WASHINGTON (CNN) -- Researchers have announced that they have successfully grown human stem cells in a laboratory, a major advance that could one day help in organ transplantation, gene therapy and treatment of such maladies as paralysis, diabetes and AIDS.

Stem cells are blank cells that can develop into virtually any kind of cell in the human body. Most cells have a specific function -- liver cells, skin cells, brain cells and so forth -- and once they have taken on this function, in a process called differentiation, they can't be adapted for any other function. Stem cells, however, have not gone through the differentiation process.

By isolating stem cells in a laboratory, scientists theoretically could grow new heart cells to repair damage from heart attacks, new liver cells to treat hepatitis and new red blood cells for cancer patients.

However, any such use of stem cells is at least a decade in the future because scientists still don't know how to customize these blank stem cells and turn them into cells with a specific function.

According to the lead researchers, James Thomson of the University of Wisconsin and John Gearhart of Johns Hopkins University, stem cells could potentially be used for such things as:

• Growing nerve cells to repair spinal injuries and restore function to paralyzed limbs.
• Growing heart muscle cells to replace useless scar tissue after a heart attack.
• Making brain cells that would secrete dopamine for the treatment and control of Parkinson's disease.
• Growing cells that make insulin, creating a lifelong treatment for diabetes.
• Growing bone marrow to replace blood-forming organs damaged by disease or radiation.
• Making blood cells genetically altered to resist specific disease, such as HIV, to replace diseased blood cells.

Stem cell research holds enormous promise
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Thomson cautioned that this doesn't mean that scientists will be able to grow a liver in a lab dish any time soon.

"We are not taking about making whole organs, but we are talking about repairing organs," he said.

According to researchers, many diseases such as Parkinson's disease and juvenile diabetes result from the death or dysfunction of just one or a few cells. Replacement cells, then, could offer "lifelong treatment," they wrote in an article to be published in the journal Science.

Thomson says huge banks of frozen stem cells could be established one day, each tissue-typed in the same way that organ donations now are. Such a bank of cells could be used not only to repair organs but to test new drugs.

The cells could also be used for basic scientific research in human development.

To isolate the stem cells, researchers took sperm from a man and an egg from a woman and fertilized them in a petri dish for several days until they became a ball of cells called a blastocyst. They then took special cells within the blastocyst and cultured them to produce the stem cells.

The research, however, may raise a number of ethical considerations. In one experiment, couples were asked for permission to use eggs and sperm they had donated. But in another experiment, researchers used cells from aborted fetuses.

Indeed, while scientists long ago isolated stem cells from mice and other laboratory animals, work on human stem cells has been slowed by opposition from anti-abortion groups who consider blastocysts to be human life.

Medical Correspondent Elizabeth Cohen and Reuters contributed to this report.