(CNN) -- Federal regulators have cleared the way for the first human trials of human embryonic stem-cell research, authorizing researchers to test whether the cells are safe to use in spinal injury patients, the company behind the trials announced Friday.

Embryonic stem cells are blank cells found in embryos, which have the ability to turn into any cell in the body.

The tests could begin by summer, said Dr. Thomas Okarma, president and CEO of the Geron Corporation. The Food and Drug Administration has approved the trials, which will use human stem cells authorized for research by then-President George W. Bush in 2001.

The patients will be those with the most severe spinal cord injuries, called complete spinal cord injuries.

"A complete spinal cord injury has no hope of recovery below the injury," Okarma told CNN. "This is significant because it's the first clinical trial of a human embryonic-based product." Watch more on stem cell research »

The primary purpose of the trial will be to see whether injecting these cells into patients is safe, but Okarma said researchers will also look for any signs of recovery. Scientists will monitor the patients for a year after the injections to see if they are regaining any function below the injured point.

"If there is any movement below the injury, they will measure that and record it," he said.

The trials will involve eight to 10 patients who are completely paralyzed below the third to tenth vertebra, and who sustained their spinal cord injury within seven to 14 days. The tests will use stem cells cultured from embryos left over in fertility clinics, which otherwise would have been discarded.

Using the stem cells, researchers have developed cells called oligodendrocytes, which are precursors to nerve cells and which produce a protective layer around nerve cells known as myelin. Researchers will inject these nerve cells directly into the part of the spine where the injury occurred.

Embryonic stem cells are blank cells found in four- to five-day-old embryos, which have the ability to turn into any cell in the body. However, when stem cells are removed, the embryo is destroyed -- which has made this one of the most controversial medical research fields in the past decade.

Federal research funds were prohibited for embryonic stem-cell research until August 2001, when Bush approved spending for research using only already-existing cell lines. Scientists later discovered that fewer than two dozen of those lines were useful for research, but abortion opponents opposed any legislation that would lift Bush's restrictions, and Bush twice vetoed congressional efforts to roll back his rules.

President Obama is expected to loosen the restrictions, which many researchers and advocates have complained severely set back work toward curing disease such as Alzheimer's, Parkinson's and diabetes.

Okarma said Geron did not use any federal funding for its research, and that the Bush restrictions had "devastated the field."

"People didn't think this would happen for another five years," Okarma said. "But it will happen soon, and it would have happened sooner if it weren't for the ridiculous Bush policies."

Geron owns patents on and licenses the procedure to make these stem cell lines. The company spent $45 million of its own to produce everything required to get approval for the upcoming trial, Okarma said.

At least two other companies have said they plan to begin conducting embryonic stem-cell tests in humans, but only Geron has received FDA approval. Another U.S. company, Reneuron, plans to conduct trials involving stem cells taken from fetal tissue in Britain this year. Other companies have developed stem cells from adult tissue, sidestepping the controversy.

In addition to producing stem cells to treat spinal cord injuries, Geron says, it has seven other types of cells derived from stem cells in its pipeline. Okarma said FDA scrutiny was stringent, and that researchers studied the cells in petri dishes and in animals before obtaining permission to test them in humans.

Getting approval was made harder and took longer because the FDA had no other peer-reviewed research outside of Geron's to consult as it reviewed the proposal, he said.

The trials are not expected to start until summer, because many of the preparations could not begin until Geron had FDA approval, Okarma said. Researchers have to be educated about how to use the stem cells, how to prepare their laboratories for the research and how to read all the magnetic resonance imaging (MRI) of the patients in the same way.

"We [also] have to train surgeons how to inject the stem cells," Okarma explained. His company even developed a device that is mounted to the operating table to help surgeons inject the stem cells.

Geron still has to select the four to seven sites for the clinical trials, Okarma said.

The first human embryonic stem cells were developed by Jamie Thomson at the University of Wisconsin-Madison in 1998.

All About Stem Cell Research • George W. Bush