Steve Connor: Huge potential balanced against ethical fears

It is easy to get confused over stem cells. It doesn't help that there are several different types of stem cell but the one involved in this proposed clinical trial happens to be the most powerful and the most controversial – embryonic stem cells.

Stem cells are cells that have yet to develop or "differentiate" into the mature, specialised cells and tissue of the body, such as cardiac muscle or nerves. But stem cells found in the adult body or human foetus are thought to have a limited ability for development – stem cells recovered from the blood may for instance only develop into blood cells.

Embryonic stem cells taken from early embryos, however, have the proven ability to develop into any of the many scores of specialised tissues and as such they possess immense powers for medical treatment. Scientists envisage that they could be used to mend any broken part of the body, from kidney and liver to brain and eye.

It is already established from work in the laboratory that embryonic stem cells have great potential. But using them in the first clinical trials is fraught with ethical problems, not just about safety but also because many people consider human embryos to be sacrosanct and inviolable.

The stem cells to be used in this trial were derived from a spare IVF embryo and have undergone an exhaustive process of investigation to make sure they are "clinical grade", meaning that they are fit and safe to be used in the treatment of human patients. In this case they will be used to treat 12 people suffering from a form of macular degeneration of the eye, causing gradual loss of vision.

The eye is a good organ to begin such work. For a start doctors can literally see how things are developing following a stem-cell transplant by looking through the "window" of the pupil to study the retina at the back of the eye.

The eye is also good in terms of the possibility of tissue rejection – after all, the embryonic stem cells come from a tissue donor. There is some evidence that, like the brain, the eye will be more tolerant of tissue transplants.

The biotechnology company behind the research, Advanced Cell Technology of Worcester, Massachusetts, is not the only organisation interested in stem cell transplants to treat macular degeneration. Professor Pete Coffey's team at University College London is also believed to be at an advanced stage of applying for a clinical trials licence.