Scientists are making progress in growing more lifelike and larger masses of tissue: researchers "have created new heart muscle with its own blood supply through the use of human embryonic stem cells. ... Despite progress over the past two decades in treating cardiac disease, there are few good ways to fix damaged heart muscle. One possibility would be to rebuild a broken heart with a transplant of healthy heart tissue. However, scientists have been stymied in these efforts by a lack of human heart tissue to work with and the failure of transplanted tissue to thrive in its new home. ... heart tissue grown by the [researchers] is threaded throughout with a network of tiny blood vessels that would improve the tissue's survival after being transplanted in a human heart ... researchers engineered the heart muscle by seeding a sponge-like, three-dimensional plastic scaffold with heart muscle cells and blood vessel cells produced by human embryonic stem cells, along with cells called embryonic fibroblasts. ... Four to six days after being seeded on the scaffold, patches of the new muscle cells began to contract together, a movement that spread until the entire tissue scaffold was beating like normal heart muscle."