Atsma notes, however, that angina is very different from a heart attack. "People who've just had a heart attack need instant methods to remove blockages, as tissue starts to die and all sorts of inflammatory processes are being unleashed," he says. "It may be that cell injections are best suited to the treatment of chronic heart conditions such as angina. In using this method to treat angina, you are treating the underlying cause of a chronic condition over weeks and months."

Atsma hopes that follow-up research will demonstrate that angina patients who receive the treatment are less at risk from associated conditions, particularly abnormal heartbeats, or arrhythmias, which kill thousands every year. He says that the small but significant improvement in blood supply seen in the placebo group suggests that the act of pricking the heart lining may encourage growth of new blood vessels.

The researchers are now investigating which types of bone-marrow cell best help repair the heart. One theory is that a type of progenitor cell called a mesenchymal cell, which gives rise to muscle and bone tissue, might encourage the growth of new blood vessels by releasing growth factors. "But the truth is we don't really know which cells--or which combination of cells--are having the effect," Atsma says.

Stefan Janssens, a cardiologist and research scientist at University Hospital Gasthuisberg, in the Netherlands, is also investigating bone-marrow treatment for heart disease, and he says that the study succeeded in demonstrating that cell injections increase blood supply to the heart. "This is a good study, and it's important that it's randomized and double-blinded," he says.

But Janssens also notes that it sheds little light on the exact mechanism of the treatment. "The injected cells are probably encouraging the growth of new blood vessels indirectly by producing growth factors. But it's not possible to rule out that they are improving the condition of old vessels."

Janssens says that boosting blood supply alone would not help the huge number of patients with heart failure, a deadlier condition in which injured hearts swell and pump blood less effectively. "For these patients, we need to know which cells will allow them to grow new heart muscle," he says.