Scientists have transformed stem cells from adult human bone marrow into nerve cells by transplanting them into damaged chicken embryos.
Stem cells could help treat disease
The University of Oslo team hopes the breakthrough could lead to a new source of cells to treat brain diseases such as Parkinson's.
It appeared that the embryos' internal repair mechanism acted on the cells to profoundly change their make-up.
Details are published in Proceedings of the National Academy of Sciences.
Stem cells are master cells with the ability to form different kinds of tissue. But those from adult bone marrow normally produce blood and immune system cells.
However, experiments have suggested it might be possible to coax them into becoming nerves.
Attempts to achieve this have, in the past, been relatively unsuccessful.
In a small number of cases, scientists have managed to identify the molecular hallmarks of neurons - but they have not been able to create properly formed interconnected cells.
However, bone marrow stem cells implanted into chicken eggs developed fully functional physical features. They were also converted at a high rate of about 10%.
Writing in PNAS, the researchers said: "This may open new possibilities for a high-yield production of neurons from a patient's own bone marrow."
The Norwegian team used a micro-surgery technique to cut out a small section of the developing spinal cord within the chicken egg.
Human haematopoietic stem cells (hHSCs) from bone marrow were then implanted into the damaged area.
Stem cells were transplanted into chicken eggs
The eggs were incubated before the embryos were removed, and spinal cord slices containing human cells dissected out and analysed.
Damage to the developing brain and spinal cord of the chicken embryo is automatically repaired through a process called regulative regeneration.
Signals from this repair mechanism provided the crucial instructions to the stem cells, making them switch development paths and become neurons.
In about 60% of the 154 embryos exhibiting regulative regeneration, bone marrow stem cells had integrated into the developing spinal cord.
There were no signs of rejection of the human cells or inflammatory reactions.
Analysis showed it was unlikely the stem cells had fused with their host to produce hybrid human/chicken cells, which would prevent them having any treatment potential.
The researchers hope it will be possible to grow nerve cells in the laboratory by mimicking the cellular signals found in chicken embryos.
Researcher Dr Marie-Claude Perreault said: "The brain and spinal cord do not undergo the kind of tissue repair that is common in many other tissues like skin, bone, and blood.
"There are stem cells in the brain and spinal cord, but evidently they are too few or too limited in capacity to provide a natural repair mechanism.
"Researchers are therefore very keen on finding ways to implant stem cells into the brain and spinal cord to boost repair.
"Bone marrow stem cells from adults are very attractive for this because they can be obtained easily, they are numerous, and they have already been studied and used in clinical treatments for blood and immune disorders for many years."
Professor Richard Gardner, chair of the UK's Royal Society working group on stem cell research and cloning, told the BBC News website: "There are a growing number of situations in which altering the environment of cells that are still able to proliferate has been shown to affect their fate."
He said there was a theory that some cells from most organs and tissues of the body found their way to the bone marrow via the blood during the development of an embryo.
It was possible that these cells may retain the ability to replace, or trigger the regeneration of, cells in their organ of origin.