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.
While this research is not the first time human neurons have been implanted in laboratory rodents, external, these scientists took that approach to a new level.
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.
The researchers then used skin cells taken from humans and “reprogrammed” them, so they grew into pieces of brain-like tissue.
These are structures called organoids – they are not whole brains grown in dishes, more collections of connected, living cells.
When these organoids were implanted into the mouse brain the cells divided and organised themselves into the animal’s existing brain circuitry, connecting with the rest of the mouse’s brain and spinal cord.
The cortex of the implanted mice is not perfect – normal cortex forms organised, structured layers. And as neuroscientist Dr Ilary Allodi put it, scans of these human-mouse brains look “a bit messy”.
After a few months though, the human cells started to look and function like the outer layer of the mouse’s brain.
About six months after the surgery, scientists put the mice through some basic behavioural tests – observing them as they moved around a small table-top arena.
Pașca said they performed “largely as [the normal] mice did”.
“They don’t have any enhancement,” he added.
Dr Sarah Chan, a reader in bioethics at the University of Edinburgh who was not involved in this research, told BBC News there was “no indication that what’s being created here are mice that can think like humans, or a human brain in a mouse body.”
But she said the study “prompts us to think about what it might mean when we start changing animal cognition”.
“How can we know what it’s like to be one of these mice? And how do we take account of that in the ways that we treat laboratory animals,” she added.
