A patient in the United States has become the first person in the world to be treated with human embryonic stem cells.
The development is being described by one researcher as the dawn of a new age in medicine.
The results of the procedure are now being eagerly awaited around the world by doctors and scientists hoping that embryonic stem cells can be used to treat conditions ranging from heart disease to diabetes and blindness.
Professor Chris Mason of University College, London, says if successful, it could be a very significant breakthrough.
"There is no doubt that this is the dawn of the stem cell age," says Mason.
"This is the first time that the really most potent stem cells have been allowed to be used in man. And this is an exciting, exciting day.
"We can only do so much to animals, now's the time to see if they really work in man."
Mason says the treatment is a "morale booster" for clinicians, researchers, and especially patients.
"These are the cells we believe have the most potential," he says.
"They can form all the cells of our body - all 200 different cell types - and they can form them in vast numbers.
"So if we are successful with the therapy it can be really manufactured at the scale required, it's a very significant breakthrough."
The patient has asked to remain anonymous. Not even their age or sex is being released.
But to qualify for the study he or she must have suffered a paralysing spinal cord injury no more than 14 days before being treated.
The embryonic stem cells that were injected had been manipulated so they have become precursors to certain types of nerve cells.
The hope is they can travel to the site of the injury and release compounds that will help the damaged nerves in the cord regenerate.
Tests in animals show that injections of the cells can improve their ability to walk and move after sustaining an injury.
The biotech company Geron Corporation is behind this trial.
It used stem cells taken from embryos left over from fertility treatments.
Dr Stephen Duncan, the director of the regenerative medicine program at the Medical College of Wisconsin, says the patient test is an important milestone.
"We've known from laboratory studies that the potential of human embryonic stem cells is enormous," says Duncan.
"But until you show you can actually translate the laboratory findings into the clinics, you are really speculating that they can be used therapeutically.
"So I think what this trial does is provide this first step towards realising the potential of human embryonic stem cells with a new method to treat patients with a variety of diseases."
The use of human embryonic stem cells is highly controversial and many pro life groups in the US are opposed to this treatment.
Geron Corporation is privately funded and no federal money has been used.
In phase one of this trial doctors will establish only whether the treatment is safe to use.
Geron will need to do more trials in the coming years to assess whether the treatment is effective in repairing spinal cord injuries.
In a statement, Geron's president and CEO, Thomas Okarma, described this trial as a "milestone for the field of human embryonic stem cell based therapies".