Small chunks of human brain tissue transplanted into rats have just passed an important milestone towards a new way to heal severe brain injuries.
Not only did the transplanted human mini-brains integrate with the surrounding rat brain tissue — the neurons in the organoids began to respond to visual stimuli: black-and-white images and lights shone in the rats’ eyes.
And that within three months.
“We didn’t expect this level of functional integration so early on,” says physician and neurosurgeon H. Isaac Chen the University of Pennsylvania.
“There have been other studies looking at single cell transplantation that show that even 9 or 10 months after you transplant human neurons into a rodent, they’re still not fully mature.”
The splicing of parts of human brains, called cortical organoids in this case, into rodent brains is becoming increasingly sophisticated. First, they were single neurons; Recently, scientists have successfully transplanted human cortical organoids into human brains baby rats And adult mice that connected to the surrounding tissue and showed signs of functionality.
Now, Chen and his team have taken the next step: transplanting human brain tissue into adult rats with large cortical injuries to see if they, too, can show functional integration.
“We focused on not just transplanting single cells, but actually transplanting tissues,” Chen says.
“Brain organoids have an architecture; they have a structure that resembles the brain. We were able to look at individual neurons within this structure to gain a deeper understanding of the integration of transplanted organoids.”
Let the human grow mini brainsthe researchers used induced pluripotent stem cells genetically engineered to express green fluorescent protein.
Induced pluripotent stem cells are generated from adult stem cells that have been reverse engineered into an embryonic, undifferentiated state; that is, they can develop into many different cell types. The green fluorescent protein gives the organoids the ability to fluoresce.
These stem cells were grown into human neurons over the course of about 80 days, which developed into small organoids. Once the organoids were grown, the researchers set about transplanting them into the brains of 10 adult male rats.
The researchers first created a cavity in each rat’s brain the size of the organoid, about 2 millimeters in diameter; This cavity represented severe brain injury. Once the cavity was created, the organoid was inserted and the rats were sutured and healed.
To see how the organoid integrates into the brain after healing, the researchers injected the rats with fluorescently labeled eyes viruses traveling along their synapses. They were then able to trace the neural connections from the rats’ retinas to the organoids transplanted into the brain.
Then, while the rats were shown blinking lights and images consisting of alternating black and white bars, the researchers used electrodes to probe activity within the organoid. About 25 percent of human neurons responded to the light stimulation.
“We saw that a large number of neurons within the organoid responded to specific directions of light, giving us evidence that these organoid neurons were not only able to integrate into the visual system, but also very specific functions of the visual cortex,” Chen says.
The experiment was limited to three months due to the limitations of the immunosuppression needed to keep the rats’ bodies from rejecting the human tissue. At the end of the experiment, the rats were euthanized.
Because of this short time, it was possible that human neurons were not yet fully mature. This could explain why the neurons’ responsiveness wasn’t higher, the researchers say.
However, the results are promising for this line of inquiry and can be used to design and refine future experiments. The team recommends using genetically immunosuppressed rodents for longer-term studies.
“Neural tissues have the potential to rebuild areas of the injured brain,” says Chen.
“We haven’t worked everything out yet, but this is a very solid first step. Now we want to understand how organoids can be used in other areas of the cortex, not just the visual cortex, and we want to understand the rules for controlling how organoid neurons integrate into the brain so that we can better control and speed up this process. “
The research was published in cell stem cell.
