Mini-human brains implanted in mice

Miniature human brains have grown successfully for months in mice – a breakthrough that could help people with neurological diseases such as strokes.

The pin-sized brains were made from stem cells and placed inside the brains of the rodents.

Around 80 per cent of the 200 mice tested survived the operation and within two weeks their brain implants were spawning new neurons. The host brains provided the mini brains with enough nutrients to keep them healthy for months.

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. Biologists have already 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 grafted human stem-cell-based organioids 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 millimetres across – a process which took around five weeks.

When inside the mouse’s brain it gave them everything they needed to grow normally.

The organiods had been genetically engineered to produce a green protein so they could see them inside the mouse’s brain. The team saw not only native blood vessels from the brain implant, but vessels with blood flowing into them – a first for organoids.

“That was a big accomplishment,” says Abed AlFattah Mansour, a Salk research associate and the paper’s first author.