A patient who was partially paralysed as a result of an injury to the spinal cord has become the first person to be injected with millions of stem cells derived from early human embryos created by IVF.

Geron Corporation, based in Menlo Park, California, said that it has enrolled the first of several patients in a pioneering study of embryonic stem cells. The phase one clinical trial will attempt to assess whether the novel treatment is safe, rather than effective.

Embryonic stem cells have the proven ability to develop into any of the 200 or more specialised tissues of the body, from insulin-making pancreatic cells to the nerve cells of the brain. Scientists believe they could be used to treat many incurable conditions, from spinal injury to Parkinson's disease.

However, there are concerns that the reality may not live up to the hype. As yet, there has been little clinical demonstration that human embryonic stem cells are safe, let alone effective, with concerns that they may lead to cancerous tumours.

Early in 2009, Geron was given a licence by the US Food and Drug Administration (FDA) to carry out the first clinical trial on spinal cord patients. But later in the year the company had to carry out further tests when the FDA became concerned about the growth of cysts in some laboratory animals injected with embryonic stem cells.

Having resolved these difficulties, Geron said it has now enrolled its first patient, who was injected with embryonic stem cells into the spinal cord within 14 days of the injury, a precondition of the licence. Researchers believe that stem cells have the best chance of repairing damaged nerves if the cells are injected as soon as possible after the injury has occurred.

Most spinal cord injuries are the result of bruising to the delicate tissues rather than severance of the nerves. The most common causes are car accidents, falls and sports injuries.

Thomas Okarma, the president and chief executive of Geron, said the start of the trial involving the injection of clinical grade embryonic stem cells into patients with spinal cord injury marks a milestone in stem cell therapies.

"When we started working with human embryonic stem cells in 1999, many predicted that it would be a number of decades before a cell therapy would be approved for human clinical trials," Dr Okarma said.

The patient, who has not been named, is attending the Shepherd Centre, a 132-bed spinal cord and brain injury hospital and clinical research centre in Atlanta, Georgia. The Shepherd Centre is one of several medical institutions in the US that have agreed to be part of the enrolment plan for Geron's clinical trial.

Donald Peck Leslie, medical director of the Shepherd Centre, said: "We are pleased to have our patients participating in this exciting research. Our medical staff will evaluate the patients' progress. We look forward to participating in clinical trials that may help people with spinal cord injury."

Professor Sir Ian Wilmut, director of the MRC's Centre for Regenerative Medicine at the University of Edinburgh, said: "This is very exciting news, however, it is very important to appreciate that the objective at this stage is to confirm first of all that no harm is done to patients, rather than to look for benefits. Once that has been confirmed then the focus moves on to development and assessment of the new treatment."

What are stem cells, and should we be using them?

What are embryonic stem cells?

Cells derived from IVF embryos that are a few days old. The cells have the power to develop into any of the 200-plus specialised tissues of the body, such as heart cells or the nerve cells of the brain.

Why do doctors want to use them?

Embryonic stem cells could be used to mend damaged tissues and organs by replacing the specialised cells that have been lost. It could revolutionise the way medicine is practised in the 21st century, leading to new treatments for a range of incurable disorders, from heart disease to Parkinson's.

Are there any ethical objections?

People who believe that human embryos are individuals with souls object to them being used in this way. Some religious groups, notably the Catholic Church, have been vocal opponents of such research and the former US President George W Bush imposed a moratorium on government funding of embryonic stem cell research.

Are there any scientific objections?

Some scientists are wary about potential cancers that could be generated when stem cells are injected into patients. This is one of the reasons why it has taken so long for the first clinical trial to go ahead.

Why are there no similar clinical trials in Britain?