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And I'm Melissa Block.

Scientists announced today that they have made mice faster learners. They did it by adding human cells to their brains, not neurons, but another type of brain cell that has long been overlooked called glia cells.

NPR's Adam Cole explains.

ADAM COLE, BYLINE: Before anyone cared about glia, they went crazy for neurons. Back in 1887, anatomy professor Santiago Ramon y Cajal looked at brain tissue under a microscope and saw what he called a dense forest, hundreds and hundreds of little branching cells. These cells soon to be named neurons were still mysterious. With beautiful ink drawings, Ramon y Cajal painstakingly maps neural networks. He developed a theory that neurons are the telegraph lines of thought that every idea and memory, every aspect of learning could be traced back to the electric signals sent between neurons.

Ramon y Cajal won the Nobel Prize for his work, and neurons were the stars of the brain show for a century. But here's the thing, only a fraction of the cells in the brain are neurons.

DOUGLAS FIELDS: Most of the cells are glia.

COLE: Doug Fields is a neuroscientist at the National Institutes of Health. He says early theories about glia weren't too sophisticated.

FIELDS: Glia was stuff, just stuff between the neurons.

COLE: Compared to flashy neurons, this stuff was so boring. I mean, glia means glue. It didn't even merit a singular noun.

FIELDS: Glia is plural. There is no singular. You know, we have a neuron, but we don't have glion.

COLE: It was only in the last decade or so that scientists realized glia were more than just support cells. Special types of glia called astrocytes have their own form of chemical signaling, and they can potentially coordinate whole groups of neurons.

FIELDS: Glia are in a position to regulate the flow of information through the brain, and this is all missing from our models.

COLE: And there's something else. This type of glia, these astrocytes have changed a lot since humans evolved from the shrew-like ancestor we share with mice, while neurons have pretty much stayed the same. A mouse neuron and a human neuron look so much alike, even experienced neuroscientists can't tell them apart.


COLE: Steve Goldman of the University of Rochester has studied brain cells for decades.

GOLDMAN: I can't tell the differences between a neuron from a bird or a mouse or a primate or a human.

COLE: But Goldman says glia are easy to tell apart.

GOLDMAN: Human glial cells, human astrocytes are much larger than those of lower species. They have more fibers, and they send those fibers out over greater distances.

COLE: So the thought is maybe these glia have played a role in making humans smarter. Goldman teamed up with his wife, Maiken Nedergaard, to test this idea. They injected some human glia cells into the brains of newborn mice. The mice grew up and so did the human glia. The cells spread through the mouse brain, integrating perfectly with mouse neurons and, in some areas, outnumbering their mouse counterparts. All the while, Goldman says the glia maintained their human characteristics.

GOLDMAN: They very much thought that they were in the human brain, in terms of how they developed and integrated.

COLE: So what are these mice like, the ones with brains full of functioning human cells? Well, their neural circuitry is just the same, so they act completely normal. They still socialize with other mice and still seem interested in mousey things. But as Goldman's team reported today in the journal Cell Stem Cell, these mice are measurably smarter. In classic maze tests, they learn faster.

GOLDMAN: They make many fewer errors, and it takes them less time to come to the appropriate answer.

COLE: It might take a normal mouse four or five attempts to learn the correct route, but a mouse with human brain cells could get it on the second try. Glia cells - those boring glia cells - somehow boost learning. In fact, they could be changing what it means to be a mouse, and that raises ethical questions for this kind of research.

ROBERT STREIFFER: Maybe bioethicists have been a little bit too cavalier assuming that, you know, a mouse with some human brain cells in it is just your normal old mouse. Well, it's not going to be human, but that doesn't mean it's a normal old mouse either.

COLE: Robert Streiffer is a bioethicist from the University of Wisconsin. He says it's not just that these mice can get through a maze more quickly, they're better at recognizing things that scare them. And perception of fear is one of the things bioethicists must weigh when they decide what types of experiments you can do on an animal.

STREIFFER: So you have to sort of step back and do some hardcore philosophy.

COLE: Like, will these types of human-animal hybrids eventually get close enough to humanity that we would feel uncomfortable performing experiments on them? The researchers in this study say we're really, really far from that point. And if you want to investigate the role of glia cells, these hybrid mice are the best tools available.

Adam Cole, NPR News.

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