‘Alien-cortical’ mice: Can human brain cells grow inside animals?

In a laboratory in California, researchers at Stanford University implanted human neurons into genetically modified mice, creating "alien-cortical mice" with human brain tissue growing inside their brains. The mice were born without parts of their cerebral cortex and hippocampus, and human organoids were injected into the empty spaces. The human tissue grew five times larger within three months, connected to the mice's blood supply, and filled over 90% of the cavities. The mice walked and vocalized normally, even though the human tissue made up about half their brain volume. The experiment may help study brain disorders like schizophrenia and cerebral palsy.

• Researchers injected human organoids into mice, which were genetically modified to lack parts of their brains.
• The mice were implanted with about 4 million human neurons, while 14 million mouse neurons were removed.
• The human tissue grew to fill over 90% of the cavities in the mice's brains.
• The mice showed normal behavior despite the human tissue making up half their brain volume.
• The experiment may help explain cerebral palsy, as the mice showed brain damage when exposed to low oxygen.

The study suggests that human brain cells can grow inside animals, but the mice are not "mice that think like humans." The researchers stopped the experiments when the human neurons were six months old, as they could not form more connections with the mice's brains. The study highlights the potential of brain organoids to study human brain function, but raises ethical concerns about the capacity of these cells to feel or be conscious. The researchers emphasize that the experiment does not itself raise major ethical issues, but it provides evidence that other animals could be created that might raise more serious ethical concerns.

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