While the excitement continues to swirl around the recent breakthrough of converting skin cells to stem cells , other researchers are quietly pursuing a new type of stem cell discovered in menstrual blood.

These menstrual stem cells could offer several advantages. They come from a source that's easy to obtain from women, they could be used to treat patients without the fear of tissue rejection, and they avoid the ethical questions associated with embryonic stem cells.

Researchers from Medistem, a biotechnology firm in Tempe, Ariz., reported the discovery of the new stem cells, dubbed endometrial regenerative cells, in this month’s Journal of Translational Medicine. Plans are already under way to investigate whether they can be medically useful.

Thomas Ichim, Medistem's chief of scientific development, said the company is currently conducting animal studies to determine the potential of the stem cells for treating several human disorders, including diabetes, multiple sclerosis and cirrhosis of the liver.

Stem cells come from two main sources: embryos or adult tissues. Embryonic stem cells can give rise to virtually any cell type in the body, but they are controversial because conventional procedures for obtaining them involve the destruction of an embryo. Adult stem cells, such as those found in bone marrow, do not pose the same ethical concerns, but they have limited powers and collecting them can require invasive procedures.

While the new technique of reverting skin cells to an embryonic stem cell-like state promises to overcome the ethical dilemmas, this approach could come with safety concerns that make the cells too risky for use in humans. The technique for converting the skin cells involves using viruses to insert several genes, one of which is known to cause cancer.

Menstrual stem cells could turn out to be a happy medium between embryonic and adult stem cells, providing an ethically acceptable alternative that is readily accessible and appears to give rise to most of the major tissue types in the body, said Ichim.

“Compared with the stem cells from other sources, such as bone marrow and cord blood, [menstrual stem cells] are easier to collect, do not cause any harm or pain to the donor and can be collected for more than 35 years, from 12 years old to 47,” said Xiaolong Meng of the Bio-Communications Research Institute in Wichita, Kan., who collaborated on the research that resulted in the discovery of the new stem cells.

Meng and Ichim's team had a hunch that stem cells may aid in the rapid expansion of the uterus lining during a woman's monthly period. To investigate this, they collected a small amount of menstrual blood from healthy women while they were menstruating. The researchers hit pay dirt, isolating a new type of adult stem cell.

Experiments in lab dishes showed that under the right conditions, the menstrual stem cells could turn into more different tissue types — including bone, blood vessel, fat, brain, lung, liver, pancreas and heart — than other adult stem cells. The new stem cells also grow readily and rapidly, which is an important advantage because it is difficult to get some types of adult stem cells to give rise to enough cells to be of any medical value.

Other stem cell researchers were split on the research, with some calling the results promising and others remaining skeptical.

Dr. Robert Lanza, chief scientific officer at Advanced Cell Technology, a company in Los Angeles developing medical treatments based on stem cells, called the discovery “exciting,” saying the menstrual stem cells appear to have several advantages over other types of adult stem cells.

“For starters, they’re easy to obtain,” Lanza said. “And since, they’re the patient’s own cells, you don’t need to worry about immune rejection, a major problem associated with the use of embryonic stem cells.”

The lack of immune rejection could extend beyond women from whom the cells were initially derived. Ichim says the menstrual stem cells seem to have an immune system-suppressing effect that could enable them to be transplanted into other people without rejection.

Arnold Kriegstein, director of the University of California San Francisco Institute for Regeneration Medicine, was unconvinced about the potential of the menstrual stem cells to transform into all the different tissue types claimed by Medistem. “Some of these claims are overblown,” Kriegstein said, adding that a more thorough study would be required to demonstrate the full capacity of the stem cells.

However, Ichim says Medistem has more data about the menstrual stem cells than they have made publicly available. He said unpublished experiments in animals and lab dishes indicated the cells give rise to different tissue types and show promise for treating disease.

Still, the company will have to convince the FDA of the medical potential of the cells before being allowed to proceed with clinical trials in people. Ichim declined to set a timeframe for when studies in humans might begin, saying that more animal research is required before they can plan for using the cells in people.

Steve Mitchell is a science and medicine writer in Washington, D.C. His articles have appeared in a variety of newspapers, magazines and Web sites, including UPI, Reuters Health, The Scientist and WebMD.