Part-human part-mouse brain developed in science breakthrough

Neuroscientists in the US have successfully adapted mice to have functioning human cells inside their brains. This breakthrough could help test treatments for psychiatric and neurodevelopmental diseases that only affect humans.

The scientists emphasize these are not "mice that think like humans." Instead, the animals are genetically engineered and surgically altered to have some human brain tissue.

The research, published in Nature, was done with independent ethical review. The goal is to better understand brain disorders that currently lack effective treatments. Some conditions cannot be studied in mice because rodents do not develop them.

Professor Sergiu Pașca from Stanford University, the lead researcher, noted that psychiatry has a low success rate for clinical trials. Drugs that seem to work well in animal models often fail in human trials. He believes this new model will provide crucial information about human biology.

The Stanford team believes this approach could be "transformative" for complex conditions like epilepsy, autism, and cerebral palsy. Pașca stated that this new model allows them to capture aspects of human brain function in a way never before possible.

Creating Human-Mouse Brains

The human brain has billions of cells and millions of circuits, making its development and disorders hard to understand at a cellular level. While human neurons have been implanted in rodents before, this research takes it further.

First, mice were genetically engineered to develop almost no cerebral cortex, the brain's outer layer responsible for higher-level thinking, memory, and senses.

Then, human skin cells were "reprogrammed" to grow into brain-like tissue called organoids. These are collections of connected, living cells, not whole brains.

When these organoids were implanted into the mouse brains, the human cells divided and integrated into the animal's existing brain circuitry. They connected with the rest of the mouse's brain and spinal cord.

Initially, the implanted cortex was "a bit messy," according to neuroscientist Dr. Ilary Allodi. However, after a few months, the human cells began to look and function like the outer layer of the mouse's brain.

About six months after surgery, the mice underwent basic behavioral tests. Pașca reported they performed "largely as normal mice did" and showed no "enhancement."

Dr. Sarah Chan, a bioethicist not involved in the research, said there's "no indication that what's being created here are mice that can think like humans." However, she noted the study raises questions about altering animal cognition and how to treat laboratory animals.

A New Avenue for Research

Dr. Ilary Allodi, a neuroscientist from St Andrews University, praised the work as "very impressive." She highlighted that cell types found only in human and primate brains spontaneously formed in the implanted mice.

Allodi described it as "keeping the human program inside the mouse environment – like the mouse is an incubator."

These engineered mice are raised under strict ethical and welfare guidelines. They will likely be used in a few labs to study specific brain disorders.

Professor James Ainge, also a neuroscientist at St Andrews University, found the development technically impressive but suggested these mice might be "of limited use." He cited the "ethical issues of raising living human brain tissue in a mouse."

Allodi noted that neuroscientists studying the human brain face similar limitations. She explained that researchers typically rely on mice, cells in dishes, or computer neural networks. This new approach offers a "new avenue" for understanding human disease.

Deep Dive & References

Human brain organoids implanted in mouse cortex respond to visual stimuli for first time - Nature, 2026