Human brain tissue implanted in rats integrated and grew with the host brain | UK News

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A research team in the US has shown that human brain tissue implanted in rats can integrate into its host’s brain, promising to offer scientists a whole new way to study brain diseases – but also raising ethical questions.

Professor Sergiu Pasca and colleagues at Stanford University in California took sesame-seed-sized clumps of human brain cells called “organoids” grown in a test tube and implanted them in the brains of baby rats.

In the research, published in the journal Nature, they report that human brain tissue not only survives, but also integrates into the rat brain, forms connections with rat brain cells, and is nourished by the rat’s blood supply.

The organoids also grew in the rat brain to about the size of a pea.

The human nerve cells in the rat are about six times larger than in the test tube.

The team then ran a series of experiments showing that human brain cells could receive sensory signals from the rat’s whiskers, but could also send instructions to other parts of the rat’s brain when trained to do so.

“They can receive sensory input, but they are also involved in some rat neural circuits,” says Professor Pasca.

The goal of the research team is the development of “in vivo” models to study the human brain and its diseases.

“One Step Closer to Seeing the Inside of the Human Mind”

The complex cellular or chemical basis of brain diseases such as autism and schizophrenia are very difficult to study in humans. Mice and rats are poor surrogates for the human brain, and research with primates is ethically questionable.

And while test-tube organoids have led to a new understanding of how nerves work at the cellular level, they never get as large or as complex as healthy human brain tissue.

But growing human brain organoids in another species is a step closer to glimpsing inside the human mind, the researchers hope. Especially when it comes to testing new drugs to treat brain diseases.

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“Imagine using this model as a non-invasive way to test the drug “in vivo” on human cells,” says Professor Pasca.

But if the research went beyond that, inserting human brain cells into animals would raise profound ethical questions. The research team said they observed no behavioral differences between the rats with human brain transplants and those without.

And given the limited lifespan of rats, human brain tissue, which takes years to mature, is limited in its ability to develop.

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Given the small size of a rat brain and the even smaller size of the transplanted human tissue, most experts seem to think that the rats don’t even become partially human — and the amount of human brain circuitry is too small to have consciousness of their own.

But as the field advances, scientists will need to be overseen, experts say.

“Critical questions concern whether an organoid can have consciousness and moral status,” write human organoid researchers J Gray Camp and Barbara Treutline in an article accompanying the publication.

“An active discourse is needed,” they write, “to develop frameworks and frontiers for research using organoids to model the circuits of the human brain.”

Human brain organoids in primates?

An obvious next step could be to introduce human brain organoids into primates.

Their brains are large enough to hold much more human brain tissue and live much longer than rats, allowing brain cells to mature.

But this is currently a red line, according to Professor Pasca: ‘Transplantation in primates is not something we would do or encourage.’



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