Stanford scientists grow human brain tissue inside bioengineered mice
Researchers at Stanford University have transplanted four million lab-grown human brain cells into bioengineered mice, replacing over 90% of the animals' cerebral cortex to model neurological disorders.
Bioengineering the rodent cortex
Researchers at Stanford University have developed a technique to grow large volumes of human brain tissue inside live rodents. In a study published on 16 September 2026 in Nature, a team led by psychiatry professor Sergiu Pașca genetically altered mice so they developed almost none of their own cerebral cortex or hippocampus. The cerebral cortex forms the outer grey matter layer that governs cognition, language, attention, and decision-making, while the hippocampus manages memory formation. Removing this developmental tissue cleared physical space within the rodents' skulls and prevented native mouse neurons from outcompeting slower-developing human cells, which develop at least 20 times slower than mouse brain cells.
Cellular growth and functional integration
To generate the grafts, the team reprogrammed human skin and blood cells from healthy donors into stem cells, guiding them to form miniaturized three-dimensional structures called cortical organoids. Scientists transplanted these organoids shortly after the mice were born. Over several months, four million human neurons grew to occupy more than 90% of the animals' cerebral cortex. The human cells divided, organized into layers, and established functional connections throughout the mouse brain and spinal cord. Scans showed that the resulting brain architecture looked somewhat irregular, though the grafts successfully produced rare cell types, including von Economo neurons, which are vulnerable in specific dementias.
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Behavioural testing and medical applications
About six months after the initial surgery, researchers evaluated the animals using table-top arena movement tests. The bioengineered mice performed similarly to unaltered mice without displaying any cognitive enhancement or altered physical capabilities. Pașca noted that the model provides direct access to living human neural networks to investigate genetic alterations linked to schizophrenia, epilepsy, severe autism, and cerebral palsy.
Sergiu Pașca explained the experimental purpose of integrating human cells into a living animal:
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.
Ethical oversight and animal welfare
The study has prompted discussions among external scientists and bioethicists regarding animal cognition and laboratory welfare standards. Emily Jackson, a professor of law at the London School of Economics who chaired a neural organoid report for the Nuffield Council on Bioethics, stated that close monitoring is required to evaluate the long-term impact on the rodents. Hongkui Zeng, director of brain science at the Allen Institute, noted that researchers must carefully consider future steps, especially if similar methods are tested in larger species.
Hongkui Zeng described the value of the platform while noting future considerations:
It's really a powerful technology to study human neurons and how human brain circuits can form in a more natural environment.
Sarah Chan, a reader in bioethics at the University of Edinburgh, pointed to the challenge of evaluating animal experiences during cognitive research:
How can we know what it's like to be one of these mice? And how do we take account of that in the ways that we treat laboratory animals?
