(CNN) Most people think experimental medical procedures are being done elsewhere, perhaps in countries where lawlessness or less supervision rule. However, two researchers who conducted a keyword-based Internet search and analysis discovered 351 businesses selling stem cell interventions from 570 American-based clinics.

Marketing directly to consumers -- in some cases desperate patients in failing health -- most advertised types of stem cell treatments do not fit within the Food and Drug Administration's regulations.

"When you go on the Internet, what comes up, the first thing you see are these businesses," said Leigh Turner, co-author of the study. He added that many sites appear to intentionally blur the distinctions between approved, evidence-based scientific practices and quack procedures. "The websites are well-done, with links to scientific studies and patient testimonials."

"It's a jungle out there," said Paul Knoepfler , co-author of the study and a stem cell scientist, adding that some treatment centers will call what they're doing "a clinical trial" when it's nothing of the sort. Instead, it's simply a for-profit center administering experimental procedures to earn income. "In college, I had friends participating in experimental trials, and they got paid," he said, adding that you should not have to pay to be involved in an official clinical trial.

Stem cells explained

The FDA describes stem cells as having the ability to divide and develop into many different cell types in the body. When a stem cell divides, each new cell can either remain a stem cell or become another kind of cell, such as a nerve cell, a skin cell or a red blood cell. These unique cells may help repair the body by dividing to replenish cells that are damaged by disease, injury or wear. Naturally, they are a hot area of research.

However, the only stem cell treatments explicitly approved for use in the United States are versions of bone marrow transplants or hematopoietic stem cell (isolated from blood or bone marrow) transplants. According to Knoepfler, the FDA considers stem cells "drugs" in most cases. However, unlike other drugs, stem cells consist of living cells that often behave in unpredictable ways that may not be controlled or reversible. They can grow inside your body, move around and change.

To address what they called an urgent need for information about stem cell research in the United States, Turner and Knoepfler performed keyword searches, text mining and content analysis of the websites they discovered.

They found 570 stem cell clinics distributed across the country, though with some clustering. "Hot spot" cities included Beverly Hills, California (18); New York (14); San Antonio (13); Los Angeles (12); Austin, Texas (11); Scottsdale, Arizona (11); and Phoenix (10).

Most of the businesses marketed autologous stem cell-based interventions, using cells derived from the patient's own body. About one in five advertised allogeneic stem cell interventions, using cells derived from someone else. Some clinics marketed both.

The clinics marketing allogenic treatments sourced their cells from amniotic material (17%), placental tissue (3.4%) and umbilical cords (0.6%).

Most of the autologous treatments use fat adipose-derived stem cells. "The fat cell stem cell area is really complicated," said Knoepfler, a professor at the UC Davis School of Medicine, explaining that the clinics tend to use these cells for conditions that have nothing to do with fat -- such as neurological conditions -- and so these interventions are higher-risk.

Some clinics advertise treatments for cosmetic applications, including "stem cell facelifts," "stem cell breast augmentation" and sexual enhancement procedures. Other centers offer interventions for 30 or more diseases and injuries, including popular orthopedic conditions, neurological disorders, degenerative conditions, spinal cord injuries and cardiac diseases. Sometimes, the doctors involved are working in an unfamiliar field; a dermatologist, say, presenting a stem cell therapy for a neurological condition.

"What a lot of us are wondering is: What's going on with the FDA?" Knoepfler said, explaining how he and Turner have emailed and talked with people at some of the clinics. A common reply was, "You can say we are not compliant, but the FDA hasn't contacted us or done anything in that area ... and absent of warning letters from the FDA, we think we're fine."

Ethical dilemma?

According to Turner, an associate professor in the Center for Bioethics at the University of Minnesota, the harm is that often people are not making a choice to participate in research; they don't even know that what they'll be receiving is something that should be deemed experimental. It's easy to imagine people with a mixture of desperation, hope and willingness to try anything to save their lives.

"The people we're talking about are just like us. They're us; we're them," Turner said. "How loose and immoral a marketplace are we willing to allow?"

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CNN contacted the FDA to learn how many people have complained about adverse events or bad outcomes following a stem cell procedure. However, a Freedom of Information Act application is required to learn the number of medical complications reported to the agency.