Mice with millions of human neurons in their brains bring scientists closer to understanding mental disorders

Neuroscientist Sergiu Pasca, 44, holds up a transparent vial filled with tiny white spheres floating in liquid. Each one is a cortical organoid, a millimetre-scale replica of the part of the brain responsible for complex thought, language and the mystery of the human mind. Pasca’s team implanted these seeds into the brains of mice that had been genetically altered so they would not develop a cerebral cortex of their own. The experiment, unique in the world, has produced healthy animals with brains that are an integrated blend of human and mouse. These animals contain more transplanted human neurons than any others ever born, the scientist tells EL PAÍS.

For the first time, these animals make it possible to investigate “features of the human brain that until now were inaccessible,” says Pasca, a physician and researcher at Stanford University in the United States. Specifically, they allow scientists to study the human neurons most characteristic of our species at the molecular and cellular level within a living organism, rather than in a cadaver sample or a laboratory cell culture, as has been the case until now. Pasca, who was born in Romania, believes the advance paves the way for new treatments for autism, schizophrenia, cerebral palsy and dementia. In the future, he says, it may be possible to create a mouse with a cerebral cortex made from an individual patient’s own neurons.

The details of the experiment, published on Wednesday in Nature, read like science fiction. It begins with skin cells taken from healthy volunteers. Scientists apply a cocktail of proteins that reprograms the cells back to an embryonic state, when they can become any tissue in the body. The cells are then guided to develop into the type of stem cells that give rise to neurons in the cerebral cortex, eventually forming organoids: the tiny white spheres with a three-dimensional structure that Pasca displays during the video interview.

In earlier experiments, the same team successfully implanted human neurons into the brains of rats and used laser light to influence their behavior. Growth, however, was limited by the lack of space inside the skull, as human neurons are much larger than those of rodents. Pasca had also shown that organoids from different regions of the brain can be created and fused together, like pieces of a living puzzle.

These spheres, each containing hundreds of thousands of human neurons, are implanted into the mice two days after birth. They then begin to grow, become vascularized and connect with the rest of the mouse brain, which controls functions such as sensory processing and movement. Because the animals lack their own cerebral cortex, the human neurons expand to make up more than 90% of the cortex and roughly half of the mice’s entire brain. Timing is crucial: this window of “plasticity,” during which the cells can integrate with their host, closes about two weeks after implantation.

The cerebral cortex that develops in these mice is comparable to that of a human fetus in the sixth month of gestation, Pasca explains, although there are important differences. At that stage, the human brain already contains more than 16 billion neurons, whereas the modified mice have around four million such cells in their cortex. A normal mouse brain, by contrast, contains about 14 million neurons, because its neurons are much smaller. What exists inside the brains of these engineered animals is not exactly a human cerebral cortex, as some cell types are still missing and neither the structure nor the size is fully equivalent. Even so, Pasca argues that it provides an invaluable model for studying our “most inaccessible organ.”

Cerebral palsy

In a proof-of-concept experiment, the researchers cut off the oxygen supply to the brains of their humanized mice. Ordinary mice are remarkably resistant to this kind of injury, whereas in humans even a brief period of oxygen deprivation can result in lifelong cerebral palsy.

The lesions observed in the animals with humanized brains resemble those seen in patients with cerebral palsy, a condition that affects around 18 million people worldwide. The team argues that these mice make it possible, for the first time, to study the effects of these currently irreversible injuries in a living brain, and perhaps even to identify ways of treating them.

In 2024, Pasca’s team implanted human brain organoids derived from the cells of a child with Timothy syndrome — a rare genetic disorder associated with severe autism and epileptic seizures — into rats. The work enabled the researchers to develop a genetic therapy that reversed the condition in the implanted animals. Pasca says the team plans to begin testing the treatment in humans in a clinical trial later this year or early next year.

Garikoitz Lerma-Usabiaga, an Ikerbasque researcher at the Basque Center on Cognition, Brain and Language (BCBL) in San Sebastián, contributed to the study by characterizing the brain’s microstructure using diffusion MRI. This technique measures the movement of water molecules to determine tissue organization and map neuronal pathways. “The technique finds that water molecules prefer to travel in the direction from the human organoid to the mouse brain, suggesting connectivity between the two,” the neuroscientist explains.

The researchers were in for another surprise. These mice generated a type of neuron that had never before been isolated in a living organism: Von Economo neurons, a rare class of neural cell first described in 1925. Initially thought to exist only in humans, they were later identified in primates and in other mammals with large, complex brains, including elephants, whales and dolphins.

These neurons are found in regions of the cortex associated with social behavior and decision-making, and their premature degeneration is a hallmark of frontotemporal dementia, a rare form of the disease that appears between the ages of 40 and 60.

The researchers believe these unusually large neurons may have emerged in the humanized mouse brains because, for the first time, they had sufficient space to develop. Their presence could now provide a valuable platform for testing treatments for the disease.

Despite having a cortex made up of human neurons, these animals do not behave very differently from ordinary mice, Pasca says. If anything, they are slightly slower in tests of memory and reflexes, because the connection between the human and mouse components of the brain “is not perfect.” That is due to the fact that the two species have been evolving separately for more than 70 million years.

Ethical issues

The outcome would likely be very different if these human organoids were implanted into monkeys, something that should not be done for ethical reasons, according to Pasca. Before carrying out the mouse experiments, his team submitted the project to an independent oversight panel made up of bioethicists, lawyers, evolutionary biologists, patient advocates and philosophers, all of whom approved the work.

“It is a very delicate subject, and it is easy for misunderstandings to arise if we do not convey the nuances well,” says Pasca, who argues against replicating this experiment in apes: “In species closer to us, human neurons would have much more space to grow, and integration would be much better. I think we should be very cautious about this kind of experiment and not carry them out unless there is a very clear justification.”

Daniel Tornero, head of the Stem Cell and Brain Injury Laboratory at the University of Barcelona, is well acquainted with Pasca’s work and describes it as “incredible.” “Psychiatric illnesses are the hardest to model because mice do not suffer them, and their cerebral cortex is much smaller,” he explains. “This new work is interesting because it allows you to model human diseases with human cells.”

In 2019, a Chinese team that included the Spanish biologist Juan Carlos Izpisua created the first human-monkey chimera: a macaque implanted with stem cells derived from humans. The aim was to explore whether animals could one day be used to grow human organs for transplantation. The term chimera comes from the fire-breathing creature of Greek mythology, with a lion’s head, a goat’s body and a dragon’s tail.

For Tornero, the animals created by Pasca are also chimeras, although the Stanford researcher is careful to avoid the term, perhaps to steer clear of controversy. Tornero notes that it is difficult to know whether there is anything human in the behavior or thought of these mice.

“Human neurons send axons [projections] into the mouse brain, but there is not enough data yet to know whether those connections are superior,” he says.

The possibility is not purely theoretical. In 2013, a research team demonstrated, on a much smaller scale, that injecting 300,000 human glial cells, which provide support and immune protection in the brain, into mice could enhance their cognitive abilities. The modified rodents had better memory and learned faster than normal ones.

For now, however, these new animal models can only be used to study disorders that arise during embryonic development. Human neurons grow about 20 times more slowly than those of mice. Even if one of these animals lived to the age of two, the upper limit of a mouse’s lifespan and roughly equivalent to old age in humans, its cerebral cortex would still resemble that of a two-year-old child.

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