Scientists who inserted human brain cells into mice say they've created animals who have proved to be smarter than their "all-mouse" siblings.

The aim, they are quick to point out, isn't to create some kind of super talking science fiction mouse, but to help research into human diseases of the brain by studying those diseases inside a mouse's brain instead of in a dish in a laboratory.

The mice in the experiments have retained all of their mouse neurons -- those "thinking" cells that account for about half their brain mass -- but almost all the so-called glial cells, which support those neurons, have been replaced with human ones, researchers at the University of Rochester Medical Center in New York say.

"It's still a mouse brain, not a human brain," explains Steve Goldman. "But all the non-neuronal cells are human."

Immature human glial cells taken from donated fetuses were injected into new-born mice, where they grew into star-shaped kinds of glial cells known as astrocytes.

Astrocytes wrap tendrils around synapses that transmit signals between neurons, supporting neighboring neurons and strengthening the synaptic connections.

Because human astrocytes are significantly larger than mouse equivalents and possess 100 times as many tendrils, they can direct signaling in the neural networks much more efficiently than native cells, the researchers said.

The effect was a significant "upgrade" of the mice brains, "like ramping up the power of your computer," says Goldman.

When tested for cognition and memory, the mice with human astrocytes proved much smarter than their "pure" mouse peers, the researchers reported in The Journal of Neuroscience.

"These were whopping effects," says Goldman. "We can say they were statistically and significantly smarter than control mice."

However, Goldman is quick to play down any suggestion the added human cells in any way make the mice more human.

"This does not provide the animals with additional capabilities that could in any way be ascribed or perceived as specifically human," he says. "Rather, the human cells are simply improving the efficiency of the mouse's own neural networks. It's still a mouse."

In an effort to learn more about how the cells might affect learning and memory, the researchers have begun putting the human cells into the brains of rats, which are more intelligent than mice.

"We've done the first grafts, and are mapping distributions of the cells," Goldman says.

He says his research team briefly considered extending the experiments to primates, but abandoned the thought of putting human cells into the brains of monkeys "because of all the potential ethical issues."