Human Cortical Tissue Grown in Mice Could Transform Brain Research

The greatest obstacle to studying the human brain is the person it belongs to.

Researchers cannot simply experiment on someone’s grey matter while that person is still relying on it. As a result, many aspects of how the brain develops and forms its connections - and how those processes can go wrong - remain insufficiently understood.

For years, scientists have sought alternative ways to investigate human brain tissue, from cells cultured in laboratory dishes to small three-dimensional structures known as organoids. Yet even highly advanced organoids lack something fundamental: the body that a brain would ordinarily control.

Neuroscientists at Stanford University have now made a major move towards addressing that limitation. They grew human cortical tissue in mice whose own cerebral cortex - the brain’s outer layer - had been largely genetically removed during early development.

After three months, the human tissue accounted for more than 90 percent of cortical tissue by volume. It had developed blood vessels, connected itself to the mouse nervous system, become electrically active, and extended some projections as far as the spinal cord.

"The most important point is that these are still mice," neuroscientist Sergiu Pașca of Stanford University told ScienceAlert.

"They have a mouse nervous system, mouse sensory organs, and mouse subcortical structures. What is unusual is that most of the cortical tissue present in these animals is human-derived and that the human neurons grow, integrate, and form functional connections with the rest of the mouse nervous system."

Human cortical tissue and the limitations of brain organoids

Brain organoids have become increasingly sophisticated over recent years.

Scientists can now cultivate clusters of brain tissue containing a wide variety of cell types seen in the developing human nervous system. They can also combine organoids representing separate brain regions into more intricate structures called assembloids.

However, there is only so much that can be achieved in a Petri dish. A standalone organoid has no blood supply, no sensory input to interpret, and no muscles to control.

Pașca and his colleagues have spent years working to overcome this constraint. In 2022, the group implanted human cortical organoids into newborn rats. There, the human neurons matured, integrated into the rats’ brains, and even reacted to sensory signals from the animals’ whiskers.

A difficulty nevertheless remained. Human neurons mature much more slowly than rodent neurons, meaning the two developing systems were competing for the same space. The researchers therefore eliminated that competition.

The team genetically modified mice so that most of the cerebral cortex and hippocampus failed to develop. They named these animals "apallial" mice.

Some of the mice were allowed to grow into adulthood in that state. Others were given transplants of human cortical organoids shortly after birth, producing what the researchers call "xenocortical" mice.

Over the following months, as the mice developed and matured, the grafts grew alongside them and expanded substantially into the unoccupied cortical area.

How the human neurons developed in xenocortical mice

The tissue that resulted was not a reproduction of a human cortex. At roughly six months, it was still developmentally immature - approximately equivalent to the human cortex at mid-gestation - and lacked the organised layers found in a mature cortex.

Nevertheless, it began producing neurons that researchers had found difficult to create using other models.

These included layer 5 extratelencephalic (L5-ET) neurons, a group of long-range projection neurons that usually link the cortex with distant regions of the nervous system.

Within this group, the scientists also identified cells resembling von Economo neurons (VENs). These distinctive elongated neurons occur in particular parts of the human brain, have been especially challenging to recreate in the laboratory, and had notably not been detected in the earlier rat study.

What is it about the mouse brain that promotes their development?

"We do not yet know, and I think this is one of the most interesting questions raised by the study," Pașca said.

"It is … possible that giving human cortical neurons access to distant targets in other parts of the nervous system provides signals that are difficult to reproduce in an organoid in a dish.

"Another possibility is that there are factors present in the in vivo environment that are missing from organoids in a dish. These could include trophic or growth factors, signals associated with neuronal activity, vascularization, or other interactions with surrounding cells."

Understanding the fate of the human brain tissue was only one part of the issue. The other was: what effect does it have on the mice?

To examine this, the scientists tested three groups in a Y-shaped maze: normal, unmodified control mice; ungrafted apallial mice; and xenocortical mice.

Mice naturally favour exploring a maze arm different from the one they have just entered. Their ability to recall where they have already been therefore offers a straightforward test of working memory.

The control mice performed better than chance. The apallial mice did not, indicating that the loss of most of their cortex and hippocampus made it difficult for them to remember the arm they had just explored.

The xenocortical mice, by contrast, also achieved scores above chance.

"That is an intriguing result, particularly because the apallial mice did not show the same performance," Pașca said. "But we should be cautious about concluding from this alone that the human neurons are directly responsible for the behavior."

Future research will need to address that question.

For the moment, the research resolves a more basic question the team wanted to investigate. It was not, “can human brain cells make mice do better in mazes?”, but rather, “can human brain grafts in mice give us a way to study what happens when the brain goes wrong?”

Potential disease research and ethical questions

That approach could offer several important applications.

"One obvious direction is frontotemporal dementia. Because we can now generate VEN-like neurons in this setting, we can derive cortical organoids from patients, ask whether these rare neurons show selective vulnerability, and test whether candidate interventions can protect them," Pașca explained.

"For epilepsy, we can ask whether disease-associated human neurons generate abnormal circuit activity or seizures and whether those abnormalities can be corrected therapeutically.

"For autism and other neurodevelopmental disorders, we can ask whether defined alterations in human cortical circuits lead to reproducible functional or behavioral consequences in the animal."

However, placing human neural tissue into another animal on this scale unavoidably presents difficult ethical issues.

The team began considering these questions before the experiments had concluded. Alongside the standard review for animal research, Pașca said that Stanford bioethicists and an external committee involving neuroscientists, ethicists, legal scholars, and patient advocates took part.

One concern relates to the animals: whether the suffering involved is warranted by the research being carried out, or whether the same knowledge could be obtained through another method.

"Another important question is whether introducing increasingly complex human neural tissue into an animal nervous system could lead to unexpected, emergent, or novel properties that would require additional ethical consideration," Pașca said.

"We considered this possibility explicitly and monitored the animals carefully, both biologically and behaviorally, as the work progressed."

At the same time, failing to develop improved models for neurological and psychiatric illnesses also carries an ethical cost.

"Neurological and psychiatric disorders affect nearly one in five people, and for many of these conditions we still do not understand the underlying biology well enough to develop effective treatments," Pașca said.

"Many of these disorders involve human neural circuits and behavioral consequences that cannot be adequately modeled in conventional animal models or in isolated human cells."

The findings have been published in Nature.

This article was fact-checked and edited by Rebecca Dyer. Although we take pride in our process, we are only human. If you notice an error, please let us know.