A near-complete human brain comparable with that of a five-week-old foetus has been grown in a laboratory dish.
The brain "organoid" was created from reprogrammed skin cells and is about the size of a pencil eraser.
Scientists hope the lumpy mass of functioning nerve cells and fibres will prove to be a valuable research tool for non-animal testing of new drugs and investigating brain disorders.
As well as neurons and their signal-carrying projections - axons and dendrites - the "brain" also contains support and immune cells. It has 99 per cent of the genes present in the fetal brain, a rudimentary spinal cord, and even the beginnings of an "eye".
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Lead researcher Professor Rene Anand, from Ohio State University, said: "It not only looks like the developing brain, its diverse cell types express nearly all genes like a brain.
"We've struggled for a long time trying to solve complex brain disease problems that cause tremendous pain and suffering. The power of this brain model bodes very well for human health because it gives us better and more relevant options to test and develop therapeutics other than rodents."
The organoid was allowed to grow to the equivalent of 12 weeks in the womb, almost matching the maturity of a five-week-old fetal brain.
"If we let it go to 16 or 20 weeks that might complete it, filling in that 1 per cent of missing genes," said Anand. "We don't know yet."
He spoke about the work at the 2015 military health system research symposium, run by the US Department of Defence, in Fort Lauderdale, Florida.