This tangled rainbow is a big accomplishment for scientists who study something mind-bogglingly small: fruit fly brains. Barely the size of a poppy seed, the adult fruit fly brain is surprisingly complex.
These little bugs have grooming rituals and complicated courtship dances. They can learn and recall their environment. And amazingly, some of the brain structures behind these behaviors are similar to those in other animals—including humans.
The new visualization, presented this week in the journal Cell, is now the largest animal brain imaged in such high resolution, allowing researchers to trace the path of a single neuron through the thicket of connections.
“I think this is where biology needs to go,” says Kate O'Neill, a postdoctoral fellow at the National Institutes of Health who was not involved in the study. “There's so much information we can get, and we're limited by technology.”
To create the new images, the scientists had to first extract a fly's brain and then marinate it in a cocktail of heavy metals. These bind to the outside of the neurons and connections between them, marking them for later analysis. Researchers then plunked the tiny brain fleck in resin and let it harden so they could cut ultra-thin slices using a diamond knife.
Far thinner than the width of a human hair, each slice was then imaged using a beam of electrons in what's known as an electron microscope. In total, the researchers captured 21 million images of just over 7,000 brain slices to assemble the stunning three-dimensional map of the fly's brain.
The fly is not the first creature to get its noggin imaged; larvae of both zebrafish and flies have had this honor. And to be clear, the researchers imaged only the fly's neural connection points and have not yet mapped all of the roughly hundred thousand neurons in the adult fly brain. Currently, such neuron tracing must largely be done by hand, but researchers are working on automating this onerous task.
“A human has to look at 2,000 images to trace this thing a hundred microns,” says study coauthor Davi Bock, a neurobiologist at the Janelia research campus of the Howard Hughes Medical Institute. “It's ridiculous.”
Because of this challenge, the dazzling rainbow above shows just 5 percent of the total brain connections. Even so, the study presents a stunningly detailed map of neuron connection points, helping researchers better understand the labyrinthine structures.
Using the data, which is available online, any scientist can now select their neuron of choice and trace out the inputs and outputs, Bock adds.
“And some of those will be surprises.”