The human brain is often described as a meager mass of tissue that is colossal in its complexity. Seeing as neuroscientists still struggle to fathom the mind of a fruit fly, whose brain is the size of a poppy seed and has only 100,000 neurons, a human’s noggin at 1.4 kilograms (3 pounds) and over 80 billion neurons is a feat on another level.
Still, progress is progress and now scientists have mapped in 3D a fruit fly’s brain in nanoscale detail in less than three days. This is compared to the years it previously took.
“You can spend years and years getting an EM image of one fly brain,” said co-author Eric Betzig, a Nobel laureate and professor at UC Berkeley, in a statement. “I can see us getting to the point of imaging at least 10 fly brains per day.”
Their new technique is called ExLLSM, which combines two state-of-the-art imaging technologies to make the brain swell like a balloon and yet keep the delicate internal biology intact. The first, called expansion microscopy (EM), expands the tissue, while lattice light-sheet microscopy (LLSM) uses focused beams of light to create a 3D image of the brain one slice at a time.
"The idea does sound a bit crude," admitted Ruixuan Gao, one of the lead authors from MIT. "We're stretching tissues apart."
In fact, when Gao and fellow team lead Shoh Asano asked to use Betzig’s microscope, he said yes but didn’t hold much hope of success.
"I was going to show them [it wouldn’t work],” said Betzig. Instead, “I couldn't believe the quality of the data I was seeing. You could have knocked me over with a feather.”
With the two techniques combined, they produced something beautiful in its intricacy and simplicity of use. The new method is high speed, high resolution, and yet still gentle. The EM makes the brain bigger by infusing the tissue with swellable gels, delicately separating molecules from each other and making the structures easier to see under a microscope.
However, they were soon confronted with another hurdle: the thicker a sample is, the harder it is to image. This is due to the need to shine light on them, but too much and you can photobleach portions of it. This is a predicament, especially considering they expanded the brain tissue by a factor of four, which pumps up the volume 64-fold.
Despite all odds, the lattice light-sheet microscope was up to the task. This is because rather than imaging the brain all at once, it “sweeps an ultrathin sheet of laser light through a specimen,” wrote the authors in Science. They were able to capture highly detailed images at a resolution of about 60 nanometers.