Blowing up the brain

Baby-diaper chemistry offers scientists a better view of our brains’ wiring

Blowing up a photo can show its details better. In the same way, enlarging a sample of brain tissue can help reveal the bigger picture of how cells in our brains are wired. A chemical similar to one found in baby diapers now gives scientists a new way to do just that.

 

For many years, disposable baby diapers have contained crystals nestled in their soft lining. Those crystals are a type of “super slurper” chemical known as sodium polyacrylate. It’s a polymer, or molecule made from long chains (hence the poly in its name). Decades ago, chemists learned that when these super-absorbers make contact with water, they suck up the liquid. Suddenly, what had been a powder made from crystals becomes a big gob of moist gel.

 

Over the years, chemists have turned super slurpers loose to tackle a host of problems beyond leaky diapers. For example, they can help pick up hazardous chemicals after a terrorist attack. But until now, nobody imagined their use in viewing the brain.

 

Credit that to a team of scientists at the Massachusetts Institute of Technology in Cambridge. They harnessed super slurpers for what they’re calling “expansion microscopy.”

 

It’s a bit like drawing a picture on a balloon’s surface, explains Edward Boyden, a neuroscientist at MIT. Blowing up the balloon makes the drawing bigger. “That’s what we were trying to do, but with a three-dimensional object like the brain.” By attaching the super slurper to molecules throughout brain cells and then adding water, the Boyden’s team can enlarge a sample of tissue to about 100 times its original volume.Science published details of the team’s technique online on January 15.

How it works

Typically, microscopy uses light and lenses to make tiny features appear bigger. Here, the team took something tiny and looked for a way to make it swell up so that it would become physically bigger.

 

The team starts by soaking a piece of preserved brain tissue in a solution with lots of the super slurper’s building blocks. “Then we add a second chemical and these building blocks start to form into long chains called polymers.”

 

This polymer attaches itself to molecules throughout neurons — nerve cells — in the tissue sample. Before long, the tissue has “all these little thin wires of polymer that are winding their way through it,” Boyden explains. It’s like the tissue has become “embedded in a sponge almost.”

 

That polymer is similar to the super slurper in diapers. “When you add water, it swells. And the brain gets bigger,” Boyden says.

 

Think about the spots on the surface of a polka-dotted balloon. When the balloon is deflated, the dots appear close together. But as it inflates, those dots begin to move farther apart. So it is with brain cells. “All the molecules in the brain get pushed away from each other,” Boyden says. Although the cells break apart, the expanded brain still has all of the same proportions as the original tissue.

 

The process also expands the spaces between neurons, called synapses. Chemical messages shoot across those synapses.

 

While some neurons can be centimeters long, synapses are usually “nanoscale,” Boyden says. They’re 100 billionths of a meter long or less.

 

With the new method, though, the synapses expand. As a result, researchers can see at the same time both big features, such as neurons, and previously nanoscale things like synapses.

 

Many scientists want to see parts of the brain more clearly, says Zayra Millan. A neuroscientist at Johns Hopkins University in Baltimore, Md., she did not work on the new project. “Expansion microscopy, which enlarges and fixes the specimens, is a novel way to do this,” she says.

 

“In an enlarged specimen, where local and long-range circuits are labeled, we have better visual access to the details,” she told Science News for Students. The ability to trace nerve fibers along their routes could help scientists trying to understand the brain and how different traumas alter it, she notes.

 

The new technique also could be a big step forward in helping scientists map the brain, Boyden says. “We can actually follow these neurons and see how they wire up.”

 

Fonte: https://student.societyforscience.org/article/blowing-brain