How did they induce individual neurons to “light up” in different colors so we can now trace their paths? Through much trial and error, they borrowed bioluminescent proteins from certain jellyfish and introduced them into the genome of a mouse. Eventually, three different-colored fluorescent molecules were encoded into the nervous system of a transgenic mouse. Like in a television screen, green, red, and cyan (blue) elements mix to create a rainbow of distinct hues. When all of the neurons are genetically modified in this way, adjacent neurons express different colors, allowing us to distinguish one from the next.
This technology has been applied in fish, fruit flies, and even worms. This Brainbow zoo provides essential insights into the underlying function and connectivity of the brain.
Featured Images. Above: Dentate gyrus. Below left: dentate gyrus. Below right: ocular motor neurons.
Livet et al., Nature 2007
Lichtman et al., Nature Reviews Neuroscience 2008