Researchers at Stanford University have implanted lab-grown human cells into the brains of mice, in an experiment that could offer a new way to study brain disorders. According to a study published in Nature, the human tissue, derived from donor stem cells, grew inside the animals’ brains and appeared to improve their performance in cognitive tests.
The researchers created the mice by genetically engineering them to lack most of their cerebral cortex and hippocampus. At two days old, the animals were injected with human "organoids", clusters of about 100,000 human cells each. Of 29 mice that received the transplants, the procedure was successful in 25. Within three months, the human tissue had grown roughly five times larger, connected to the mice's blood supply and filling over 90 percent of the cavities left by the missing mouse brain.
Studying the human brain has long been extremely difficult due to ethical barriers and the organ's biological complexity. While researchers have used lab-grown "organoids" as models, these lack the connections and environment of a living brain. Senior author and Stanford professor Sergiu Pasca said the work is part of a long-running quest. "For the past two decades, there's been a quest to try to build models of the human brain outside of the human body," he said. "This is not going to replace all the models we had before, but it's going to provide us access to other aspects of human brain function that would be very difficult to study otherwise."
The mice implanted with human tissue performed better in maze tests than those without, suggesting the human cells contributed to cognition. Unexpectedly, some of the implanted tissue developed into von Economo neurons, a type of long, rare cell normally absent in mice and found in highly social animals such as humans, apes and whales. The experiment might also help explain cerebral palsy: when the modified mice were exposed to low oxygen, they showed brain damage, an altered gait and poor limb coordination. The condition has been hard to study in rodents.
The researchers stopped the experiments when the human cells were six months old, before they could form more neural connections or show signs of consciousness. John Evans, a sociology professor at the University of California, San Diego, who was not involved in the study, said the work does not itself raise major ethical issues, but provides evidence that other animals could be created that might raise more serious concerns. He argued that the lack of clear stopping points on the gradual path from mice to non-human primates is a problem, and suggested researchers should not go beyond rodents for now.
Analysis
Why This Matters
- Provides a new, more biologically relevant model for studying devastating brain conditions like schizophrenia, cerebral palsy and dementia.
- Raises fundamental questions about the nature of consciousness and what constitutes a "human" brain, even in part.
- The success of this experiment creates pressure to define clear ethical boundaries for future neurohybrid research, potentially involving animals closer to humans.
Background
The human brain is one of the most challenging biological systems to study directly due to ethical restrictions and its immense complexity, containing roughly 86 billion neurons. Scientists have developed lab-grown organoids as models, but these lack the structural and functional environment of a living brain. The Stanford experiment represents a significant step in the long-running effort to build more accurate models of the human brain.
Key Perspectives
Sergiu Pasca, Stanford University: Says the work provides access to aspects of human brain function that are very difficult to study using other models, and is the result of a long quest to build models of the human brain outside the body.
John Evans, UC San Diego: Argues that while this specific experiment does not raise major ethical issues on its own, it opens a path toward creating animals that could, and cautions researchers against moving beyond rodents to avoid creating ethically controversial neurohybrid organisms.
What to Watch
- Where the scientific community and regulators draw the lines for the next steps, particularly the proposed barrier against moving into non-human primates.
- Whether the insights into cerebral palsy gained from this model lead to clinical trials or new treatment approaches.
- The emergence of formal ethical oversight or guidelines specifically addressing the integration of human brain tissue into animals.
Checked
12 claims in this story checked against 1 source: 11 supported.
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- SupportedHuman neurons were successfully grown inside mouse brains in a Stanford study.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “Researchers at Stanford University have implanted lab-grown human cells into mice genetically engineered to lack parts of their brains, including the cerebral cortex”
- SupportedThe mice with implanted human cells did better in maze tests, suggesting the cells contributed to cognition.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “while those with human cells did better, suggesting the human tissue contributed to cognition.”
- SupportedThe experiment could offer a new way to study disorders such as cerebral palsy and raises ethical questions.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “these so-called “alien-cortical mice” may offer a new way to study neural circuits and brain disorders such as schizophrenia, cerebral palsy and dementia.”
- SupportedThe researchers genetically modified mice to lack most of the cerebral cortex and neighbouring hippocampus.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “Researchers genetically modified the mice so they were born and grew without most of the cerebral cortex and the neighbouring hippocampus”
- SupportedAt two days old, the mice received human organoids, each containing about 100,000 cells grown from donor stem cells.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “When the animals were two days old, researchers injected human “organoids” into the empty spaces. Organoids are miniature masses of cells grown in a laboratory from stem cells or tumour tissue”
- SupportedThe transplants worked in 25 of the 29 mice that received them.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “They tried the transplants on 29 mice, and it worked on 25 of them.”
- SupportedWithin three months, the human tissue grew about five times larger, connected to the mice’s blood supply and filled more than 90 percent of the cavities.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “Within three months, the human tissue had grown about five times larger inside the mice’s brains, connected to their blood supply and filled more than 90 percent of the cavities.”
- SupportedThe human tissue developed into von Economo neurons, normally absent in mice and found in highly social animals.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “some of the tissue developed into von Economo neurons, which are long, rare cells normally absent in mice and almost impossible to make in a lab. They are found in highly social animals such as humans, apes, whales and elephants.”
- SupportedLow oxygen exposure in the modified mice caused brain damage, an altered gait and poor limb coordination, potentially helping researchers study cerebral palsy.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “When the modified mice in this case were exposed to low oxygen, they showed brain damage, an altered gait and poor limb coordination.”
- SupportedPasca said there had been a two-decade quest to build models of the human brain outside the body.Al Jazeera – Breaking News, World News and Video from Al Jazeera: ““For the past two decades, there’s been a quest to try to build models of the human brain outside of the human body,” he said.”
- SupportedPasca said the approach would not replace existing models but could provide access to other aspects of human brain function.Al Jazeera – Breaking News, World News and Video from Al Jazeera: ““This is not going to replace all the models we had before, but it’s going to provide us access to other aspects of human brain function that would be very difficult to study otherwise.””
- Partly supportedResearchers stopped the experiments when the human cells were six months old, before they could form more neural connections or show signs of consciousness.Al Jazeera – Breaking News, World News and Video from Al Jazeera: “The researchers stopped the experiments when the human nerve cells implanted in the brains of mice were six months old, before they could form more neural connections with the animals’ brains, and before any signs of consciousness appeared.”
