When Human Neurons Live in a Mouse

On organoids, autism, and the question of where the human actually resides

I recently read a news story in Science that made me stop.

Researchers at Stanford University had done something that, only a few years ago, might have belonged to science fiction. They had genetically engineered mice so that large parts of the cerebral cortex did not develop, and then transplanted small organ-like structures grown from human stem cells—so-called brain organoids—into the space left behind.

The human tissue grew.

After three months, it made up more than 90 percent of the cortical tissue. The human neurons established connections with other parts of the mouse nervous system and sent nerve fibers through the brain and toward the spinal cord.

These were no longer simply human cells in a laboratory dish.

They were living as part of an animal.

And then a question arises that cannot be answered by a microscope alone:

Where, exactly, does the human reside?

A Human Neuron

At first, the question seems simple.

A human neuron is a neuron of human genetic origin. The cells in the Stanford experiment were grown from human stem cells. Biologically, there is therefore no doubt about where they came from.

But origin is not necessarily the same as meaning.

A neuron does not exist alone. It receives signals from other cells, sends signals onward, is affected by its chemical and electrical surroundings, and participates in larger networks.

In the experiment, the human cell grows inside the body of a mouse. It receives its blood supply there. It forms connections with the mouse nervous system. Its activity is influenced by the organism of which it has now become a part.

The cell is human.

Its context is not.

What, then, is it?

Perhaps the question has already been wrongly posed.

The Parts and the Whole

We have a strong tendency to look for the human in the parts.

In the genes.

In the brain.

In the neurons.

Somewhere in there, we think, there must be something that makes us who we are.

The new research makes this idea more complicated. If human neurons can live and function within the nervous system of a mouse without the mouse thereby becoming human, then the human cannot simply be located in the neurons.

It emerges within a context.

This is not an argument against biology. On the contrary. It is biology itself that shows us how decisive context can be.

Among other things, the researchers found a rare type of nerve cell with characteristics associated with von Economo neurons. These are found in particular regions of the human brain, including regions associated with social awareness and decision-making, and they have previously been extremely difficult to grow successfully in the laboratory.

Something became possible because the cells were allowed to develop within a living nervous system.

The environment was not merely a container.

It helped shape what could come into being.

An Old Hermeneutic Question

Here, the new biology moves surprisingly close to an old hermeneutic problem.

We cannot understand the part without the whole. But neither can we understand the whole without the parts.

This is the hermeneutic circle. For Gadamer, it is not merely a method that we can choose to use or not to use. It describes something fundamental about understanding itself. A word acquires meaning through the sentence in which it appears. The sentence acquires meaning through the text. And the text is understood within a language, a history, and a world in which the reader already finds himself.

We move from the part to the whole and back again.

Each time, our understanding may shift.

The human neuron in the mouse brain makes this movement almost visible.

We can isolate the cell and examine it. We can map its genes, measure its electrical impulses, and describe the proteins it produces. We can compare it with other cells and follow its development.

All of this gives us genuine knowledge.

But what the cell does cannot be understood independently of the network in which it participates. And the network, in turn, cannot be understood independently of the organism of which it is a part.

Here, the analogy must be used carefully.

A neuron does not interpret its context in the way a human being interprets a text, a situation, or another person. Biology is not hermeneutics.

But the research shows us something that hermeneutics has long taught us to notice:

Context is not merely background.

It helps determine what the part can become and what the part can do.

This also changes the question with which I began.

Where does the human reside?

If the answer were simply “in the human neurons,” we ought to be able to point to those cells in the mouse brain and say:

There is the human.

But we cannot.

The cells are human.

The mouse is still a mouse.

This does not mean that biology has failed. It means that the question of the human cannot be answered by examining one part of the human being alone.

And Then Autism Enters the Picture

It is precisely here that the researchers’ interest in autism becomes interesting.

Not because the new study has discovered anything specific about autism.

It has not.

Sergiu Pașca’s research group has previously used a different transplantation model to study Timothy syndrome, a rare genetic condition that can, among other things, be associated with autistic traits. In the 2022 study, human brain organoids were transplanted into rats. That work demonstrated how such models could be used to investigate particular genetic and cellular mechanisms within a living nervous system.

The new 2026 study is different.

Here, mice are genetically engineered so that large parts of the cerebral cortex fail to develop. Human tissue is then able to grow into the space where mouse tissue would otherwise have been.

Among other things, the researchers investigated what happened under conditions of oxygen deprivation. The human neural tissue was damaged, and the animals developed difficulties with gait and balance. This may make the model useful for research into conditions in which disruptions early in brain development play a role.

The researchers also point to possible future applications in areas including autism.

But so far, the experiment has found nothing specifically about autism.

That is precisely the point.

The model makes it possible to study human neurons as they develop as part of a living nervous system. It may later be used to investigate biological processes that could be relevant to autism.

But the model is not autism.

Researchers have not found “autism” inside a neuron.

Nor has a mouse with human neurons thereby acquired human autism.

The distinction is crucial.

For autism does not reveal itself only in cells. What we call autism also becomes apparent in a human being’s ways of sensing, orienting themselves, communicating, and being with other people. It appears in the encounter between a human being and a world.

Biology is part of this.

But knowledge of the part is not yet knowledge of the whole.

The Organoid and the World

This brings us back to the organoid.

An organoid in a laboratory dish has no world in the sense in which a living human being has a world.

It lies in a nutrient medium. Researchers can add chemical substances, measure electrical activity, and study how its cells develop.

But the organoid goes nowhere.

It hears no voice.

It encounters no other.

When human cells are placed inside a living animal, something else happens. They become part of a functioning nervous system. Signals come and go. The body moves. The sensory organs are affected by their surroundings. The organism responds.

The cells have acquired a context.

Yet they have not acquired a human world.

That distinction matters.

The researchers themselves emphasize the limitations. The transplanted tissue does not develop in the same way as a human brain. Its connections are different. Its sensory inputs are different. And organoids reproduce aspects of early fetal development more successfully than the later stages after birth, when much of the brain’s network of connections is formed.

This is human neural tissue.

It is not a human mind.

When the Boundary Becomes Unclear

But precisely for this reason, an ethical question also arises.

How far should we go?

As human brain tissue becomes increasingly integrated into animals, it is hardly sufficient simply to ask what percentage of the tissue is of human origin.

Ninety percent human cortical tissue does not make a mouse human.

But percentage alone cannot be all that concerns us.

We must also ask what the tissue can do.

Could increasingly complex networks alter the animal’s perception or learning? Could future models develop properties we did not anticipate? And if research were one day to approach forms of experience or suffering that are difficult for us to recognize, how would we know?

There is currently no reason to claim that the mice in this experiment have acquired human consciousness.

Nor should we suggest that they have.

But the ethical problem is interesting precisely because we are entering a territory in which old categories become less self-evident.

Human.

Animal.

Human cell.

Mouse brain.

The words remain clear when taken separately.

It is the connections between them that have become more difficult.

And once again, we encounter the problem of the part and the whole.

Where Is the Human?

I began with a mouse.

A mouse in which large parts of the cerebral cortex consist of tissue grown from human stem cells.

I end somewhere else.

For perhaps the experiment tells us as much about our desire to locate the human in one particular place as it tells us about neurons.

We search in DNA.

We search in neurons.

We search in particular regions of the brain.

And of course we should continue searching there. Biology is real. Genes are real. Neurons are real. Differences in brain development are real.

But perhaps the human is not found in any of these parts alone.

A human being comes into being in a body, in a history, in a language, and in a world shared with others.

That is also why I become cautious when this research approaches autism. The more we learn about neurons, the more precisely we may understand particular biological mechanisms.

That is valuable knowledge.

But understanding the human being requires another movement as well.

From the part back to the whole.

And from there, back to the part again.

The human neuron growing inside the mouse brain makes this movement visible in a way I had not expected.

It is unquestionably human.

But it is not a human being.

Between those two sentences, an entire landscape opens.