Heart attack victims given injections of cells from their bone marrow showed striking signs of recovery, say researchers.

The scientists from the University of Rostock in Germany believe that the cells may help new tissue to grow within the organ.

When the heart is deprived of blood during a heart attack, heart muscle cells die because of the lack of oxygen.

Even if the patient survives the attack, the heart may never recover to the level it was prior to the attack.

Laboratory experiments have shown that if bone marrow cells are injected into damaged heart tissue, it triggers the growth of fresh blood vessels to supply the damaged region.

Bone marrow contains "stem cells" - master cells that have the ability to grow into many different types of cell.

In the heart tissue, scientists think they are prompted to grow into new tissues, helping the organ compensate for the damage it has suffered.

The German team injected these cells into the hearts of six patients who had suffered attacks.

Because the treatment is unproven, these patients were also given conventional procedures, such as heart bypass operations, to help keep their hearts going.

However, all the patients did well after surgery - and five had unusually good blood flow to the heart.

While this is not proof that the stem cells had anything to do with it, it is encouraging for the researchers.

The next step may be to give the injection on its own to patients who have suffered an attack.

Professor Gustav Steinhoff, who led the team, said: "We have shown that local bone marrow stem cell implantation together with a bypass operation is safe.

"Controlled studies are needed to clarify the role of cell transplantation in myocardial regeneration."

Another team from the University of Hong Kong carried out a similar experiment, also described in The Lancet.

They injected stem cells into eight patients - all of whom had improved heart function three months later.

However, a commentary accompanying the papers, from Drs Roger Latham and Peter Oettgen, from Harvard Medical School in the US, warned against over-optimism by either doctors or patients.

They wrote: "Wherever this field leads, it is likely to follow the well-known pathway of incredible results leading to unrealistic expectations followed by sobering complications and disappointments - and ultimately, cautious optimism."