Seeing through to a mouse's nervous system

Neuroscientists have developed a way to turn an entire mouse, including its muscles and internal organs, transparent while illuminating the nerve paths that run throughout its body. The process, called uDisco, provides an alternate way for researchers to study an organism’s nervous system without having to slice into sections of its organs or tissues. It allows researchers to use a microscope to trace neurons from the rodent’s brain and spinal cord all the way to its fingers and toes.

“When I saw images on the microscope that my students were obtaining, I was like ‘Wow, this is mind blowing,’” said Ali Erturk, a neuroscientist from the Ludwig Maximilians University of Munich in Germany and an author of the paper. “We can map the neural connectivity in the whole mouse in 3D.” They published their technique in the journal Nature Methods.

The technique has been conducted only in mice and rats, but the scientists think it could one day be used to map human brain. They also said it could be particularly useful for studying the effects of mental disorders like Alzheimer’s disease or schizophrenia.

Of the neurons
Ali and his colleagues study neurodegenerative disorders, and are particularly interested in diseases that occur from traumatic brain injuries. Researchers often study these diseases by examining thin slices of brain tissue under a microscope. “That is not a good way to study neurons because if you slice the brain, you slice the network,” Ali said. “The best way to look at it is to look at the entire organism, not only the brain lesion but beyond that. We need to see the whole picture.”

To do this, Ali and his team developed a two-step process that renders a rodent transparent while keeping its internal organs structurally sound. The mice they used were dead and had been tagged with a special fluorescent protein to make specific parts of their anatomy glow.

First, they dumped the mouse in a glass of alcohol to dehydrate it. Water acts like a mirror and reflects light, so they needed to rid the mouse’s muscles and tissues of it. Then they soaked the mouse in an organic solvent that dissolves its fats like a dishwashing detergent.

While the researchers were soaking the outsides of the rodent in alcohol and the organic solvent, they were simultaneously pumping the liquids through its blood vessels to douse its insides as well. It takes about four days for the mouse to become transparent.
Another effect of the uDisco formula is that it also shrinks the mouse to about half or a third of its size. That makes it small and flexible enough to fit under a microscope.

Ali admits that the process is simple enough that any scientist can perform it. But the challenge, he said, was in finding the right combination of chemicals — among hundreds of thousands of possibilities — that would make the mouse transparent while retaining the fluorescent protein and keeping its internal structure normal.

This is the first such technique to meet all of those requirements; other methods either made the organism larger or did not retain the fluorescence. “The applications of this method are countless,” Dr Ingo Bechmann, a professor of anatomy at Leipzig University in Germany, who was not involved in the study, said in an email. “While at present, we have to prepare individual organs for histopathological evaluation, the future will be in many cases to use uDisco.”

Matthias H Tschop, research director of the Diabetes Center at Helmholtz Zentrum, Munich, who researches how the nervous system interacts with organs to control metabolism, praised the technique in an email, but was sure to note that it would not be used on live humans in the future, though it could be applied to cadavers. He was not involved in the study.

“The fact that most biomedical scientists would have associated such technology with a science-fiction movie rather than daily lab work at the bench,” he said, “reflects the transformative quality of this advancement.”