A Massachusetts company announced today that it has grown large numbers of a rare type of human embryonic stem cell that can transform into blood and blood vessels. When injected into mice, the cells congregated around damaged blood vessels in the eye, heart and limbs, where they seemed to help rebuild injured tissue, according to a report published online today. The company says it plans to request approval by the end of next year to conduct human trials of the cells, called hemangioblasts.

If the cells replace blood vessels in people, they might lead to new treatments for diseases that result from a lack of blood to some part of the body, including heart attack, stroke and the complications of diabetes.

Researchers believe that hemangioblasts exist for a brief time in embryonic development, during which they give rise to the blood-forming tissues of the body. "Theoretically, this should be the ideal cell to use to get vascular repair," says stem cell researcher Mervin Yoder of the Indiana University School of Medicine in Indianapolis, who was not part of the research.

Other groups had discovered hemangioblasts in mouse and human embryonic cells as well as in adult human bone marrow and umbilical cord blood. But they were unable to harvest them in large enough numbers to evaluate the cells' healing properties.

To get more cells, researchers from Advanced Cell Technology in Worcester, Mass., grew clusters of human embryonic cells in a precise cocktail of growth factors and other cell-regulating chemicals that took several years to work out, says Robert Lanza, the firm's vice president of research and scientific development.

After a few days, they divided the clusters into individual cells, a small percentage of which grew into so-called blast colonies of up to 400 million cells, the team reports in Nature Methods. In lab dishes, the blast cells formed tissues that resembled blood-forming stem cells as well as blood vessels, the group found. "We've figured out for the first time how to produce these cells from human embryonic stem cells literally by the billions and billions," Lanza says.

To test the cells' abilities, the researchers injected them into the injured body parts of rodents with diabetes or injuries that cut off blood flow to the retina, heart or hind leg. The hemangioblasts turned up in the damaged blood vessels, and mice that received them survived heart injury in twice the numbers of control mice, the group reports.

Success in mice is no guarantee of success in humans, Yoder says, adding that the cells will also have to be tested against hemangioblasts isolated in other ways. "We really don't know if these cells will be better," he says. At least now they should be plentiful, he says, "and that's exciting."