Photo Credit: The Salk Institute

For the first time, a human mini brain has been implanted into the skull of mice and surprisingly has continued to grow. 

Scientists at the Salk Institute are responsible for the successful implantation of human cerebral organoids into the brains of another species, reports STAT News. The host brain even managed to supply the lentil-sized mini cerebrums with enough blood and nutrients to keep them alive and developing for months. 

This isn’t the last time such an approach is put to use, as the team is excited to use this method to understand human brain development and test whether the tiny entities might one day serve as cortical repair kits, in turn replacing regions of the brain that have been injured or failed to develop normally. 

It’s “an important technical advance,” explained neuroscientist Mical Stachowiak of the State University of New York, Buffalo, who created cerebral organoids to study schizophrenia, and “an important initial step toward using organoids in regenerative medicine.”

However, when the team first revealed their plans last November at the annual meeting of the Society for Neuroscience, they weren’t met with the same excitement. Bioethicists raised questions about what implants of human brain organoids would do to mice’s intelligence, consciousness, and even their own identity as mice. The published paper does nevertheless give details about how successfully the human organoid integrated into the mouse brain and addresses one of those concerns. At least in the tests the scientists ran, the mice with human brain organoids seemed no different and no smarter, than standard lab mice. 

The first human brain organoids were created from stem cells back in 2013, and now researchers are hoping the organoids will be better than lab animals or cells growing in culture at revealing how the human brain develops, both normally and when things go awry and identify potential therapeutic or genome-editing targets. 

The verisimilitude of human brain organoids has been limited, largely due to the fact that once they grow more than a few millimeters across, oxygen and nutrients can’t get to their innermost cells. “In our hands, the organoids stop growing around five weeks,” said Salk’s Fred Gage, who led the study. “It’s a function of size rather than time. We see some cell death even in the edge of the organoids starting at 10 weeks, which becomes really dramatic over time. This is an obvious hurdle for longtime study,” if the goal is to follow brain development for longer than a trimester or two prenatally, the stage at which the current crop of brain organoids start to wither. Thus, implanting human brain organoids in a mouse brain gives them everything they need to grow and develop.

In a bid to do so, the team took human brain organoids that had been growing in lab dishes for 31 to 50 days and implanted them into mouse brains (more than 200 so far) from which they had removed a small piece of tissue to make room. Because the human cells had been genetically engineered to express green fluorescent protein, the tiny blobs showed up in bright lime through the transparent window that the scientists glued into the mice’s skull. 

80% of the implants successfully took. Within two to 12 weeks, the organoids were sprouting additional neurons, including ones found in very specific regions of the human cortex; glia cells including astrocytes; and neural stem cells. By 14 days almost all the organoids had developed a rich network of blood vessels carrying nutrients and oxygen, allowing them to survive for up to 233 days. Their structure and cell maturation were that of a newborn. 

In addition, the implanted organoids were also sending axons – the biological wires that carry brain signals from one neuron to another – into both sides of the mouse brain, not only the side with the window, and forming such strong synapses with mouse neurons that the neural activity of the two species was synchronised. 

“When neurons fire together, that means they are connected and [engage in] cross-talk,” added Abed Mansour, the paper’s lead author, forming “neuronal networks.”

However, even after all the human-mouse mixing, the rodents were no better learners than those with mouse-only brains. The scientists put the mice into a circular platform with 20 holes around the edge, guiding them to the one hole that led to an escape tunnel and then letting them try it themselves to test their memory. On the first day of testing, the mice with human brain organoids made fewer mistakes, being able to find the right hole more often, but this ability disappeared by the second day.

“We are just scratching the surface” in terms of behaviour tests, Gage concluded. “I think it’s early to provide solid answers to this important question.” The escape-hole is very specific, he added, “I can’t exclude at this point other behavioural impacts.”