“Breakthrough Study Finds Human Brain Clumps Transplanted into Rat Brains React to Light and Sound”

An image of a rat brain with a transplanted human brain organoid (cell)

Human brain cells implanted in rat brains can respond to visual stimuli such as light, a new study has shown.

Research shows that human brain cell “organoids” — clumps of lab-grown neurons — can integrate into rat brains.

The researchers cultured human stem cell-derived neurons in the laboratory for about 80 days before transplanting them into the brains of adult rats that had suffered injuries to their visual cortex.

The research shows promise for the treatment of injured human adult brains – although whether the organoid grafts can be functionally integrated into the visual system of injured adult brains remains to be determined.

Previous research has shown that we can transplant single human and rodent neurons into rodent brains, and more recently it has been shown that human brain organoids can integrate into developing rodent brains.

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H. Isaac Chen, physician and assistant professor of neurosurgery at the University of Pennsylvania, said, “We didn’t just focus on transplanting single cells, we actually transplanted tissues.

“Brain organoids have an architecture; They have a structure similar to the brain. We were able to look at individual neurons within this structure to gain a deeper understanding of the integration of transplanted organoids.”

In the experiment, the transplanted organoids had integrated into their host’s brain within three months: they became vascularized, increased in size and number, emitted neuronal projections, and formed synapses with the host’s neurons.

The team used fluorescently tagged viruses that jump from neuron to neuron at synapses to detect and track physical connections between the organoid and the host rat’s brain cells.

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Chen says, “By injecting one of these viral tracers into the animal’s eye, we were able to trace the neural connections downstream of the retina.

“The tracer made it to the organoid.”

The researchers used electrode probes to measure the activity of individual neurons within the organoid when the animals were exposed to flashing lights and alternating white and black bars.

Chen says, “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.”

The team was surprised at the extent to which the organoids were able to integrate in just three months

Chen says, “We didn’t expect to see this level of functional integration so early.

“There have been other studies looking at single cell transplantation that show that even 9 or 10 months after human neurons are transplanted into a rodent, they are not fully mature.”

“Neural tissues have the potential to rebuild areas of the injured brain. We haven’t got everything worked out yet, but this is a very solid first step. Now we want to understand how organoids might be used in other areas of the cortex, not just the visual cortex, and we want to understand the rules that guide how organoid neurons integrate into the brain so that we can better control and manufacture this process it’s faster.”#

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