Five years ago Viviana Gradinaru was slicing thin pieces of mouse brain in a neurobiology lab, slowly compiling images of the two-dimensional slivers for a three-dimensional computer rendering. In her spare time, she would go to see the Body Worlds exhibit. She was especially fascinated by the “plasticized” remains of the human circulatory system on display. It struck her that much of what she was doing in the lab could be done more efficiently with a similar process.
“Tissue clearing” has been around for more than a century, but existing methods involve soaking tissue samples in solvents, which is slow and usually destroys the fluorescent proteins necessary for marking certain cells of interest. To create a better approach, Gradinaru, at the time a graduate student, and her colleagues in neuroscientist Karl Deisseroth's lab focused on replacing the tissue's lipid molecules, which make it opaque.* To keep the tissue from collapsing, however, the replacement would need to give it structure, as lipids do.
The first step was to euthanize a rodent and pump formaldehyde into its body, through its heart. Next they removed the skin and filled its blood vessels with acrylamide monomers, white, odorless, crystalline compounds. The monomers created a supportive hydrogel mesh, replacing the lipids and clearing the tissue. Before long, they could render an entire mouse body transparent in two weeks.
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Soon they were using transparent mice to map complete mouse nervous systems. The transparency made it possible for them to identify peripheral nerves—tiny bundles of nerves that are poorly understood—and to map the spread of viruses across the mouse's blood-brain barrier, which they did by marking the virus with a fluorescent agent, injecting it into the mouse's tail and watching it spread into the brain. “It's like seeing the whole world versus individual slices of it,” Gradinaru says. The process reduces opportunities for human error, makes lab work move faster, produces richer data and requires fewer lab animals. Gradinaru offers the recipe for her hydrogel solution to any lab that requests it. Her next step is to use the technique to find, map and learn more about cancers and stem cells.
*Erratum (11/19/14): This sentence from the print edition version was edited after posting. The original reported the work was done in the late immunologist Paul Patterson's lab.