Stanford scientists transplanted lab-grown human brain tissue into newborn rats to learn how human neurons mature and work inside a living brain. The grafted tissue grew, received sensory information, and influenced learned behavior in controlled experiments.
The 2022 Nature study used cortical organoids, small clusters of brain-like tissue grown from reprogrammed human cells. Organoids mimic parts of the cerebral cortex, the outer brain layer involved in memory, thought, and sensory processing.
Why Transplant the Tissue?
Brain organoids grown in a dish are useful, but they lack blood flow, immune support, and sensory input. Those missing conditions limit how fully the cells can develop. By placing the organoids in living rat brains, researchers gave human neurons access to a more natural environment.
The team implanted the tissue into the sensory cortex of rats two to three days after birth. Because the animals’ brains were still developing, the human organoids could grow with the rats’ own neurons and form connections.
After six months, the grafts occupied about one-third of the implanted brain hemisphere. Rat blood vessels entered the human tissue, bringing oxygen and nutrients. The transplanted neurons also grew larger and produced more complex branches than similar neurons left in a laboratory dish.
What Did the Team Discover?
The grafted neurons became part of active neural circuits. When scientists blew air across the rats’ whiskers, neurons inside the human tissue responded. This showed that sensory signals could travel from the rat’s body to the transplant.
Researchers then used optogenetics, which activates specific cells with light. They paired light stimulation of the human neurons with water. Over time, the rats learned to approach a drinking spout when the light appeared, showing that the transplant could influence behavior through the host brain’s circuitry.
“This is the most advanced human brain circuitry ever built from human skin cells,” said Sergiu Pașca, the study’s lead researcher. “Our platform provides, for the first time, behavioral readouts for human cells.”
The model could help scientists investigate conditions including autism, epilepsy, and schizophrenia. The study did not create human-like rats: researchers found no clear changes in memory, movement, or seizure risk.
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