U.S. scientists say they have isolated a new type of stem cell, a discovery they believe could someday provide a ready means of repairing or replacing diseased organs in patients.

The researchers have already used these "amniotic fluid-derived stem (AFS) cells," as they've been dubbed, to grow human bone inside laboratory mice, nerve cells that show function and liver cells that secrete urea, a substance converted by the organ from toxic ammonia.

"It has been known for decades that both the placenta and amniotic fluid contain multiple progenitor cell types from the developing embryo, including fat, bone, and muscle," said Dr. Anthony Atala, head of the Institute for Regenerative Medicine at the Wake Forest University School of Medicine.

"We asked the question: 'Is there a possibility that within this cell population we can capture true stem cells?' The answer is yes."

Atala's team isolated the stem cells from amniotic fluid and placental tissue left over from routine prenatal tests used to detect fetal abnormalities — both amniocentesis and chorionic villus sampling, or CVS (the latter involves taking a tiny snip of the placenta for analysis).

They found that about one per cent of all the cells were indeed stem cells, but of a kind not previously identified.

The AFS cells have characteristics of both human embryonic stem cells and adult stem cells. (Stem cells are those cells which give rise to all the different cell types in the body, from those that make up heart muscle to organs like the pancreas to neurons in the brain.)

'It's really coming from a fetus'

"The cell type is totally different, which makes sense because it doesn't come from the embryo and it doesn't come from the adult — it's really coming from a fetus," Atala said in an interview from Winston-Salem, N.C.

The AFS cells grow like human embryonic stem cells, doubling in number every 36 hours in laboratory dishes, but do not form tumours when implanted in lab animals, as embryonic cells can do.

What's more, specialized cells generated from AFS included all three classes of cells found in the developing embryo — called ectoderm, mesoderm and endoderm. Their high degree of flexibility suggests AFS cells are similar to embryonic stem cells, which are thought to be able to generate every type of cell in the body.

So far, Atala's group has been able to prod the AFS cells into forming muscle, fat, bone, blood vessel, nerve and liver cells in the laboratory.

"It's still in its early stages, but long-term our goal would be to develop these cells to provide therapy for patients," said Atala, who has spent seven years working on the project.

If they do prove to have the hoped-for therapeutic value, he suggests stem cells from amniotic fluid and placental tissue — in other words, afterbirth — could be frozen and banked for future use, ideally for donors' offspring, or even across most of the population.

"The nice thing about them is there is a ready supply because in the U.S. there are about four and a half million births per year," he said.

"So you have all of a sudden a readily available resource of cells, which has been a challenge with some cell types."

Statistically, a bank of 100,000 AFS cell specimens would be enough to supply 99 per cent of the U.S. population with a perfect genetic match, Atala said.

Research called interesting

Mick Bhatia, director of the Stem Cell and Cancer Research Institute at McMaster University, called the research "very interesting" in that the AFS cells appear to fulfil the criteria for "pluri-potency" — the ability to give rise to different cell classes.

"But having said all that, in my opinion, it's not completely novel," Bhatia said from Hamilton.

"I think the source is novel, but … other people have shown that there are cells in adult animals — non-embryonic — that seem to when cultured for a period of time have properties of embryonic stem cells."

"It's too early to tell whether this [research] will generate cells that will be as robust, i.e., can you really freeze, thaw and send them to other labs and get them to grow easily?" he said. "It's very complex."

And although harvesting the cells from amniotic fluid gets around concerns about destroying embryos for stem cells, Bhatia said there will still need to be rigorous ethical scrutiny regarding donor consent to use the AFS cells for research — and later, possible therapy.

"It's important work, it's interesting and time will tell."

Atala predicted it would be many years before scientists will know whether AFS cells can be harnessed to help patients — but he remains hopeful.

"We're looking at many different types of therapy that could be useful in the future," he said.

"For example, nerves that could be used for Parkinson's or stroke, [insulin-producing] islet cells that could be used for diabetes, liver cells that could be used for liver transplantation or heart muscle cells that could be used for patients with heart disease."

"There are many different areas of investigation that we are pursuing."