Part-human part-mouse brain developed in science breakthrough

The research has raised questions about what it means to alter the way laboratory animals think and feel

Neuroscientists in the US have successfully adapted mice to have functioning human cells inside their own brains.

While mice with brains that are partly human might sound like a Kafkaesque experiment, the scientists said these are not "mice that think like humans".

The scientists said their work, published in the journal Nature, external, was carried out with independent ethical scrutiny.

In scans of the implanted mice, researchers were able to see connections between the human brain cells and the rest of the mouse brain

The aim of this ethically complicated breakthrough was to better understand the biology of brain disorders for which there are currently no effective treatments.

As lead researcher Prof Sergiu Pașca from Stanford University explained in a press conference, psychiatry has "one of the lowest success rates for clinical trials".

The Stanford researchers said that, for some complex conditions, including epilepsy, autism and cerebral palsy, it has the potential to be "transformative".

The hope is that this will provide a new way to investigate the biology of some human brain disorders

The human brain is made up of billions of cells, interconnected in millions of circuits, making it difficult to understand its development and what exactly is happening – at the cellular level - when things go wrong.

First, they genetically-engineered mice to develop almost none of their own cerebral cortex – that is the outer layer of the brain sometimes referred to as "grey matter". It handles higher-level thinking, memory and senses.

These are structures called organoids – they are not whole brains grown in dishes, more collections of connected, living cells.