“Waiting for a Supernatural Being” .. Successful Transfer of Human Brain Cells to Mice

New research shows that human neurons transplanted into the brains of rats continue to grow, form connections with the animals' brain cells and help direct their behaviour, raising ethical and scientific questions as to whether it will result in a supernatural being, or an improved rat with unique abilities.

According to a study published in the journal Nature, clumps of human brain cells formed in the laboratory were transplanted into the brains of newborn mice, and it was found that these cells grew and merged with the nerve circuits of the rodents, eventually forming about one sixth of their brains.

The researchers say that these animals can be used to learn more about human neuropsychiatric disorders.

The team used organoids made from skin cells.

These cells were made into stem cells in the lab before being encouraged to form layers of cells similar to those in the human cortex, the folded outer part of the brain that contains areas responsible for thought, vision, hearing, memory and environmental sensing, among other things.

This process took about two months in the laboratory.

The resulting 3D organelles were then injected into the brains of day-old mice through an incision in the skull. The organelles were implanted in the sensory cortex, an area that plays a role in helping the animals sense their environment.

Within four months, brain scans showed that the organelles had grown to about nine times their original size and made up about a third of the cerebral hemisphere.

The cells appeared to have formed connections with rat brain cells and were integrated into brain circuits.

"It's just a very massive step," said Sergio Paschka of Stanford University.

In another experiment, the team specifically released human cells using optogenetics — a technique that involves highlighting brain cells that have been genetically modified to respond to light.

By doing this, the researchers were able to influence how often the mice sought a reward.

Paschka hopes that studying human brain organelles in mice can help researchers understand human diseases.

In another experiment, his team created organelles using cells taken from people with Timothy syndrome, a rare genetic disorder that affects the brain and heart.

These neurons looked different from healthy cells, and they seemed to work differently, too.

The research team is currently testing potential new treatments for the syndrome in these mice.

But the most important question is, are the mice that contain brain cells still mice?

Participating researchers think they are, and Paschka says the animals showed no signs of enhanced cognition or any other human-like behaviors.

Another co-researcher says that the human brain is incredibly complex - much more than the brains of mice, even with a portion of human cells in it.

Julian Savulescu, a bioethicist at the National University of Singapore, who was not involved in the study, commented on the trial, "It's an important step forward in progress toward [understanding] and treating brain diseases."

But evolution also raises ethical questions, he says, particularly regarding what it means to "humanize" animals.

He finds that mice, at least at the cellular level, are not entirely mice anymore.

Savulescu has no ethical concerns about this study though, because only a small portion of the cells were transplanted, and placed in the part of the brain that deals with sensing the environment, rather than consciousness.

But in the future, scientists should be aware of the potential effects of transplanting larger organoids into other areas of the brain. “It raises the possibility of creating an improved rat that may have greater cognitive abilities than a normal rat," he says.

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