U.S. researchers have found stem cells in amniotic fluid (the liquid that cushions babies in the womb ) of pregnant women, which has many of the same traits as embryonic stem cells, while avoiding their knottiest ethical, medical and logistical drawbacks.

The stem cells, easy to harvest from the fluid left over from amniocentesis tests given to pregnant women, were able to transform into new bone, heart muscle, blood vessels, fat, nerve and liver tissues, a US study says.

The finding points to a promising avenue of research that sidesteps the hurdles facing embryonic stem cell research, which has been stymied by moral objections to the destruction of embryos that occurs when cells are harvested.

The study also suggests another advantage: unlike embryonic cells, which can form tumours when implanted in laboratory animals, amniotic fluid stem cells do not appear to do so, says Dr Anthony Atala, the director of the Institute for Regenerative Medicine at Wake Forest University School of Medicine in North Carolina, and senior author of the report published online by the journal Nature Biotechnology.

However it is still unclear whether stem cells from amniotic fluid can give rise to the full range of cell types that embryonic stem cells can produce, says Dr Robert Lanza, an embryonic stem cell researcher and the head of scientific development at Advanced Cell Technology in Massachusetts.

Two Swiss scientists, Dr Dorthe Schmidt and Dr Simon Hoerstrup, of University Hospital Zurich, have used amniotic fluid stem cells to grow heart valves. They are testing them in sheep.

According to Dr Atala, embryonic stem cells are derived from days-old embryos. Nearly all of the development is still to come, so those cells must be extremely flexible. Amniotic fluid stem cells, which are sloughed off by the developing foetus, are different kind of a stem cell, they lie somewhere between the two major categories of stem cells, embryonic and adult.

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