Advances in tissue engineering augur a day when scientists can build a human kidney or heart right in the lab. But that day may still be decades away. One reason: an organ needs blood, and so far no one’s engineered a complete network of blood vessels. A University of Michigan team, however, has taken an important step in that direction, harnessing the body’s own resources to grow new vessels.

Most researchers have been trying to build blood vessels in much the same way they engineer organs-lining three-dimensional templates with cultured cells and growing them in incubators. But the Michigan team, led by materials scientist David Mooney, built a polymer material impregnated with growth proteins that coaxes the body to do the work. The researchers implanted the spongy material in rats, in dime-sized patches that released the proteins according to a careful schedule. The proteins-which normally play a role in tissue development-stimulated the rats’ cells to migrate into the material and organize themselves into new blood vessels that were as mature as the rats’ original ones.

“While we’re investigating this technology in the context of engineering liver tissue and bone tissue, it may be useful in other situations as well,” says Mooney. He envisions, for example, a patch that could be used to grow new vessels that restore blood flow to heart muscle damaged by a blocked artery-a condition for which about three million people annually undergo coronary bypass operations in the United States alone. MIT biomedical engineer Robert Langer sees similar potential applications in repairing damaged sections of livers or lungs. However, MIT biomedical engineer Linda Griffith cautions that the jump from lab to clinic could be tricky. “Proteins are inherently difficult to work with,” she says, and the precisely timed release of growth proteins so effective in rats may be hard to replicate in people.

Though no time frame for human studies has been set, the University of Michigan has licensed the material to Cambridge, MA-based biotech firm Curis. If further experiments prove successful, researchers could be that much closer to actually growing human organs.