Mesenchymal Stem Cells
Are Not Stem Cells

The accepted dogma of the day is that pluripotent stem cells that can make many different tissues can be induced to form bone. This process is called osteoinduction.

As the dogma goes, mesenchymal stem cells (MSCs) that have the ability to form bone, muscle, fat, etc. can be directed by molecules such as BMP-2 to form bone. However, mesenchymal stem cells are not stem cells, they are not pluripotent, and they do not form any type of tissue.

The stem cells that do exist in our bodies are tissue specific. This means, the stem cells in a tissue will only produce that type of tissue and no other type of tissue. A bone stem cell can only form bone and as such, osteoinduction appears to be an inappropriate term. Any compound that stimulates osteogenesis will stimulate both the bone stem cell and the osteoblast to make bone. Stimulating bone formation is complex but, in my opinion, it is not a two-part process but one single metabolic process.

So, if you are thinking about having fat stem cells grafted into your knee to repair bone and cartilage, they can only produce fat. If you are thinking about having mesenchymal stem cells grafted into your knee, they will not produce any type of tissue at all. Mesenchymal stem cells play an important part in the regenerative process, but they require in-situ regenerative cells to provide any benefit.

We have known that freeze dried bone allograft was not osteoinductive or stimulated osteogenesis in humans for 20 years, but because it was a compelling story for the promotion and use of these materials, our profession has chosen to ignore the facts. Old dogmas die hard, but as we move past them we find an exciting future on the other side. The following is an open source article for anyone interested in gaining a better understanding of bone regeneration.

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American Society for Bone and Mineral Research (ASBMR)

Tissue Engineering and Regenerative Medicine International Society (TERMIS)