Hundreds of companies across the United States are selling unapproved stem cell treatments directly to patients, raising concern about safety and scams in a fast-growing industry, researchers said Thursday.

The report in the journal Cell Stem Cell found that at least 351 companies across the United States are marketing unapproved stem cell procedures at 570 individual clinics.

The clinics claimed to offer ‘stem cell interventions’ for muscle and bone disorders, heart disease, problems with the immune system, injured spinal cords, and cosmetic reasons, and more.

Sixty-one percent of the stem cell procedures marketed involved fat-derived stem cell interventions, and 48 percent offered bone-marrow-based treatments.

Only one company was found that advertised embryonic stem cells.

Researchers compiled the list by searching online, and said their results ‘should serve as a baseline for future studies of US businesses engaged in direct-to-consumer advertising of purported stem cell interventions,’ according to the report.

‘In almost every state now, people can go locally to get stem cell 'treatments,'‘ said co-author Paul Knoepfler, a researcher at the University of California, Davis, and Shriners Hospital For Children.

Clinics advertising stem cell treatments were most common in California (113 clinics), Florida (104), and Texas (71).

‘This is a marketplace that is dramatically expanding before our eyes,’ said Leigh Turner, a bioethicist at the University of Minnesota and co-author of the study.

Turner said such businesses have entered the marketplace routinely since 2009.

‘Does that mean that people are getting access to safe and efficacious interventions or is there basically unapproved human experimentation taking place?’ Turner asked.

‘Brakes ought to exist in a marketplace like this, but where are the brakes? Where are the regulatory bodies? And how did this entire industry come into being in a country where stem cell-based interventions and the medical devices that produce them are supposed to be regulated by the FDA?’

The US Food and Drug Administration (FDA) has approved the use of stem cells for leukemia treatment -- commonly known as bone marrow transplants -- and some bone, skin and corneal diseases or injuries, according to the International Society for Stem Cell Research.

‘Other stem cell treatments, while promising, are still at very early experimental stages,’ says the society's website.

The study did not delve into potential harms caused by unapproved stem cell treatments.

In January, a US patient sued a US stem cell clinic alleging damage to her eyes from fat stem cell injections, in what Knoepfler said may have been the first such lawsuit of its kind.