Mice made transparent by a dye that allows their organs to be observed

Massage a common food coloring on the skin of live mice have made their tissues transparent, allowing us to see their blood vessels and organs at work. The technique could one day help doctors look deeper into our bodies to diagnose conditions.

Monitoring the internal environment of a live animal is not easy. When it is dead, we can take slices of its tissue or use chemicals to remove proteins and fats from it to get a better look. With live animals, some things can be seen through scans and endoscopy, but to monitor living tissue, it often has to be cut open.

Now, Zihao Ou at the University of Texas at Dallas and his colleagues made the tissue of living mice transparent by rubbing the food coloring tartrazine, also known as E102 or Yellow 5, on their skin. When the skin absorbs the dye molecules, it changes the tissue’s refractive index, or the speed at which light passes through it.

The mice then became transparent, which allowed the researchers to see peristalsis, the muscle contractions that move food through the digestive tract and observe the blood vessels on the surface of the brain.

To understand how this technique works, think of sparkling water, says Ou. Light Light passing through the fluid changes direction every time it goes from water to a gas bubble or vice versa, he says. This means that the light scatters in all directions and cannot penetrate the fluid as easily as it can through air or still water alone. Biological tissue behaves similarly because it contains a lot of water, but also other molecules like lipids and proteins, which usually have a higher refractive index than water.

Adding the dye changes the refractive index of water in tissue to more closely match that of lipids and other molecules, reducing the amount of light that scatters. “That means you can see deeper, you can probe deeper,” Ou says.

The dye can be washed off and does not appear to have harmed the mice.

The work strikes at the heart of one of the biggest problems in microscopy, he says. by Christopher Rowlands at Imperial College London. “If you wanted to see more than a millimeter or so from the surface of the tissue, forget it, it didn’t happen, and now suddenly it’s a possibility,” he says. “You’re seeing through a centimeter, whereas before you could see a millimeter. That centimeter makes a difference for many applications.”

Tartrazine could be potentially toxic if applied in large amounts to the skin, Rowlands says, but neurobiologists routinely insert probes and lenses into the skin. brain or remove pieces of bark. Using a dye on the skin that is widely accepted as safe for consumption would probably be even less harmful, he says.

But while the technique makes skin more transparent, it won’t give doctors a completely clear view of a person’s insides. “It’s not going to be a Harry Potter invisibility cloak,” Rowlands says. “It’s going to be something where the skin looks more glassy than it should be.” Even if the effect were to occur throughout the body, bones and specialized structures called organelles inside cells would still be visible, he says.