Fluorescent image of a section through an adult mouse heart. Muscle cells are shown in red; the outer layer of progenitor cells and the blood vessels which nourish the muscle tissue are green.

Credit: Dr Paul Riley/Institute of Child Health

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PARIS: A new group of stem cells that can give rise to heart muscle cells, known as cardiomyocytes, have been discovered by researchers in the United States.

The stem cells are located in the outermost layer of the heart and could one day play a critical role in regenerating injured heart tissue, the researchers say.

"In heart failure, you lose cardiomyocytes, so the only way to reverse heart failure is to make more of these cells," said William Pu, the study's lead researcher and a pediatric cardiologist at Children's Hospital in Boston, Massachusetts, USA.

The new findings come on the heels of two earlier breakthroughs.

In 2006 scientists identified another cardiac stem cell - marked by the expression of a gene called Nkx2-5 - with the potential to become either heart muscle or cells lining blood vessels in the organ's left-sided chambers.

Gene expression is the process by which information encoded in the DNA of a particular gene is transformed into a protein or RNA, which plays a key role in protein synthesis.

In parallel, other U.S. researchers discovered a related progenitor heart cell - so-called because of its capacity to generate different types of tissue - that produces the same cell types in the right-sided heart chambers.

Pu's study, published online in the British journal Nature, shows for the first time that new heart stem cells can also be derived from a third type of cardiac stell cell, located within the surface of the organ and identifiable through its expression of a gene called Wt1.

The results were independently verified by another team of scientists at the University of California in San Diego, whose research was published in the same issue of the journal.

Pu and colleagues showed that the cells from the heart's outer lining, called the epicardium, can not only metamorphose into cardiomyocytes but also into smooth muscle cells, endothelial cells, which line the interior of blood vessels, and fibroblasts, found in connective tissue.

"If you are going to regenerate tissue, you need to regenerate the whole tissue, not just the cardiomyocytes," Pu said.

The discovery of the new stem cells was accidental. In order to study the role a different gene in the epicardium, the researchers labeled cells in live mouse embryos with red fluorescent protein.

"Unexpectedly, we saw that these epicardial cells were becoming cardiomycytes - it was a lucky observation," said Pu.

The next challenge, he added, is trying to figure out how a progenitor stem cell decides to become a certain type of functioning cell within the heart, and then how to develop methods to trick the stem cells into transforming into the desired tissue.

"We still don't know how we can manipulate these progenitors," he said.