The team transplanted the human brain organoids into rats that were each only a few days old. The tissue was placed in the somatosensory cortex, a region responsible for receiving sensory information and passing it on to other parts of the brain. As the rats grew, the organoids grew too, eventually taking up a third of the somatosensory cortex. Because the rats were so young when they received the organoids, the transplanted tissue was able to form neurological connections with the rats’ natural brains.
The stem cells used to create some of the brain organoids were derived from people with Timothy syndrome (TS), a genetic condition that affects a person’s nervous, cardiovascular, and immune systems and is sometimes associated with autism. The scientists saw that the TS organoids didn’t grow to quite the same size as those derived from people without Timothy syndrome. The TS organoids’ neurons also fired “abnormally” when the rats’ whiskers were stimulated or when they were exposed to blue lights.
Though the procedure is expensive, Pasca and his team believe transplants like the ones they performed could help researchers monitor the development of neurological conditions in a non-invasive way—for humans, at least. Critics of the procedure worry about the ethical implications involved in “editing” rats’ brains in such a way, as well as the possibility of giving non-human animals human consciousness. Pasca’s team says they were careful to ensure the rats behaved normally and didn’t experience disruptions like seizures or memory defects. Meanwhile, some scientists claim brain organoids can’t become conscious in the way full human brains can…but others aren’t so sure.