Mice become transparent thanks to dye that allows their organs to be seen

Rub regular food coloring into your skin leather living mice have made their tissue transparent, allowing us to see their blood vessels and organs at work. The technique could one day help doctors peer deeper into our bodies to diagnose conditions.

Monitoring the internal environment of a living animal is not an easy task. When it is dead, we can take sections of its tissue or use chemicals to remove proteins and fats from it to get a better picture. In living animals, some things can be seen with scans and endoscopy, but to monitor living tissue, it often has to be cut out.

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

The mice were then made transparent, allowing the researchers to see peristalsis – the contractions of the muscles that move food through the digestive tract and look into the blood vessels on the surface of their brain.

To understand how this method works, imagine carbonated water, Ou says. Light He says that light passing through a liquid changes direction every time it passes from water to a gas bubble or vice versa. This means that the light is scattered in all directions and cannot penetrate the liquid as easily as it can air or water, which do not fizz. Biological tissue behaves similarly because it contains a lot of water, but also other molecules such as lipids and proteins, which typically have a higher refractive index than water.

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

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

The work gets to the heart of one of the biggest problems in microscopy, he says. Christopher Rowlands at Imperial College London. “If you wanted to see anything more than a millimetre or so from the surface of the tissue, forget it, it wasn’t happening – and now suddenly it’s possible,” he says. “You can see a centimetre, whereas before you could see a millimetre. That centimetre matters for a lot of applications.”

Tartrazine can be potentially toxic if applied to the skin in large quantities, Rowlands says, but neuroscientists regularly insert probes and lenses into it. brain or remove chunks of bark. Using a dye on the skin that is widely recognized as safe to consume would probably still be less harmful, he says.

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