WASHINGTON (Reuters) — Researchers looking for ways to turn stem cells into the types of heart cells they want said they had found the key to making one important type in mice.

They found the cells that give rise to the muscles of the ventricles — the chambers that pump blood out of the heart — in mice and said they will try to use this information to turn ordinary skin or blood cells into this important heart tissue.

These so-called progenitor cells, described in a report in the journal Science, should also lead to better ways to study heart disease and to test drugs, the researchers said.

"This is the beginning of making heart parts for heart disease," said Kenneth Chien of the Harvard Stem Cell Institute in Massachusetts.

"We have the pure cells; they can be expanded, and they can make fully functional strip of muscle."

Stem cells are the body's master cells, giving rise to the other, "differentiated" cells and tissues in the body. They multiply wildly in the lab and live almost forever, which makes them a powerful tool.

When directed correctly, these cells can be made to form heart tissue, bone tissue, blood or other cells. But as they differentiate into these tissues, they lose their immortality and ability to proliferate.

So scientists want to get from patients embryonic stem cells, or cells that resemble them called induced pluripotent stem cells, grow them in the lab and then use them for research and medical treatments.

Being identical or close genetic matches, they would be easy to transplant back into patients.

Chien's team genetically engineered mice that had fluorescent tags in their heart cells that made the right ventricle glow red.

They could then find and isolate progenitor cells in mouse embryos that exclusively gave rise to ventricular muscle — one of several types of muscle cell in the heart.

"That's the type of muscle in the heart we're trying to regenerate," Chien said in a statement.

They used these cells to make batches of tissue that beat as a heart cell should.

Because the hearts of mammals are all very similar, it should now be possible to find the human versions of these cells for study, they said.