Stain And Label-Free Microscopy Enables Imaging Of Extracellular Vesicles

Researchers at the Beckman Institute of the University of Illinois at Urbana–Champaign have developed a method that employs ultrashort laser pulses to visualize extracellular vesicles in tissue samples without using stains or labeling compounds. An increase in the number of these small vesicles is known to be associated with cancer, whereby cancer cells use them to communicate with each other. Therefore, visualizing them microscopically could be useful in detecting cancer progression.

At present, researchers use labels and stains so that they can see specific structures in tissue samples, such as extracellular vesicles. This creates extra work and expense, and frequently means that the tissue sample is not usable for other types of analysis afterwards. These researchers have developed a new imaging technique, described in Proceedings of the National Academy of Sciences, that does not require staining or labels.

Their imaging method involves directing ultrashort laser pulses at the imaged tissue sample, which allows the researchers to detect the “optical signature” of structures. “We can acquire information about the structure and metabolism of the living tissue,” said Stephen Boppart, a researcher involved in the study. “The imaging system allows us to capture all this information simultaneously, allowing us to see many more details about the tissue, cells, and their functions than the current ways of imaging.”

Different structures within the tissue provide different signals, helping the researchers to distinguish between them. “There are two mechanisms involved in the imaging,” explained Sixian You, another researcher involved in the study. “Some of the tissue components emit different kinds of fluorescence which comes in different colors. The other mechanism involves molecular structures that when aligned in a certain way, will give you an entirely different set of colors.”

The technique results in some striking images, but it also has the potential to help with diagnostics. So far, the researchers have used it to analyze extracellular vesicles, a marker of tumor progression. “Cells use these extracellular vesicles to communicate with each other, even under normal conditions,” said Boppart. “Tumor cells will alter these extracellular vesicles, and release more throughout the body. For this reason, they have the potential to be used as markers for cancer progression.”

Another application involves using the technique to discern between cancerous and healthy tissue in tissue samples from the tumor border, obtained during surgery, to help surgeons determine if they have removed all of a tumor. “We are trying to use the label-free technique to look at the tissue right away in the surgical room,” said You. “After we get the images, we use deep learning, which can be used to differentiate between cancerous and normal breast tissue.”

Here’s a 3D visualization of extracellular vesicles in a tissue sample from 40 μm to 120 μm below the surface:

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