Stem cells are getting serious. Two decades after they were discovered, human embryonic stem cells (hESCs) are being tested as a treatment for two major diseases: heart failure and type 1 diabetes.

Treatments based on hESCs have been slow coming because of controversy over their source and fears that they could turn into tumours once implanted. They have enormous potential because hESCs can be grown into any of the body's 200 tissue types, unlike the stems cells isolated from adult tissues that have mostly been used in treatments until now.

In the most rigorous test of embryonic stems cells' potential yet, six people with heart failure will be treated in France with a patch of immature heart cells made from hESCs, and 40 people with diabetes in the US will receive pouches containing immature pancreatic cells made from hESCs.

The hope is that the heart patch will help to regenerate heart muscle destroyed by heart attacks. Trials in monkeys showed that the patch could regenerate up to 20 per cent of the lost muscle within two months.

The pancreatic cells are supposed to mature into beta cells, which produce the hormone insulin. These would act as a substitute for the cells that are destroyed by the immune systems of people with type 1 diabetes.

Although treatments based on hESCs have already been given to people with a type of age-related blindness and with spinal paralysis, the latest trials are the therapy's first foray into major fatal diseases. Heart disease is the biggest killer in the world, and cases of type 1 diabetes are growing.

"Both are landmark studies, and are different from what we've had up to now," says Chris Mason, head of regenerative medicine at University College London. "The blindness already being treated is serious, but diabetes and heart failure are killers, and things we don't have solutions for, so this brings hESCs into the mainstream."

Some people with heart disease and diabetes have received experimental treatments based on stem cells isolated from adult tissue, often from bone marrow, with varying degrees of success. These mesenchymal stem cells, or MSCs, can mature into several tissues including muscle, bone, cartilage and fat but there is no guarantee that they will grow into cardiac muscle.

A recent review of 23 trials involving 1255 people with heart disease found that there is some evidence that recipients of stem cell therapy are less likely to die or be readmitted to hospital a year or more after treatment than people who received standard treatment.

The hope is that using hESCs in place of MSCs will improve these outcomes further because they can be grown from scratch into cells exactly suited to their medical purpose. "We think our cells are more committed to the heart lineage," says Philippe Menasch, head of the French trial at the Georges Pompidou European Hospital in Paris.

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Embryonic stem cells to tackle major killer diseases