Adult stem cells form blood vessels. (Retina Rescue).

By using stem cells derived from bone marrow, two groups of scientists have induced the growth of new blood vessels in the eyes of mice. The marrow cells zeroed in on spots in the eye where blood vessels had been damaged.

Replacing these vessels with new ones presents a possible therapy for people, says Martin Friedlander of the Scripps Research Institute in La Jolla, Calif. Or, ironically, the stem cells might work best as delivery vehicles for compounds that would halt the excessive growth of poor-quality blood vessels, a common problem in damaged eye tissue, he says. Both these approaches might lead to treatments to prevent blindness in elderly people and patients with diabetes.

To establish that the marrow cells--also called adult stem cells or endothelial precursor cells--can colonize the eye, Friedlander and his colleagues first transplanted stem cells from an adult mouse into the eyes of newborn mice. Some were healthy; others had a condition in which blood vessels degenerate in the retina, the light-sensing portion of the eye. In both groups, the transplanted stem cells grew into a high-quality web of blood vessels, Friedlander says.

Next, the Scripps researchers blocked blood flow to a portion of the eyes of several adult mice. The blockage mimics the deterioration seen in people who have diabetic retinopathy and in elderly people who have the so-called wet form of age-related macular degeneration. Stem cells transplanted into the mouse eyes formed a web of functional capillaries at the injury site.

The team's report is scheduled to appear in the September Nature Medicine.

In a separate study, Edward W. Scott of the University of Florida in Gainesville and his colleagues also took marrow stem cells from adult mice and intravenously injected them into other mature mice. The cells sought out damaged eye tissue and built functional capillaries there. Even mice given a single stem cell formed some vessels traceable to that cell, the researchers report in the June Nature Medicine.

"It's really amazing that these cells know where to go," Friedlander says. The stem cells may be attracted to proteins produced by cells at an injury site, he says.

John S. Penn, a molecular biologist at Vanderbilt University in Nashville, says it's "astounding" that bone marrow cells can grow into a new, improved vascular system in a damaged retina. But he says the key advance in these studies could be the observation that marrow stem cells can navigate their way to trouble sites in the eye.

In diabetic retinopathy and the wet form of age-related macular degeneration, the body reacts to disrupted retinal blood flow by building a rash of fragile, leaky vessels that do more damage than the initial injury (SN: 10/2/99, p. 215).

Drugs being developed to stop blood vessel growth, or angiogenesis, may provide a tool to contain such leakage, Penn says. He notes that in one experiment, Friedlander's team used stem cells to deliver angiogenesis inhibitors to eye tissues.

Ideally, Friedlander says, genetically engineered stem cells would build a new web of functional vessels even as the cells deliver inhibitors that shut down leaky ones.