Researchers have advanced a technique known as xenocortication, enabling human brain tissue derived from stem cells to grow inside the skulls of newborn mice that have been genetically modified to lack their own cortex and hippocampus. The study, published in Nature, reports that the human grafts expand substantially, form long-distance neural connections, and respond to injuries, offering a living model to observe how human brain circuits develop and react to disease.
“The main motivation was a fundamental limitation in studying disorders of the human brain,” senior author Sergiu P. Pașca, the Uytengsu Family Founding Director of the Stanford Brain Organogenesis Program at Stanford University, told PsyPost. “Many neurological and psychiatric conditions begin during development, but living human brain tissue is largely inaccessible, and animal models cannot fully reproduce human genetics or aspects of biology that may be specific to our species.”
Pașca noted that conventional lab models—neural organoids grown in dishes—have advanced the study of human brain development but cannot replicate the full biological environment, including a vascular system, sensory inputs, or motor outputs. “Over the years, organoids and assembloids have given us increasingly powerful ways to study human brain development in the laboratory,” he explained. “But they still lack many features of an intact nervous system, including a normal blood supply, sensory inputs and motor outputs.”
The team’s approach aims to overcome limitations of studying human brain biology outside the body, building on earlier work showing transplanted human neural clusters can mature in living animals. Pașca pointed out that it is difficult in a dish to link changes in human neural circuits to functional consequences. “It is also difficult in a dish to connect changes in human neural circuits to functional consequences,” he said.
To create a model in which human cortical tissue can develop more extensively within an intact nervous system, the researchers sought to remove competition between developing human tissue and the mouse’s own brain. “So the question was whether we could create a model in which human cortical tissue could develop more extensively, integrate into an intact nervous system and allow us to study human biology from cells and circuits all the way to functional readouts,” he told PsyPost.
In traditional transplants, human cells compete for space inside the animal’s skull. The researchers therefore engineered mice to lack a neocortex and hippocampus. “The two parallel developing systems are in competition for turf,” Pașca noted. The resulting apallial mice survive and move about despite the missing major brain structures, prompting Pașca to suggest that early cortical loss may be offset by other brain regions during development.
The human cortical organoids were transplanted into the mice’s empty brain cavities in a procedure described as xenocortication. Over the following months, the grafts expanded dramatically; by the third month, the human tissue had increased in size by 4.7 times and occupied about 92 percent of the available cortical space in the mouse brain.
The human grafts differentiated into a diverse array of mature neural cell types. Notably, the transplanted tissue generated layer 5 extratelencephalic projection neurons—cells that in humans include von Economo neurons, which are implicated in social behaviour and certain psychiatric conditions. Pașca explained that organoids grown in a dish rarely produce these neurons, but the grafts in mice did so abundantly. “This cell type appears to be particularly vulnerable in frontotemporal dementia, a neurodegenerative disorder that can begin in midlife,” he said. “Yet here they were, sitting in the xenocortical mice’s human tissue. Now we can generate these rare cells from a healthy person and study them in a living, behaving animal to learn more about what they’re doing.”
Researchers also observed extensive physical integration between human tissue and the mouse host. Fluorescent tracing showed human nerve fibres extending deep into the mouse brain and even down into the cervical spinal cord, while fibres from the mouse’s lower brain regions projected up into the human graft.
To test functionality, the team used advanced imaging and electrical recording to track activity within the graft. They detected synchronized bursts of electrical activity spreading through the human tissue, resembling brain waves seen in early development. This spontaneous activity strongly correlated with the mouse’s facial movements, indicating the graft was electrically active and integrated into the animal’s nervous system.
Behavioural testing revealed nuanced effects. Both apallial mice and xenocortical mice could walk and explore normally, but the two groups showed distinct patterns. In a Y-maze working memory task, normal mice typically remember previously explored arms and alternate choices. The apallial mice failed to alternate above chance, suggesting impaired working memory, while the xenocortical mice performed above chance, indicating partial retention of exploratory memory function.
The researchers then investigated whether the xenocortical model could mimic human brain injury using hypoxia. Exposing mice to five hours of low-oxygen conditions (five percent oxygen) was intended to simulate injuries linked to cerebral palsy and related motor deficits. They assessed gait changes with the automated CatWalk system, which measures footprint patterns, limb support and timing. Post-injury, the human grafts showed increased cellular stress and inflammation, and the xenocortical mice altered their walking patterns, spending more time bearing weight on three or four paws and adopting a wider stance than before the injury.
Pașca said the findings could help illuminate why human neural tissue shows particular vulnerability to oxygen deprivation and how cerebral palsy might arise, while offering a platform to test potential therapies. He stressed the individualised aspect of the model: “The cells we implant carry the genetic material of the person they’re derived from — whether that person is a patient or a healthy individual — allowing us to study downstream disease effects in brain cells and circuits.”
Alison Singer, president of the Autism Science Foundation, welcomed the potential for precision medicine, noting: “The idea that you can make an organoid model with an individual’s unique genetic character and use that to learn what’s gone awry in that individual’s brain is a critical step toward precision medicine.”
Ethical considerations were foregrounded in the work. Pașca described ongoing engagement with ethicists, neurobiologists, patient advocates, philosophers and legal scholars over several years, adding that an overriding argument justified the research in light of the suffering caused by currently incurable neurological disorders: “Throughout several years of experimentation, we have received input from ethicists, neurobiologists with expertise in primate and human cortical biology, patient advocates, philosophers, and legal scholars… An overriding argument questioned the ethics of not conducting this research in the face of the suffering of hundreds of millions of people afflicted with neurological disorders that today are uncurable but tomorrow could yield treatments we discover by using this model.”
The study, titled Developmental xenocortication using human-derived organoids in mice, has a broad author list led by Konstantin Kaganovsky and Sergiu P. Pașca, with collaborators from multiple laboratories.
