The first human clinical trial of a therapy involving embryonic stems cells has been approved to proceed, Geron Corporation announced Friday.

The U.S. Food and Drug Administration had originally approved the study in January 2009 and was scheduled to have begun last summer.

But the plans for the trial came to a halt when Geron discovered that animals in a preclinical study had developed a higher frequency of cysts than had been seen in other studies, the company said in a statement Friday. The FDA placed a hold on the trial. Since then, Geron has developed new markers and assays for the therapy, and conducted another preclinical animal study.

The purpose of this first phase of research in humans is to test the safety of a therapy in patients with spinal cord injury. Candidates for the trial are those with the most severe injuries.

Dr. Thomas Okarma, president and CEO of the Geron Corporation, told CNN in 2009 that scientists would also look for any signs of recovery, in addition to verifying safety, in the Phase 1 trial.

Despite approval in 2009, the plans for the trial came to a halt when Geron discovered that animals in a preclinical study developed a higher frequency of cysts than had been seen in other studies, the company said in a statement Friday. The FDA placed a hold on the trial. Since then, Geron has developed new markers and assays for the therapy, and conducted another preclinical animal study.

If successful, Geron's therapy, called GRNOPC1, may also have applications for Alzheimer's disease, multiple sclerosis, and a fatal neurological disorder called Canavan disease.

The company can't speculate on when the trial will begin, but the hope is to start this year, Geron spokeswoman Anna Krassowska said.

The field of embryonic stem cells has been highly controversial because the research process involves destroying the embryo, typically four or five days old, after removing stem cells. These cells are then blank and can become any cell in the body.