Out on a limb: Pioneering scientists grow monkey arms in the lab

In a U.S. laboratory, a monkey arm is stripped down as far as its individual cells. All that's left behind is a bare, frail scaffold.

But that's not the end of the road for this arm. The scaffold is rebuilt with infusions of cells from another being -- be it a monkey, or a human -- which grow and transform the limb.

The aim is ultimately to restore the limb to its fully functional form. But this transformed limb will contain the blood, bones, muscles, cartilage -- and more -- not of the animal it once was, but instead, the animal providing these new cells.

The hope is to eventually use human cells to make limbs that can be transplanted in humans -- and the technology is already being trialled in monkeys.

There are no good options to replace lost limbs," says Harald Ott, director of the organ repair and regeneration lab at Massachusetts General Hospital (MGH), in Boston, who is leading this research.

In the United States, it is estimated that approximately 185,000 amputations occur each year, and more than 2 million people are currently living with limb loss, according to the Amputee Coalition.

The current options for amputees are a diverse range of prosthetics -- incorporating many new forms of technology to help them feel real -- or transplants, when matching donors are available. But with these options come limitations in terms of movement and control. In the case of transplants, the limiting factor is the need for life-long immunosuppressive drugs to stop a recipient's immune system from attacking their new limb. Suppressing immunity in this way opens up the risk of new infections and certain cancers.

Ott's ambitious technique therefore has an ambitious goal -- to one day provide amputees with fully functional limbs that can be transplanted as if they were their own.

The idea is to create limbs made up of cells from the amputee's own body to produce an arm -- or one day leg -- tailored to them and therefore unlikely to be attacked by their immune system.

"If it works out you could regenerate ... on demand," says Ott.