Scientists Grew Human Brain Tissue Inside Mice and It Actually Worked

The transplanted cells formed circuits, reached the spinal cord and responded to injury.

A mouse brain is not much larger than a thumbnail. In a new experiment, scientists genetically prevented much of its cerebral cortex and hippocampus from developing, then gave millions of human brain cells room to grow in the vacant space.

The crazy thing is that the human cells actually survived. What’s more, they expanded until human-derived tissue made up most of the animals’ cortical tissue, connected with the mouse brain and spinal cord, and fired in coordinated waves.

The point wasn’t to make some weird human-rodent hybrid (which isn’t even possible anyway). Instead, the human brain cells trapped inside the mouse are meant to act as a test bed for studying human neurons inside a functioning nervous system. Mice do not naturally develop many disorders that afflict the human brain, and drugs that look promising in rodents often fail in people.

“Even drugs that actually make it to clinical trial — that seem to be working really well in animal models — fail dramatically in clinic,” Sergiu Pașca, a Stanford University neuroscientist who led the research, said at a press conference.

Making Room for Human Cells

Using genetic engineering, the researchers caused most of the developing rodent neocortex and hippocampus — regions involved in sensation, memory and higher-order processing — to disappear. The manipulation was quite extreme, cutting total brain tissue volume by roughly half.

The team then implanted four small human cortical organoids (clumps of neurons) into newborn mice. The organoids had been grown from human induced pluripotent stem cells, which can be made by reprogramming adult cells such as skin cells.

All of this solved a timing problem. Human neurons mature much more slowly than mouse neurons. In previous transplant experiments, the mouse brain developed its own circuitry so quickly that the human cells were left with little room or opportunity to grow and make long-distance connections.

So, the team created room in advance by removing most of the cerebral cortex and hippocampus, leaving a large empty space for the transplanted human organoids. The strategy worked: between two and three months after transplantation, the human tissue grew about 4.7-fold. By three months, it made up about 92 percent of the cortical tissue present in the transplanted mice.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

This is not the first time human neurons were grown in mouse brains. In 2018, researchers showed that human brain organoids could become vascularized inside mouse brains. In 2022, Pașca’s group transplanted cortical organoids into newborn rats. The tissue matured, received sensory input and influenced behavior, but had to grow alongside an intact host cortex.

A Working Circuit, but Not a Human Brain Inside a Mouse

By about five to six months, the human neurons resembled the human cortex in mid-gestation, around the late second trimester. Although they did not form the orderly layers of a normal human cortex, related cell types locally clustered together, and the graft generated cell classes difficult to produce in a dish.

One of the more striking findings was that the human tissue began producing cell types that mice do not normally have. Researchers spotted a small number of cells resembling von Economo neurons, large nerve cells found in humans and some other big-brained mammals. Some of the human neurons also sent long extensions all the way into the mouse spinal cord.

And yes, the human neurons worked inside the mouse brain. The transplanted tissue became part of the animals’ nervous system, with the human neurons firing together in coordinated bursts. Some of that activity rose and fell alongside the mice’s facial movements.

But the human cells didn’t seem to affect the mouse’s behavior too much. Sadly, perhaps, there was no Pinky and the Brain moment.

The rodents’ movement was broadly normal. The genetically modified mice born without most of their cortex struggled with a simple working-memory maze. Those given human brain tissue did better on that test, suggesting the graft restored some function, but they still performed poorly on a more demanding test of longer-term learning.

RelatedPosts

Our brains don’t like having too many options to pick from because they’re lazy
Pill for your thoughts: what are nootropics?
Memories are stored in specific brain cells, MIT Inception-like research finds
Activating one gene in the brains of male mice helps reverse depression

“There is no indication that what’s being created here are mice that can think like humans, or a human brain in a mouse body,” Sarah Chan, a bioethicist at the University of Edinburgh who was not involved in the study, told the BBC.

Disease Models Meet an Ethical Line

To see whether these mice could help scientists study brain injury, the researchers briefly deprived them of oxygen, mimicking one kind of stress that can damage the developing brain around birth. The human neurons reacted strongly, and two days later the mice showed subtle changes in the way they walked. That suggests the model could be useful for studying conditions linked to oxygen deprivation, including cerebral palsy.

There’s still a long way to go, though. The human tissue was still immature and did not organize itself like a normal human cortex, which forms in neat layers. It also lacked many of the cell types and connections found in a fully developed human brain.

At the same time, if the organoids resemble the developed brain, there may be some serious ethical problems at stake. A May 2026 report from the Nuffield Council on Bioethics called for stronger governance as neural organoids become more complex, including updated guidance for experiments that place human neural tissue into animals.

Nita Farahany, a Duke University bioethicist who served on an external ethics board for the Stanford work, told NPR that the team had been “incredibly thoughtful.” Still, she said, “this brings us into new gray areas for which there are not clear ethical guidelines or norms.”

For Pașca, the scientific promise is equally clear. “Here we have a new model that allows us to actually capture aspects of human brain function in a way that has not been possible before,” he told the BBC.

The findings were reported in the journal Nature.