Experiments Done With Heart Valves
Daniel Q. Haney,
AP Medical Editor

2:16 PM EST; November 7, 1999; Atlanta, GA (AP) -- In search of better spare parts, scientists for the first time have grown heart valves from scratch in a test tube, then shown that they work like nature's own valves -- at least in animals, researchers said Sunday. The approach, called tissue engineering , is intended to create a fresh source of heart valves to replace those that wear out or are faulty from birth. Using the recipient's own cells, researchers hope to construct valves that will grow as the recipient does and work without blood-thinning drugs.

So far, the experiments have been conducted on lambs with the valves grown at Children's Hospital in Boston by Dr. Simon Hoerstrup, who described the results at the annual scientific meeting of the American Heart Association. "What's exciting is the possibility of making valves that are exactly like our own. That's the potential here," commented Dr. Valentin M. Fuster of Mt. Sinai Medical Center in New York City. Heart valves open and shut so blood will flow in only one direction through the heart. When they deteriorate or leak, surgeons replace them with either mechanical valves -- made from metal, ceramics, plastic, Dacron, and other materials -- or valves taken from pigs and other animals. Neither kind is ideal. The animal valves tend to wear out, so they must be replaced. And the mechanical ones, while more durable, can trigger the development of blood clots, so recipients must taking blood-thinning drugs that can cause unwanted bleeding. Young patients with heart defects currently must undergo several potentially life-threatening valve replacement operations as their hearts outgrow their mechanical valves, which is why the Boston researchers are especially interested in valves that will grow with the recipient. The test-tube valves appear to answer the problem, "but, in all likelihood, it will be five years or more before they will be ready for human use," Hoerstrup said.

The scientists start by removing cells from one of the lamb's arteries and growing a mass of the cells in a test tube. Next, they use biodegradable polymers to create a scaffolding in the shape of a valve. Then they attach the cells to the form. Within two weeks in a nutrient-rich culture, the cells multiply and completely envelop the scaffolding, which then decays. Past attempts to make working valves the same way have failed because the valves, while they looked normal, were too weak to withstand the pressure of pumping blood. This time, the researchers pumped fluid through the scaffolding as the valve grew, mimicking the effects of a pulse and conditioning the valve so it developed the strength of a normal valve.

Six animals have had the new valves for about five months. "This is not long enough to evaluate how the valves will hold up over a lifetime, but they seem to be working without the need for blood thinners, although there has been some leakage of blood back through the opening covered by the valves," Hoerstrup said. "We have good reason to think that the valves are growing in the sheep," Hoerstrup said, but he cautioned that much more work needs to be done. Heart valves are constructed of flap-like structures called leaflets. In the lambs, the scientists created three-leaflet copies of the pulmonary valve, which separates the right ventricle from blood vessels that lead to the lungs. Hoerstrup's team hopes to design more sophisticated valves that will better mimic the intricacies of natural valves.