What would Stuart Little make of it? Mice have been created whose brains are half-human. As a result, the animals are smarter than their siblings.

The idea is not to mimic fiction but to advance understanding of human brain diseases by studying them in whole mouse brains rather than in laboratory dishes.

The altered mice still have mouse neurons — the “thinking” cells that make up around half of all their brain cells. But practically all their glial cells, the ones that support the neurons, are human.

“It’s still a mouse brain, not a human brain,” says Steve Goldman of the University of Rochester Medical Center in New York. “But all the non-neuronal cells are human.”

Goldman’s team extracted immature glial cells from donated human fetuses. The researchers injected them into mouse pups, where they developed into astrocytes, a star-shaped type of glial cell. Within a year, the mouse glial cells had been completely usurped by the human interlopers. The 300,000 human cells each mouse received multiplied until they numbered 12 million, displacing the native cells.

(Joel Page/AP)

Astrocytes are vital for conscious thought, because they help to strengthen the synapses, which are connections between neurons. Their tendrils are involved in coordinating the transmission of electrical signals across synapses.

Human astrocytes are 10 to 20 times the size of mouse astrocytes and carry 100 times as many tendrils. This means they can coordinate all the neural signals in an area far more adeptly than mouse astrocytes can. “It’s like ramping up the power of your computer,” Goldman says.

A battery of standard tests for mouse memory and cognition showed that the mice with human astrocytes are much smarter than their mousy peers.

In one test that measures ability to remember a sound associated with a mild electric shock, for example, the humanized mice froze for four times as long as other mice when they heard the sound, suggesting their memory was about four times better. “These were whopping effects,” Goldman says. “We can say they were statistically and significantly smarter than control mice.”

Goldman first reported last year that mice with human glial cells are smarter. But the human cells his team injected then were mature, so they simply integrated into the mouse brain tissue and stayed put.

This time, he injected the precursors of these cells, glial progenitor cells, which were able to divide and multiply. That, he says, explains how they were able to take over the mouse brains so completely, stopping only when they reached the physical limits of the space.

This story is an excerpt from New Scientist magazine.