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After more than a decade of waiting, the first results of a trial involving human embryonic stem cells have been published in a medical journal.

The Lancet reports how two women in the USA with eye disease were injected with stem cells and both apparently showed some slight improvement in vision. The company Advanced Cell Technology (ACT) says the patients are doing well four months on from the trial.

This is a significant moment because there has been so much expectation about human embryonic stem cells - which have the potential to turn into any tissue in the body.

It has been a controversial area of research in the United States. In 2001 President Bush imposed restrictions on federal funding for embryo research on moral grounds.

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You can read more about this area of research and about other types of stem cells here.

A previous trial involving spinal patients was abandoned by the US biotech firm Geron. The company said the safety trial had gone well and it had to stop the trial because of financial problems.

But the end result was that the data never made it into a peer reviewed journal.

That means the ACT trial is now the first official human embryonic stem cell trial to have reported.

One of the patients suffers from Stargardt's disease - which leads to progressive deterioration of vision. The other has age-related macular degeneration, the main cause of blindness in the developed world.

Both patients had such poor vision they were registered blind.

They both had retinal cells, derived from human embryonic stem cells, injected into the back of the eye.

After the treatment they showed some slight improvement in vision. You can read more about the trial here.

This has prompted one extraordinary newspaper headline: .

I realise there is a lot of excitement about this area of research, but I would urge caution. The authors of the Lancet paper make no special claims about the study which was designed to show whether the treatment was safe.

They say: "So far, the cells seem to have transplanted into both patients without abnormal proliferation, teratoma formation, graft rejection, or other untoward pathological reactions or safety signals."

In other words the treatment did not appear to do any harm. Of particular concern was the risk of the cells forming tumours, which did not happen. This is enough to give scientists encouragement to continue the trials.

Although the study was simply designed to show safety, the researchers could not ignore the apparent slight improvement in vision of the two women.

But the authors say: "We are uncertain at this point whether any of the visual gains we have recorded were due to the transplanted cells, the use of immunosuppressive drugs, or a placebo effect."

That is crucial and shows why further larger trials are needed. As I've reported before, the European arm of this trial is being conducted at Moorfields Hospital in London.

The first British patient - a man with Stargardt's disease - was treated last week. Again, this was a safety trial rather than an attempt to improve vision.

So there is a long way to go before there's talk of miracle treatments and curing blindness.