Mesenchymal Stem Cells (MSCs) Explained in Plain English

If a clinic is offering you an injection or an infusion, these are almost certainly the cells you would receive. Understanding what they are — and are not — is the single most useful piece of knowledge for evaluating any offer in this field.

What they are

Mesenchymal stromal cells are found in bone marrow, fat tissue, and umbilical cord tissue. In a laboratory dish they can be prompted to become bone, cartilage or fat cells. That laboratory capability is the origin of the entire industry.

The name changed for a reason

They were originally called mesenchymal stem cells. Many researchers, including the scientist who named them, have argued the term is misleading, and the International Society for Cell and Gene Therapy has recommended calling them mesenchymal stromal cells instead.

The reason is that what they do in a dish is not what they do in a body.

In the laboratory, they differentiate into cartilage. In a human knee, injected MSCs do not appear to survive long, engraft, or become new cartilage in meaningful quantities. Most are cleared within days.

What they do instead is secrete signalling molecules — cytokines and growth factors that modulate inflammation and influence surrounding cells. Researchers call this the paracrine effect. It is a real biological activity, and it is a much more modest claim than regeneration.

Where they come from

Bone marrow. Aspirated from the pelvis under local anaesthetic. The best-studied source. MSCs are a small fraction of what is collected, and their number and quality decline substantially with age.

Adipose tissue (fat). Collected by mini-liposuction. Higher cell yield than marrow. Regulatory status in the US is restrictive — stromal vascular fraction is not lawfully marketable for these uses.

Umbilical cord tissue. Donated after healthy births. Younger, more consistent cells, no harvest procedure. Attracts more FDA scrutiny as a donor product.

What they are used for

Approved: nothing, in the United States, for the uses clinics market. No MSC product has FDA approval for orthopaedic, neurological, pulmonary or anti-ageing use. Some MSC products are approved in other jurisdictions for narrow indications such as graft-versus-host disease.

Studied: knee osteoarthritis, degenerative disc disease, graft-versus-host disease, Crohn’s fistulas, and others.

Marketed: a much longer list than either of the above.

What the strongest evidence shows

For knee osteoarthritis — the most tested indication — a 2025 meta-analysis of eight randomised controlled trials in 467 patients found that approximately 63% of pain improvement at six months was attributable to contextual and placebo effects, with the cells contributing "only a modest incremental benefit." Certainty of evidence: low.

That is the best-studied use of MSCs in this sector. Everything else has less evidence than that.

Questions this lets you ask

Now that you know what MSCs are, three questions become much sharper:

1. "Do you expect these cells to regenerate my cartilage, or to reduce inflammation?" An honest clinician says the second. If they say the first, ask for the imaging evidence.
2. "How many viable cells will I receive, and how is viability measured?" Cell number and viability vary hugely and are rarely disclosed.
3. "How long do you expect these cells to survive in my joint?" The honest answer is days.

Sources

1. Contextual effects of mesenchymal stem cell injections for knee osteoarthritis: systematic review and meta-analysis of RCTs. Frontiers in Medicine, 2025.
2. International Society for Cell and Gene Therapy — position statements on MSC nomenclature and clarification of terminology.
3. US Food and Drug Administration — Approved Cellular and Gene Therapy Products.

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