Researchers in the US are the first to successfully create hybrid brains of 2 different species using a technique called blastocyst complementation.
According to a new study published in the journal Cell, the team has created mice that can sense odours with their rat neurons, which successfully integrated into part of the olfactory system.
This is the first time an animal has been able to use the sensory apparatus of another to sense and respond accurately to the world.
“These experiments have told us that we are somewhat limited in when and how we can add brain cells to an existing brain,” says lead researcher Kristin Baldwin, a professor of genetics and development at Columbia University.
“If the brain has developed to a certain point, the transplanted cells don’t necessarily wire together appropriately.”
Baldwin and collaborators injected rat pluripotent stem cells into mouse blastocysts – an early stage of embryo development that occurs hours after fertilisation. Pluripotent stem cells have the ability to differentiate into most, or all, cells of an organism.
“You could see rat cells throughout almost the entire mouse brain, which was fairly surprising to us,” Baldwin says.
“It tells us that there are few barriers to insertion, suggesting that many kinds of mouse neurons can be replaced by a similar rat neuron.”
To determine if rat neurons had been integrated in a functional neural circuit, the researchers genetically modified the mouse embryo to either kill or inactivate its own olfactory neurons.
They could then determine whether the rat neurons restored the animal’s sense of smell.
“We hid a cookie in each mouse cage, and we were very surprised to see that they could find it with the rat neurons,” says Baldwin.
However, mice that had retained their own, silenced olfactory neurons were less successful than those whose neurons had disappeared during development.
“This suggests that adding replacement neurons isn’t plug and play,” says Baldwin.
“If you want a functional replacement, you may need to empty out dysfunctional neurons that are just sitting there, which could be the case in some neurodegenerative diseases and also in some neurodevelopmental disorders like autism and schizophrenia.”
Hybrid brains will allow researchers to better understand how brain cells get sick or die, and how to repair and replace parts of the brain.
