Scientists create mice with half-human brains to study mental illness

What happened

Scientists have grown human brain tissue inside the skulls of living mice to create animals with part-human brains, in a major step forward for studying complex neurological and psychiatric disorders.

The researchers, led by Sergiu Pașca at Stanford University, genetically engineered mice to be born without a cerebral cortex or hippocampus — the areas responsible for memory, thought, and language. This left a physical void. Soon after birth, the team injected these mice with human brain organoids, which are tiny, three-dimensional structures grown in a laboratory from reprogrammed human skin cells.

Key numbers behind the research

Each newborn mouse received several injections containing roughly 100,000 human brain cells. In total, the rodents lacked about 14 million of their own brain cells and gained around 4 million human ones.

Within three months, the human tissue had hooked up to the mouse's blood supply and expanded to fill the empty space, making up about half of the animal's total brain volume.

The resulting mice look normal, though they are physically cautious and forgetful. The transplant did not give them human-like cognitive abilities, but it did slightly improve their movement and memory compared to how they behaved with empty skulls. Tests showed that some of the human neurons successfully wired themselves into the mouse's nervous system and spinal cord.

Why it matters

"We’ve been trying really hard as a community to find therapeutic solutions for these conditions, but the reality is that in psychiatry and neurology we’ve been left behind by every single branch of medicine," Pașca said. He explained that studying the human brain has always been held back because the living organ is largely inaccessible. Growing human tissue inside a living animal offers a way around this barrier.

The Stanford team has already used the mice to study how human cells react to physical trauma. By exposing the rodents to five hours of low oxygen, they observed the precise way human nerve cells degrade during oxygen deprivation — a process that can cause cerebral palsy during pregnancy and birth.

The researchers also found that the transplanted tissue developed rare "von Economo neurons." These cells, which have previously only been seen in postmortem human brains, are the first to die off in frontotemporal dementia.

What happens next

While the breakthrough could transform how we study brain diseases, it has reignited ethical debates over animal welfare and the boundaries of bioengineering. Emily Jackson, a law professor at the London School of Economics who recently chaired a report on neural organoids for the Nuffield Council on Bioethics, emphasised that the welfare of these engineered animals must be closely monitored to evaluate how the procedure affects them.

Other scientists question whether the technique is the best way forward. Madeline Lancaster, a group leader at the MRC Laboratory of Molecular Biology in Cambridge, pointed out that the setup is highly artificial and may not accurately represent how human brains develop naturally. She noted that because animal experimentation is ethically sensitive, the scientific field is still aiming for fully lab-grown, "in vitro" solutions that do not require living hosts.