In a sceptical era, when few people still believe that miracle cures can be poured out of medicine bottles, stem cells still fire the imagination and create hope. Maybe that's because it is not just the patients who want to believe that stem cells may one day enable them to walk again, but also the doctors and the scientists.

There is genuine scientific excitement over the concept of using the body's own cellular building blocks to regenerate damaged or ageing organs. "Regenerative medicine" is the scientists' preferred terminology. And although it is still very early days, the lab work and limited experiments so far carried out in animals – and humans – have not disappointed.

The great hope, which the paralysed former Superman star Christopher Reeve shared and to which he dedicated a fortune, is that stem cells may one day repair a damaged spinal cord and allow paralysed patients to walk again.

It may happen. The Geron Corporation in the US has begun experiments on humans. It has now injected two patients with its stem cell product, catchily codenamed GRNOPC1. So far the company has revealed only that neither patient, injected between seven and 14 days after their catastrophic injury and given just two months of therapy, had any serious side-effects from the treatment. There are all sorts of imponderables, from the size of the dose to the timing and location of the injection to the critical issue of whether the stem cells will survive inside the body, which mean it will be years before we have any clear idea as to whether this is going to work.

But some stem cell treatments have been spectacularly successful, such as the rebuilding of Claudia Castillo's windpipe. Tuberculosis had wreaked such damage that the 30-year-old mother of two was barely able to climb the stairs before an enthusiastic, pioneering international group of doctors chose her to be the lucky subject of a stem cell experiment.

The operation took place in Barcelona, but with input from some of the world's top scientists in London and Italy as well as Spain. They took stem cells from bone marrow in her hip and tricked them into becoming cartilage. Then they seeded them on to a piece of donated windpipe, which was transformed into something her body recognised as one of its own organs. Now she goes dancing.

Castillo's treatment involved adult stem cells. Bone marrow cells are particularly responsive to persuasion to become something else, but the clock can also be wound back on skin cells and other adult cells in the laboratory. Embryonic cells, which have not yet developed into anything, are best of all – removed from the blastocyst, which is the embryo at only a few days gestation when it is the size of a full stop. The disadvantage is the religious and moral dismay such work can excite.

The basic stem cell concept is so simple that it is hardly surprising that they have been touted as a potential cure for so many diseases. There is great hope for Parkinson's disease, but huge caution on the part of reputable scientists – not least since the disastrous outcome of an early experiment in 40 patients in Denver, reported in the prestigious New England Journal of Medicine in 2001. Embryonic nerve cells were injected into the brain, in the hope that they would supply the missing dopamine. Some of the younger patients improved, but 15% of the group had a stunning and distressing response about a year in. They began to writhe, jerk their heads and flail their arms uncontrollably as the foetal cells went into overdrive. One scientists said they were "absolutely devastated".Patients who are in a desperate state easily fall prey to quacks and charlatans who offer injections of "stem cells" that may be nothing of the sort – or if they are, have probably not been differentiated to turn into the type of cell that is needed. Such were the doctors who injected stem cells into patients with multiple sclerosis – at one point offering treatment on the Cork to Swansea ferry, the Guardian discovered, when they were forbidden by the authorities from holding a clinic on the Irish mainland.

But the excitement over the potential of stem cells has not been dampened by such abuse, because those who really know – the scientists in the field – really believe in the cures to come. Sadly for those who need help now, it is going to take a long time, but happily for humankind, the future looks unusually hopeful.