process with simple improve­ments to their tools, including a higher-powered light source and a more sensitive camera. Colla­borators at Osaka University, other depart­ments at the University of Tokyo and the Japan Science and Tech­nology Agency also contr­ibuted to this research. (Source: U. Tokyo)

Reference: M. Tamamitsu et al.: Label-free biochemical quantitative phase imaging with mid-infrared photothermal effect, Optica 7, 359 (2020); DOI: 10.1364/OPTICA.390186

Link: Institute for Photon Science and Technology, University of Tokyo, Tokyo, Japan

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