With the same type of laser used in eye correction surgeries in humans, scientists can cut precise holes in the outer cuticle of fruit flies. The technique, described for the first time in a new PNAS Early Edition article, allows researchers to see into the brains of living fruit flies—using microscopes—in a faster and more repeatable way than has been possible in the past.
“Traditionally, a fly experimentalist would anaesthetize a fly, pick it up with tweezers, attach it to a fixture, then use a scalpel to cut through muscles and the cuticle. It was quite tedious,” explains Supriyo Sinha, first author of the new paper.
A well-trained scientist, he says, could surgically cut a window into the fly brain in about 15 minutes. Then, the fly would be examined under a microscope—through the hole, researchers can study the placement and arrangement of brain cells and their patterns of activity.
But Sinha, who works with biologist Mark Schnitzer at Stanford University, wants to be able to look at fly brains in a more high-throughput way. Rather than look at one fly at a time and try to draw conclusions on how the brain works from a sample size of one, the scientists want to compare the same spots in the brains of up to a hundred living flies at once. “We think that imaging dozens of flies simultaneously could lead to a paradigm shift in this field,” Sinha says. “We could gather statistics much more quickly.”
Manually cutting into the brains of a hundred flies, though, would be a long process. So Sinha, Schnitzer, and their colleagues turned to lasers. They found that a laser set to the right wavelength—193 nanometers, the same setting used in eye surgeries—could rapidly cut through fly cuticles. Then, they designed the laser system so that the location on the fly’s head that it cut into could be programmed and automated. Using the fly’s eyes as markers, the laser can open a window into the identical spot in the brain of dozens of flies. The error, Sinha says, is only about 10 microns.
“It’s obviously much faster; it takes about 5 seconds to do the laser surgery on one fly,” he says. “We also can open up much smaller holes than is possible with forceps.”
And smaller holes means less impact on the flies. For one thing, they live longer than they would if larger sections of their cuticle were peeled off with a scalpel and forceps. In the new paper, the researchers showed that the laser microsurgery had no impact on fly health for at least 18 hours. Hand-dissected flies survived for as short a time as an hour or two after surgery. “This improved viability allows longer experiments,” Sinha points out. “We can study a fly multiple times, as it learns something, for instance.”
Now, Schnitzer and Sinha are building an array of 100 microscopes, so that once they have windows into the brains of a hundred living flies, they can simultaneously image their brain cells. And they’re still fine-tuning the laser microsurgery—they don’t know, for example, whether it works as precisely in all areas of the brain.