The techniques pioneered by the winners of this years Nobel Prize in medicine, John B. Gurdon and Shinya Yamanaka, have already allowed scientists to generate stem cells and clone animals.

But it is the potential these discoveries hold that truly boggles the mind. If stem cells the primitive cells that develop into tissue like skin, blood, nerves, muscle and bone can be harnessed, the belief is they can be used as a repair kit for the body.

In theory, a few skin cells could be harvested to rebuild a spinal cord damaged by trauma, to replace brain cells destroyed by dementia, to rebuild heart muscle damaged by a heart attack or to grow a new limb ravaged by diabetes. It is the stuff of science fiction, so close we can taste it.

But these dreams of miracle cures must be tempered with a strong dose of realism.

Despite billions of dollars in investment in research, from government agencies and biotech companies, there is little evidence that stem cell therapies work.

Yes, some hearing has been restored in gerbils and there have been modest improvements in paralyzed lab rats using stem cell treatments, but these are baby steps. In humans, the gains have been far more modest.

We can treat some forms of cancer, like leukemia and multiple myeloma, with stem cell transplants. But this is simply a refinement of an earlier technique, bone marrow transplant. And to perform such a transplant, the immune system must, for all intents and purposes, be destroyed a punishing regime with a significant mortality rate.

It is a far cry from the notion of an injection of magic stem cells that allow people to walk again or restore their memories.

The International Society for Stem Cell Research says that while there are hundreds of conditions that can purportedly be treated with stem cells, the treatments that have actually been shown to be beneficial are extremely limited. Aside from the cancer treatments mentioned above, some bone, skin and corneal conditions have been treated by grafting stem cells, growing them in the lab and transplanting them.

But in all these cases, the stem cells are tissue-specific, meaning the cells are carrying out a function they were designed to do. This is very different from the notion that undifferentiated stem cells can be used to treat a broad range of conditions.(And we wont delve into potential problems, such as rejection and the concern that stem cells could grow out of control and cause cancerous tumours.)

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Stem cell therapy a miracle cure? Not quite yet