For an organ that has been scanned millions of times by experts using high-end imaging technology, the brain remains in large part a shrouded landscape, as lost in darkness as the ocean floor.
One reason has less to with the brain's complexity than its uniformity: it contains billions of identical-looking cells, most sprouting multiple identical-looking branches to other cells, near and far. A needle in a haystack at least looks different from the strands around it; finding and mapping large numbers of neurons is more like working out the root system beneath a tropical rain forest.
But last week, researchers at Harvard published pictures in which all those anonymous gray cells glowed in distinctive colors, like a bougainvillea bush gone haywire.
The scientists bred mice so their brain cells had genetic inserts containing genes for three colors of fluorescent protein, red, green and blue. They prompted each insert to randomly express one color, using a genetic trigger. Because there were multiple copies of the three-gene insert in each cell, the cell itself expressed a random mixture of the three colors, some 90 shades in all. What emerged was a kind of beaded rainbow belt of neurons, with the fluorescent glow radiating out through each cell's neural branches. The researchers called the technique "Brainbow."
Scientists can use this technique in animals, whose brain systems work in ways similar to those of humans, to see exactly where each cell begins and ends, both within the brain and out through the spine and the limbs - and what happens in between.
"I take a view that this is like the Hubble telescope," said Dr. Jeff Lichtman, a professor of molecular and cellular biology at Harvard who is the paper's senior author. "We've never been able to look at the brain this way before. Why not just start looking and see what we observe?"