For the first time ever, US scientists have grown miniature human brains in a new species, in the skulls of mice and have suggested the breakthrough could help with stem cell research and give an insight into neurological disorders such as autism, dementia, and schizophrenia.

The pin-sized brains were made from stem cells and placed inside the brains of the rodents. Around 80% of the 200 mice tested survived the operation and within two weeks their brain implants were generating new neurons.

The brain implants survived for an average of 233 days, but began the process of dying much earlier. Researchers hope these tiny implants could be used as cortical repair kits that could replace parts of the brain that have failed to develop normally.

Earlier, biologists had success growing tiny, stem-cell-based brain-like ‘organoids’ in dishes or test tubes. However, now researchers can develop more sophisticated organoid models by transplanting them into rodents. This means they receive sufficient oxygen and other nutrients to grow and develop properly.

Researchers from the Salk Institute implanted human stem-cell-based organoids into a blood-vessel-rich area of the mouse brain. They grew human stem cells, differentiated them into brain cells and let them grow to be just a few millimeters across – a process which took around five weeks. When inside the mouse’s brain it gave them everything they needed to grow normally. The organoids had been genetically engineered to produce a green protein so they could see them inside the mouse’s brain.

“That was a big accomplishment. We saw infiltration of blood vessels into the organoid and supplying it with blood, which was exciting because it’s perhaps the ticket for organoids’ long-term survival,” lead author Abed AlFattah Mansour, a research associate at Salk, said in a statement.

The study can help in the development of cures for brain disorders, speed up the testing of drugs, and even pave the way for someday transplanting healthy populations of human cells into people’s brains to replace damaged or dysfunctional tissue.