WASHINGTON (AP) -- Stem cells taken from bone marrow can grow new blood vessels in the eyes of mice, a development researchers say raises the possibility of treating some diseases that often lead to blindness in humans.

In tests in mice, the stem cells injected into the eye became incorporated into the eye's structure and formed new blood vessels.

If the process turns out to work in humans, the scientists hope to use it to treat eye diseases affecting the blood vessels in the retina. They include diabetic retinopathy and age-related macular degeneration, two leading causes of blindness.

Dr. Martin Friedlander, who headed the research team at the Scripps Research Institute in La Jolla, Calif., said it may be possible to use the process to rescue sick blood vessels or, in modified form, inhibit the growth of abnormal vessels in the eye.

His research will be published in the September issue of the journal Nature Medicine.

Peter A. Dudley, director of the retinal diseases program the National Eye Institute, said it is "extremely interesting" that the team was able to take certain precursor stem cells that can form blood vessels and then target them.

He said it seems reasonable this could lead to human treatments. But he cautioned that the work only involved mice and that many details need to be worked out before moving on to humans.

Dr. John S. Penn, who teaches ophthalmology at Vanderbilt University, said the work adds to the fundamental understanding of biology, adding that the finding that the cells can home in on specific parts of the eye "is pretty cool stuff."

He also cautioned that the work is in mice and much work needs to be done before it can be applied to humans.

Stem cells are a type of cell that can differentiate into many different cells depending on what is needed. They form in the embryo and are also found in adult bone marrow.

Friedlander's team used a type of stem cell called an endothelial precursor cell taken from mouse bone marrow.

When these cells were injected into the eyes of mice, they attached to cells in the retina called astrocytes and then formed new blood vessels.

"What's exciting about this, and surprising to us, is they don't target mature vessels, they go where vessels are going to form," Friedlander said.

Newborn mice, for example, do not have blood vessels in their retina but have astrocytes forming a sort of template for future vessels, Friedlander explained.

In adult mice, he said, if the retina is injured, it encourages the development of astrocytes. By injecting the stem cells, the researchers can help stabilize a degenerating blood vessel system.

Friedlander said he was "flabbergasted" at the improvement when the stem cells were injected into the eyes of a type of mice that have eye degeneration and normally go blind within 30 days of birth.

Friedlander said he believes that because the stem cells target astrocytes, genetically modifying the stem cells before injection may make it possible to block the growth of unwanted blood vessels, which are also a factor in some eye disease.

He also suggested that the cells could be used as a drug delivery system for the eyes, something Penn said would be an exciting development.

Diabetic retinopathy is the leading cause of blindness in working age Americans, and almost all people who have had diabetes for more than 30 years will show signs of poor eyesight.

Age-related macular degeneration is a common cause of vision loss among people over age of 60. Both conditions are caused by damage to blood vessels of the retina.

Copyright 2002 The Associated Press. All rights reserved.