Scientists at Stanford University have created genetically engineered mice whose brains contain substantial amounts of functioning human tissue, developing a new experimental model intended to improve research into neurological and psychiatric disorders.
The researchers hope the approach could help scientists investigate conditions that are difficult to reproduce in conventional laboratory animals, including epilepsy, cerebral palsy, schizophrenia, intellectual disability and rare forms of dementia.
The work, published in Nature and conducted with ethical oversight, does not indicate that the animals developed human-like cognition.
Researchers genetically engineered mice so that key brain regions, including the cerebral cortex and hippocampus, developed minimally, creating space for transplanted human tissue.
Human skin cells were reprogrammed into brain-like structures known as organoids before being implanted into newborn mice.
The animals ultimately contained about four million human brain cells, with the transplanted tissue occupying roughly half of the brain by volume.
Within three months, the human tissue connected to the animals' blood supply and largely filled the available cavity, while some human neurons formed connections with mouse brain cells and the spinal cord.
The tissue remained immature and lacked the organized structure found in a normal human cortex.
Behavioral testing also found no evidence that the transplants enhanced the animals, although some existing movement and cognitive difficulties improved slightly.
The model could allow researchers to take cells from patients with neurological disorders, grow corresponding brain tissue and observe its development inside a living animal.
Experiments involving low oxygen demonstrated the vulnerability of implanted human nerve cells to oxygen deprivation, while researchers also identified rare von Economo neurons in the transplanted tissue.
Those cells have previously been observed in postmortem examinations and are affected in frontotemporal dementia.
Researchers said the model could therefore provide new opportunities to investigate specific diseases and potential treatments that require interactions across a living biological system.
Independent experts nevertheless highlighted continuing ethical questions surrounding animal welfare and the implications of growing living human brain tissue inside rodents.
Researchers emphasized that continued oversight would be necessary as the technique develops.