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Mice injected with human brain cells grow to have 'half human brains' that make them smarter than other rodents, scientists have found.

Researchers claim that giving mouse pups a type of immature human brain cell, known as glial cells, caused their brains to grow differently so they became more human-like.

These human glial cells, which are the support cells of the brain providing it with structure and nutrients, multiplied and grew to replace a similar type of cell in the brains of the mice.

Mice injected with human brain cells grow to have 'half human brains' that make them smarter than other rodents, scientists have found. Human astrocytes (or glial cells, pictured) have around 100 times as many tendrils as mouse astrocytes, which are thought to improve the efficiency of communication

While the mice still had their own neurons - the cells that transmit and store information in the brain - the support cells were almost entirely human, according to the researchers.

Tests in these mice showed that they appeared to have better memories than those that did not have these hybrid brains. It is thought the human cells improved the efficiency of the mice brains.

The results have raised the prospect that it may be possible to make animals smarter by injecting them with human brain cells.

While it might raise fears that scientists may seek to create highly intelligent creatures, like those seen in science fiction films like Deep Blue Sea and Rise of the Planet of the Apes, the researchers behind the work insist the affects would be limited.

They have also raised serious ethical issues about what this 'hybrid brain' research should be used for. 

Dr Steven Goldman, who led the research at the University of Rochester Medical Centre in New York, said they had decided not to put human cells into monkeys.

He said: 'While the human cells are essential making the mice better, there are limits to how far that can go - the neural networks still belong to the mice.

While the mice still had their own neurons - the cells that transmit and store information in the brain - the support cells were almost entirely human, according to the researchers 

'So we are not creating a human brain in the mice, but it seems to improve the efficiency of the mouse neural network that is already there. They do not get any additional capabilities that could be seen as human.

'It can be suggested that we could try this technique in monkeys and apes but we are not trying to make animals more intelligent.'

'We really questioned about what the purpose of doing that would be - it has to justify the question you are trying to answer.'

WHAT IS A GLIAL CELL? 

Some glial cells also perform repairs in the brain, helping to maintain the myelin sheaths that surround neuron

Glial cells are the most abundant cells in the brain and help provide the support structure for the nervous system.

These star shaped cells provide the physical structure of the brain while also helping to supply valuable nutrients and oxygen from the blood supply to the neurons, allowing them to operate.

Some glial cells also perform repairs in the brain, helping to maintain the myelin sheaths that surround neurons.

Astrocytes are the most abundant type of glial cell and these not only tie neurons to their blood supply but also help to regulate the external environment around the cells - helping neurons to communicate with each other.

This allows signals to be sent from neuron to neuron more efficiently and so improves the ability for information to be sent and stored around the brain. Astrocytes are considered essential for conscious thought and memory recall.

Dr Goldman, whose work is published in the Journal of Neuroscience, instead believes mice with these hybrid brains could prove valuable in studying neurological conditions like schizophrenia and testing new treatments for diseases like multiple sclerosis.

To create the mice, Dr Goldman's team and their colleagues at the University of Copenhagen extracted immature glial cells - known as glial progenitor cells - from aborted human fetuses.

These were then transplanted into the brains of mouse pups, where the human cells grew and expanded until they replaced the mouse glial cells.

The researchers found that the human cells grew to become a type of mature glial cell known as astrocytes, which are responsible for providing nutrients and repairing damage to the brain.

While each mouse received 300,000 human cells, these multiplied over the course of a year until they numbered 12 million and had displaced the mouse glial cells.

The researchers wrote: 'After a few months we start to see the glial progenator cells being replaced by the human cells. 

'The mouse glial progenator cells either die off or flee to the outer regions of the brain.

'After a year almost all of the glial progenator cells have been replaced with human cells and so have most of the astrocytes. These have a natural regular turn over in the brain and are replaced by glial progenator cells that mature. So the human cells out compete and replace the mouse cells.

'This ultimately generates mice with humanised brain that supports the neurons.'

The mice were also tested for memory and cognition and those with the human cells were found to be much smarter than their peers.

One test examined their ability to remember a sound associated with a mild electric shock saw the humanised mice freeze for four times as long as other mice when they heard the sound.

This suggests that their memory was far better than in the normal mice.

Human astrocytes are about 10 to 20 times the size of astrocytes in mice and have up to 100 times as many tendrils - thread like structures that help improve the connections between neurons.

Human astrocytes also produce growth factors that have been shown to be important for learning and memory.

'It's like ramping up the power of your computer,' Dr Goldman told New Scientist, .

Human glial progenitor cells grew to replace the mouse's own glial progenitor cells, spreading through the brain. The brain section from a mouse that received the implant shows the human glial cells in green

He added that the precursor human cells transplanted into the mice appear to adapt to the environment in the mouse brain according to the conditions there. 

They believe that hybrid brain mice could allow researchers to study a range of neurological disorders that affect humans but are difficult to replicate in animals.

It would help overcome concerns that results in animal models can often not be applied to humans as the biology that underpins the disease can be different while mice also do not naturally develop some conditions, like Alzheimer's Disease, at all.

Dr Goldman believes that astrocytes may play a key role in many neurological diseases. They have already begun trying to use hybrid human/mouse brains to study a rare and deadly viral infection of the brain called progressive multifocal leukoencephalopathy.

They have also started a new project using the hybrid mice to study schizophrenia.

In their most recent study Dr Goldman and his colleagues injected human cells into the brains of mice who were missing the myelin sheaths that insulate their neurons.

In their most recent study Dr Goldman and his colleagues injected human cells into the brains of mice who were missing the myelin sheaths that insulate their neurons. Pictured are nerve connections in a mouse brain

These mice tended to grow more human cells that specialise in making myelin than the healthy mice, suggesting they were compensating in the affected mice.

This could lead to new treatments for diseases like multiple sclerosis, which is where the myelin sheath is damaged.

Dr Goldman has applied for permission to treat MS patients with glial progenitor cells and hopes to begin a trial in around a years time.

Other researchers have said it was 'amazing' that the implanted human brain cells were still able to work when implanted in the mice given the differences between the species. 

However they have also warned against putting them into other animals.

Professor Wolfgang Endard, from Ludwig-Maximilians University Munich in Germany, said: 'If you make animals more human-like, where do you stop?'