US scientists say they have discovered a new source of stem cells that could one day repair damaged human organs.
The cells came from the fluid surrounding the developing foetus
Researchers successfully extracted the cells from the fluid that fills the womb in pregnancy and then grew them in lab experiments.
The types of stem cell with potentially the most use have so far been derived from specially grown human embryos.
But this has created ethical concerns because the embryos are destroyed in the process.
Opponents say this is tantamount to cannibalism.
Supporters say stem cells offer real hope in treating illnesses like diabetes, Parkinson's and Alzheimer's.
Implanted in mice
Writing in Nature Biotechnology, the scientists said it should be possible to harness the cells' ability to grow into different tissue to treat disease.
However, UK experts had doubts about the feasibility of the technique.
They said gathering amniotic fluid from large numbers of women might be difficult.
Amniotic fluid contains a large number of cells, many of which come from the developing foetus.
The team from Wake Forest University School of Medicine, in North Carolina, extracted these from fluid samples taken as part of unrelated diagnostic tests during pregnancy, then encouraged them to grow in the laboratory.
They found that they had the potential to turn into a wide variety of different cells - the hallmark of potentially useful stem cells.
They then transplanted them into mice, and carried out further tests to look at how they performed in a living creature.
Again, the results were encouraging, with the stem cells spreading and starting to produce key body chemicals in both brain and liver.
Bone stem cells introduced onto an artificial 'scaffold' then implanted into mice also appeared to behave in a similar way to normal bone cells, forming bone even months later.
The conclusion of the researchers was that the amniotic cells were 'pluripotent' - capable of becoming many different cell types, and that they held the potential for treatment - particularly on the child from whose mother they were taken, for whom they are an exact tissue match.
Dr Paolo De Coppi, now of Great Ormond Street Hospital, who worked on the study, said the amniotic stem cells were similar to, but not identical to, embryonic stem cells.
He said: "Our research suggests that for some clinical applications they may work better than embryonic stem cells.
"For example, embryonic stem cells injected into muscle can form teratomas - amniotic stem cells do not do this.
"However, the range of applications for these stem cells may be more narrow than for embryonic stem cells."
Dr De Coppi it might be possible to take amniotic stem cells from a child diagnosed before birth with a problem, and use them to grow new tissue in the laboratory, which would be ready to use to treat the child when it was born.
In theory, it might also be possible to genetically modify a foetus' own stem cells and inject them back into the amnioitc fluid to correct gene disorders.
Professor Colin McGuckin, from Newcastle University, is researching the use of similar cells taken from the umbilical cord at birth.
He welcomed the report, saying that it was 'thorough' and demonstrated the potential of amniotic stem cells.
"The best thing is to have a variety of stem cell sources to provide the best stem cell for patients. Unless researchers do work to demonstrate there are alternatives to embryonic stem cells, the wider public won't understand that.
"It shouldn't be seen as a race between embryonic stem cells and other sources."
However, he said that harvesting amniotic fluid presented particular difficulties in many cases.
"If it is a natural birth, the waters break and they are all over the floor, and you've lost them. In this country, the majority of women give birth naturally, which means that fluid could not be collected.
"You could conceivably gather amniotic fluid during a caesarean section, but that process could interfere with the experience of giving birth."