WASHINGTON (Reuters) - Japanese researchers who invented a way to make powerful stem cells out of ordinary cells say they have now found a safer way to do it.

An undated image showing mouse embryonic stem cells stained with a flourescent green marker. REUTERS/National Science Foundation/Handout

Shinya Yamanaka of Kyoto University in Japan and colleagues invented a new way to transform ordinary cells into embryonic-like stem cells called iPS cells, using a ring of genetic material called a plasmid.

Working in mice, they generated induced pluripotent stem cells, or iPS cells, and said they believe the method can work in people, too, and is an important step toward a new field called regenerative medicine.

Stem cells are the body’s master cells, giving rise to all the tissues, organs and blood. Embryonic stem cells are considered the most powerful kinds of stem cells, as they have the potential to give rise to any type of tissue.

But they are difficult to make, requiring the use of an embryo or cloning technology. Many people also object to their use, and several countries, including the United States, limit funding for such experiments.

In the past year, several teams of scientists have reported finding a handful of genes that can transform ordinary skin cells into iPS cells, which look and act like embryonic stem cells.

To get these genes into the cells, they have had to use retroviruses, which integrate their own genetic material into the cells they infect. This can be dangerous and can cause tumors and perhaps other effects.

Last month U.S. researchers did the same thing using a harmless virus called an adenovirus, but the method was not efficient -- and introducing any virus into the body can pose risks.

Yamanaka’s team tried several different methods but eventually looped three of the genes needed into one plasmid and the fourth into another, and transplanted these into cells from a mouse embryo.

The mouse embryonic cells reverted to a stem-like state and began behaving like embryonic stem cells.

Yamanaka’s team said the method was also not as efficient as using retroviruses but said they plan to try their method using human cells.

If it works, some day doctors may be able to make tailor-made transplants to treat diseases in people by removing a few cells, transforming them in the lab and transplanting the new tissue or organs back in.