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The report describes how the cells were harvested from backup amniotic fluid specimens. These had been obtained for amniocentesis, a procedure that looks for genetic disorders before a baby is born.

Similar stem cells were isolated from "afterbirth". This is the placenta and other membranes that are expelled after a baby is born normally.

The potential of stem cells is enormous. In principle a bank of just 100,000 specimens could supply 99% of the US population with perfect genetic matches for transplants, Atala says. There are more than 4 million live births each year in the United States.

Besides being easily obtained AFS cells can be grown in large quantities. This is because the cells divide in two every 36 hours.

They also don't need guidance from other cells. And they don't produce tumours. These can occur with certain other types of stem cells.

Specialised cells made from AFS cells include all three types of cell found in a developing embryo. These are known as ectoderm, mesoderm, and endoderm.

Their great flexibility and potential for growth, means that AFS cells are very similar to human embryonic stem cells. These are able to generate every different type of cell in the body.

The full range of cells that AFS cells can produce is not known yet, said Atala. "So far we've been successful with every cell type we've attempted to produce.

"The AFS cells can also produce mature cells that meet tests of function, which suggests their therapeutic value."

These tests included implanting neural cells from AFS cells into mice that had a degenerative brain disease. The cells grew and "re-populated" the diseased areas. Bone cells produced from AFS cells were successfully used to grow bony tissue in mice. Liver cells were able to secrete urea, which the liver produces from ammonia.

Stem cells can generate a wide range of mature cells. These can replace damaged cells in a human body. This is why scientists are so keen to study stem cells.

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