Harvesting stem cells from amniotic fluid in the womb could be an easy and productive alternative to the practical and ethical complications of isolating cells from human embryos, US researchers believe.

In a report published online by US journal Nature Biotechnology, a team from Wake Forest University School of Medicine and Harvard Medical School described how they used amniotic fluid-derived stem cells to grow a variety of body cells in the laboratory, including mature cells with therapeutic potential. For example, when neural cells developed from the AFS cell source were implanted into mice with a degenerative brain disease, these cells grew and ‘repopulated’ the diseased areas.

The researchers found that around 1% of the cells in amniotic fluid samples taken for amniocentesis – the procedure used in prenatal testing for genetic disorders – were stem cells with markers matching those in both embryonic and adult stem cells. A similar type of stem cell was isolated from the placenta and other membranes expelled after birth.

AFS cells might represent an intermediate stage between embryonic and adult stem cells, the researchers suggest, adding that they are capable of extensive self-renewal, a defining property of stem cells. Moreover, their therapeutic potential could be broad: so far the Wake Forest–Harvard team has been successful with every cell type it has tried to create from the AFS source, including bone, fat, blood vessel, nerve and liver cells.

Not only were AFS cells readily available, but they can be grown in large quantities, as they typically doubled every 36 hours, the authors noted. According to lead researcher Dr Anthony Atala, director of the Institute for Regenerative Medicine at Wake Forest University School of Medicine, a bank with 100,000 specimens could theoretically supply 99% of the US population with perfect genetic matches for cell transplantation. There are more than 4 million live births in the US each year.

Another potential advantage was that AFS cells do not require guidance from other cells (known as 'feeders'), nor do they produce tumours, which can occur with certain other types of stem cells, the researchers said.

As Atala pointed out, it had been known for decades that both the placenta and amniotic fluid contained multiple progenitor cell types from the developing embryo. It took seven years’ work, though, to establish that some of these progenitors were true stem cells.

While other researchers have cautioned that AFS cells are no substitute for the more adaptable human embryonic stem cells, the technique described in Nature Biotechnology should enable scientists to circumvent the strong ethical objections to harvesting stem cells from embryos that are destroyed in the process. This has particular relevance in the US, where last year President George Bush vetoed a bill easing the tight restrictions on federal funding of stem-cell research.

In the UK, scientists have raised concerns about a proposed ban on ‘hybrid’ embryos – created by implanting human DNA in animal eggs – included in the government’s recent White Paper on overhauling the Human Fertilisation and Embryology Act 1990. They argue that hybrid embryos are a better source of stem cells and are needed to overcome the shortage of human eggs available for research. By Peter Mansell