Fluorescently labelled brain cells, visualised using expansion microscopy Credit: Professor Ed Boyden
The basics behind capturing an image haven’t changed in a long time. Home cameras and telescopes still use the same type of lenses, and microscopes in schools have been pretty much the same for decades.
This visible light technology is relatively cheap, but it’s limited by the laws of physics as to how much detail we can see. Scientists have found new ways to bend these rules with advanced technologies that see objects without using visible light. These include gamma ray telescopes and electron microscopes, but this technology comes with a price. And its own disadvantages.
What if we could find new ways to view images more clearly with the same cheap and easy-to-use equipment scientists have had for years? This is exactly where Professor Ed Boyden comes in.
The new concept
Boyden is a Professor in Neurotechnology at the Massachusetts Institute of Technology (MIT) in the US. Having already pioneered the use of light to control cells in the brain (optogenetics), he and his team have gone on to make waves in microscopy too.
The team’s concept was a simple one. If we can’t improve microscopes to see smaller and smaller samples, why don’t we just make the samples bigger?
Theoretically, if scientists can make something larger, then the same laws of physics no longer apply. And this is exactly what he set out to do.
Boyden and his team were trying to separate the crowds of proteins in brain cells from one another, so they could be labelled better. They came up with the initial idea in 2007, but it wasn’t until 2012 when things started to click into place.
“Two great graduate students in my lab, Fei Chen and Paul Tillberg, were experimenting with nano-imaging,” he says. “But we realised it would be hard to apply these techniques to large, 3D specimens like those of the brain. So, we started thinking about how to do the opposite – to blow up the tissues!”
The challenge here was not only how to make the sample larger, but also to have it grow evenly in all three dimensions. It would be no use having the sample stretch in one direction like a rubber band, or non-uniformly like a popped corn kernel.
Boyden needed to expand his samples in a controlled way, so they looked the same, only bigger.
Expanding tissues using nappies
Surprisingly, the answer for this problem lay in the homes of parents around the world: babies’ nappies.
“We had been reading the papers of MIT physicist Toyoichi Tanaka,” Boyden says. “He had been working in the 1980s on the physics of swellable gels. And we thought maybe we could embed a piece of brain tissue into the same gels he’d been using.”
And that’s where they found their solution. It turned out that Tanaka’s gel was the perfect candidate for expanding microscopy samples. It’s also the chemical, or polymer, that’s responsible for absorbing babies’ pee in nappies. By just adding water, this chain-like polymer (called sodium polyacrylate) expands evenly in all directions and can swell to up to around a thousand times in volume.
All Boyden then had to do was find a way to embed this polymer within the tissue samples so that when it expands, the sample expands with it. And that meant fusing it to structures within the tissue. Boyden and his team found a way to “feed” cells the building blocks of the chains, which could then be joined together by triggering a chemical reaction.
After that, you simply add water.
The samples are then ready to be viewed using a standard light microscope, completing the process known as ‘expansion microscopy’.
The image above shows the ultrafine structures within kidney samples. The more detailed image, after expansion, allowed them to observe previously unseen processes.
The colours come from fluorescently labelled antibodies and small molecule DNA binders that glow under light or ultraviolet radiation.
Using expansion microscopy to tackle cancer
These images show Boyden’s first tests using expansion microscopy to look at cancers. The top image is a sample from a prostate cancer sample and the one below is from an ovarian cancer sample. The expanded images revealed features of filaments within cancer cells that are critical for their growth and spread, which couldn’t previously be seen with optical microscopes.
Boyden is now applying this technology to look at cancer in more detail as part one of our Grand Challenge teams. He’s leading on the expansion microscopy work linked to Professor Greg Hannon’s project to create virtual reality maps of tumours.
“Imagine trying to understand human life but all you can see are the cities glowing on the earth’s surface from outer space,” says Boyden. “You can’t see people, or cars, or buildings. Would we really understand societies, economies, factories, transportation?”
According to Boyden, the same logic applies to cancer.
“The building blocks of life, genes and proteins, are nanoscale in dimension,” he says. “But by expanding cancers, we hope to map the fundamental building blocks of life.”
“Without doing that, how can we really understand what a cancer is?”
Boyden hopes that by seeing cancers with a resolution equal to the smallest building blocks of life, we’ll be able understand them at the most fundamental level, improving our ability to diagnose, treat and manage different cancers.
Carl Alexander is senior science media officer at Cancer Research UK