Those small threads you can see in the video above are individual nerve cells. Until now, scientists couldn’t see them in place in the whole mouse brain.
But a new technique can now turn the entire bodies of small animals transparent, allowing scientists to trace the paths of nerve cells and blood vessels from nose to tail. The new tool, published today in Nature Methods, is the first to offer such a detailed view of individual cells in an intact body.
“Now…we can look into the wiring of the whole mouse in high resolution,” says coauthor Ali Ertürk, a neuroscientist at Ludwig Maximilian University of Munich. This will allow researchers to better understand how the nervous system is assembled, and how injuries or illnesses can mess with this circuitry.
Generally, scientists can view entire organs in low-resolution with techniques like MRI, or cut tiny pieces of tissue into fine slices and pop it under a microscope for a detailed look at a very small area. But cutting tissue up into thin wafers is time-consuming and doesn’t capture the full complexity of the nervous system, whose cells are often too long to fit onto a tissue section.
“This is usually sufficient to study tumor cells or inflammatory cells because they are small circles…but neurons are not like this,” Ertürk says. “You don’t see the entire picture, you are cutting the wires.”
With the new method, which he and his colleagues have dubbed “ultimate DISCO” or uDISCO, those wires can be viewed whole and in place. Ertürk likens the process to learning how pipes in a wall are organized by turning everything to glass and filling the pipes with colored water.
To make a dead rat see-through with uDISCO, the animal is soaked in solutions that remove the water and lipids from its tissues, leaving a glassy scaffold behind. This process also shrinks the rodents’ bodies by up to 65 percent, making them easier to fit under a microscope, and renders tissue hard but flexible for easy positioning.
In rodents engineered to carry fluorescent proteins, a peek under the microscope can then offer a high-resolution glimpse at whichever cells or areas the researchers decided to illuminate.
Previously, Ertürk and his colleagues created a similar technique called 3DISCO (3D imaging of solvent-cleared organs). 3DISCO could clear a mouse brain or spinal cord within a few hours, but the solutions quickly damaged the glowing proteins. To turn a whole mouse transparent takes days, at which point there was no signal left to light up the cleared tissues.
“They would destroy this fluorescent color,” Ertürk says. “So basically we would make [the mice] transparent, but at the end we would lose what is painted inside.”
uDISCO preserves the fluorescent proteins in a body for months, making it possible to clear and view entire rodents. The whole process takes about eight days. By contrast, using an electron microscope to map a mouse brain from tissue slices would take about 50 years, and a human brain 1000 years, Ertürk says.
He hopes to eventually map an entire human brain with uDISCO. Ertürk and his colleagues also plan to apply uDISCO to investigate how brain injury or psychiatric conditions can affect the rest of the body. Another use would be to follow the path of metastasizing cells in tumors, or understand how transplanted stem cells migrate to unintended areas of the body. Using uDISCO to create atlases of whole rodents might even cut down on the number of lab animals used in future research.
With uDISCO, scientists can also compare how the nervous system looks in healthy mice compared to those with Alzheimer’s-like diseases. “This will give us hints how the miswiring is happening,” Ertürk says. “And how we can then tackle it to make it correct.”
