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Human Brain Organoids Transplanted Into Rats Respond to Visual Stimuli

By: April Carson

On February 2, a study released in the journal Cell Stem Cell unveiled that when incorporated with rat brains, brain organoids--groups of lab-cultivated neurons--could react to visual stimulation such as glaring lights. This astonishing discovery highlights the potential and power of these sophisticated clumps created in laboratories.

The experiment was conducted by Seung-Wan (Gavin) Oh and his colleagues at the University of California, San Francisco (UCSF). They transplanted human brain organoids created from stem cells into rat brains. The implanted organoids were then tested with visual stimuli such as a flashing light and monitored for their response. To the researchers' astonishment, they found that the organoids similarly responded to these stimuli as a normal human brain would.

Years of research have confirmed that transplanting single human and rodent neurons into rodents' brains is feasible, plus more recently it was proven that human brain organoids can blend with developing rodent brains. But whether these transplanted grafts can functionally link up to the visual system of damaged adult brains remains unknown.

The successful transplantation of brain organoids into rats to form neural networks and respond to visual stimuli suggests the potential for future therapy that could restore vision in people with damaged eyes or optic nerves.

H. Isaac Chen, a physician and Assistant Professor of Neurosurgery at the University of Pennsylvania emphasized that their research went beyond transplanting individual cells - they also transplanted brain tissue to recreate the architecture and structure found in an actual human brain.

To promote successful integration, scientists grew human stem cell-derived neurons in the lab for 80 days before transplanting them into the brains of injured adult rats. Miraculously, within three months after grafting, these organoids became vascularized and started to grow and expand; additionally, sending neuronal projections to their host's brain cells while forming various synaptic connections.

In a remarkable display of integration, the human brain organoids even responded to visual stimuli. When scientists exposed the rats to a flickering light in one eye, they observed responses similar to those seen when using electrical stimulation - indicating that the organoid was likely perceiving and responding to what it saw.

To locate the connections between the rat's brain cells and organoids, our team utilized fluorescent-tagged viruses that hopped from neuron to neuron. According to Chen, "We injected one of these viral tracers into the eye of an animal and were able to trace its neuronal connection downstream from the retina—ultimately leading straight to the organoid." This incredible feat enabled us a more intimate view of how physical interconnections are made up in this system.

Subsequently, the researchers deployed electrode probes to evaluate the activity of individual neurons within the organoid when animals were exposed to bursts of light as well as alternating white and black bars. The team found that the neurons in the organoid reacted to these visual stimuli, with some neurons being more active than others. This indicates the neurons were forming a representation of the visual stimulus, and that the brain organoid is capable of producing responses similar to those in an actual animal.

The team was astonished by the level of integration that took place within a mere three months. Chen expressed, "We didn't anticipate seeing such vast functional integration at this stage! Research from other studies regarding transplantations involving single cells reveals that even after 9 or 10 months post-transplanting human neurons into rodents, they still hadn't matured completely."

The team is excited to see how the brain organoid progresses and is eager to test new stimuli to better understand its functionality. They hope that their research will lead to further developments in this field and help us gain more insight into how our brains work.

Chen emphasizes the remarkable potential of neural tissue to repair and restore damaged areas of the brain, noting that though there is still more development required before it can be fully realized, this early stage presents a solid foundation for future progress.

This research could have major implications for the treatment of neurological diseases, such as Alzheimer’s and Parkinson’s. By understanding how these organoids respond to various stimuli, scientists may be able to uncover new ways to treat neurological diseases or even develop cures.

Ultimately, this work shows great potential in advancing our understanding of the brain and treating diseases that affect it. With the help of continued research, we may soon uncover ways to repair damage to the brain with the help of organoids, thus improving people’s quality of life around the world.

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April Carson is the daughter of Billy Carson. She received her bachelor's degree in Social Sciences from Jacksonville University, where she was also on the Women's Basketball team. She now has a successful clothing company that specializes in organic baby clothes and other items. Take a look at their most popular fall fashions on

To read more of April's blogs, check out her website! She publishes new blogs on a daily basis, including the most helpful mommy advice and baby care tips! Follow on IG @bossbabymav



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