Scientists have stolen a trick from fireflies and jellyfish to make animals with cells that glow so brightly they can be seen from outside the body.

The Japanese team created mice and marmosets whose brains contain nerve cells that produce light which can be picked up by a camera to study the tissues inside the living animals.

The breakthrough means that researchers can now track individual cells in animals with unprecedented accuracy to reveal, for instance, how brains respond to changes in the world, and how tumour tissues, immune cells, and stem cells move around the body.

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Scientists have long used natural bioluminescence molecules to “tag” cells in living animals, but the light they produce is not bright enough to pass through thick tissues to cameras outside. But writing in the journal Science, Atsushi Miyawaki at the Riken Brain Science Institute describes how he created synthetic molecules that shine 1,000 times more brightly than natural ones.

In tests, the scientists showed that they could make brain cells glow by injecting the synthetic bioluminescent molecules into the animals or simply adding them to their drinking water.

In one demonstration, the researchers gave mice glowing brain cells to show how neurons in part of the organ called the hippocampus responded to the animals being moved to new cages. In another experiment, the same process allowed the team to track neurons deep inside the brains of marmosets for more than a year. Miyawaki said it was the first time that a small group of neurons involved in learning deep inside the brain had been visualised from outside an animal.

In an accompanying article, Robert Campbell at the University of Alberta in Canada called the work a “substantial leap forward” that could shed light on the spread of cancer around the body, and, beyond tracking other cells, reveal how well gene therapies have reached their targets.

Bioluminescent insects, fungi and sea creatures have been known since antiquity, but it was not until the late 19th century that scientists began to understand that the phenomena in insects such as the firefly was created by a chemical reaction.