A new understanding of the genes that make muscle cells may change the way researchers think about stem cell transplants for muscular dystrophy and muscle injuries, U.S. researchers said on Wednesday.

In a surprise finding, they said genes important for forming muscle cells in embryos and newborns are not normally active in adult stem cells.

And researchers hoping to use muscle stem cells in stem-cell transplant therapies should not assume genes that control early muscle development serve the same purpose in repairing adult muscle, Christoph Lepper and colleagues at the Carnegie Institution in Baltimore reported in the journal Nature.

Earlier studies have shown that two genes — Pax3 and Pax7 — control cells that give rise muscle in embryos, and Pax7 also helps build muscle in newborn mice.

To get a better understanding of their function, Lepper and colleagues studied these genes at various stages of development in live mice.

"I thought that if they are so important in the embryo, they must be important for adult muscle stem cells," Lepper said in a statement.

The team used genetic engineering to suppress both the Pax3 and Pax7 genes in adult muscle stem cells, and they found that adult stem cells were still able to function normally.

"I was totally surprised to find that the muscle stem cells are normal without them," Lepper said.

The researchers then looked at whether the same was true in injured muscles, when muscle stem cells go to work making new muscle tissue.

To study this, they injured mouse leg muscles between the knee and ankle, and found the muscle stem cells were able to make new muscle, even without the two key embryonic muscle stem cell genes.

The team said the embryonic muscle cell genes appear to only be active in mice within the first three weeks after birth. After that, they believe the genes go quiet and allow a different set of genes to take over.

Finding those genes will be important as scientists pursue new treatments for diseases like muscular dystrophy, a genetic, degenerative disease that affects voluntary muscles, they said.

And they said teams should look at other types of stem cells to see how age might affect their properties, and they should take age of stem cells into account in transplant-based treatments.