Vunjak-Novakovic is enthusiastic about the research. "They've made nature work for them," she says. "And they've demonstrated that vascular supply makes all the difference for the functionality of engineered heart tissue."

In some ways, the blood vessels might be more important than the myocytes. "That's the elephant in the room that we don't tend to talk about," says Harvard's Schoen. "Nobody knows if the myocytes are necessary. Perhaps if you can inject something that revascularizes the damaged area of the heart, that might be all you need."

Indeed, in Cohen's study, rats that received a vascularized patch without myocytes also showed improvement in their cardiac function. These myocyte-free patches also integrated into the local tissue and thickened the scar that's left after an infarction. That strengthening alone may relieve some of the stretching of the damaged heart muscle wall and thus improve contractility, Cohen says.

With or without myocytes, the approach is not yet ready for the clinic. "It's a significant research advance that demonstrates an approach to growing vasculature in an engineered tissue," Schoen says. "But we're not significantly closer to making engineered heart muscle patches for patients who have heart disease." For one, the strategy requires two rounds of surgery: one to implant the patch in the abdomen and a second to move it to the heart. And Cohen points out that patients with coronary disease are generally in no condition to tolerate that sort of invasive treatment.

But the model could help scientists better understand the molecular mechanisms that drive vascularization--and that could allow the growth of a ready-made patch with blood vessels in place prior to implantation. Better still, Cohen says, would be a material that could induce regeneration in the heart itself--something she and her colleagues are working on. "I think all these approaches should be technically possible," says Cohen. "We just need to do more good science to find the best one."