All agree that the potential is huge. Stem cells have the capacity to develop into any of the specialised cells of the body, thus offering for the first time a means of repairing damage caused by injury, accident or disease.

But the issue is whether we should plunge ahead with clinical trials while stem cells are only partly understood, or hold back and do years of careful scientific study first.

The new trial belongs — with no offence to anybody involved — to the “quick and dirty” school. Medical history is full of examples of treatments that have worked before they were fully understood, from Jenner’s vaccination against smallpox onwards, so there is good precedent for this approach.

Although described as a stem-cell trial, nobody really knows if the material to be injected actually contains stem cells able to develop into muscle cells. Earlier work suggests that there is a benefit from the treatment, but it is hard to prove that this is how it occurs.

An alternative possibility is that the bone marrow cells contain growth factors that encourage the development of new blood vessels in the heart, or even that sticking the needles into the heart produces a beneficial mechanical effect. Frankly, nobody really knows.

To some, this is a call for caution. They argue that the stem cells could develop undesirably, into immune system cells triggering inflammation, for example, or even into tumour cells.

It is true that the German trials give no indication this will happen, but we have not had enough time to be absolutely sure.

Among those arguing for more research is Roger Pedersen, of the University of Cambridge. Professor Pedersen has said that we know little about what triggers cells to differentiate, and that “simple alchemy” will not work: “We’re going to have to do some science.”

On the other side of the argument, Anthony Mathur, senior lecturer and consultant cardiologist at Bart’s, argues: “If we were to wait for the mechanisms of every drug to be worked out, we would be withholding a lot of wonderful treatments that are already in use.”

There are other approaches, such as taking immature muscle cells from the leg and injecting them into the heart. Tony Gershlick, a consultant cardiologist from Leicester, is trying this method, reasoning that cells already differentiated into muscle are more likely to behave as muscle.

The risk of holding back is that people will lose enthusiasm — while the risk of plunging on is that something nasty will happen and stop the field in its tracks.

Gene therapy, greeted a decade or more ago with all the same enthusiasm as stem-cell therapy today, is now under a cloud. It did not work and one patient in the US died as a side-effect of using a virus as a vector to carry the right gene to his lungs.

It would be wonderful if the London trial did produce an easy way to limit the damage done by heart attacks. Medical history suggests it may be wise not to be overoptimistic.

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