What Are Stem Cells? Types, Sources, and How They Work

"Stem cells" appear in headlines, beauty ads, and clinic menus alike — promising everything from younger skin to repaired joints. Behind the marketing, though, stem cells are a real and genuinely fascinating part of biology. This guide explains, in plain language, what a stem cell actually is, the main types, where they come from, and — importantly — how they are really thought to work, which is often quite different from how they are sold. For the field they underpin, see our overview of regenerative medicine.

What is a stem cell?

A stem cell is a cell with two defining abilities that ordinary, specialised cells (like a skin or muscle cell) do not have:

• Self-renewal — it can divide and make more copies of itself.
• Differentiation — it can turn into one or more types of specialised cell.

Together, these make stem cells the body's built-in repair and maintenance system: a reservoir of unspecialised cells that can step in to replace ones that are lost or worn out. That is the real, established biology. What happens when you take them out, process them, and inject them — the basis of "stem cell therapy" — is a separate and much less settled question, which we come to below.

The main types of stem cells

Not all stem cells are the same, and the differences matter a lot:

• Embryonic stem cells are pluripotent — they can become virtually any cell in the body. That power also makes them ethically and legally complex, and they are essentially a research tool, not something used in a wellness clinic.
• Adult (tissue) stem cells, including mesenchymal stem cells (MSCs), are multipotent — they can become a limited range of cell types and mostly maintain the tissue they live in. These are the cells regenerative clinics actually work with.
• Induced pluripotent stem cells (iPSCs) are ordinary adult cells "reprogrammed" in a laboratory back to an embryonic-like state — a Nobel-winning breakthrough, but still overwhelmingly a research technology.

Where do adult stem cells come from?

The MSCs and adult stem cells used in research and clinics are harvested from the body's own tissues, most commonly:

• Bone marrow — the classic, well-studied source.
• Adipose (fat) tissue — abundant and relatively easy to access, now a very common source.
• Blood, and other tissues such as dental pulp and skin in smaller numbers.

Because these come from your own body, they are autologous, which is part of why MSCs have a relatively favourable safety profile in research compared with cells from a donor (allogeneic).

What makes MSCs the ones clinics use?

Of all the stem cell types, mesenchymal stem cells have become the workhorse of regenerative clinics for a few practical reasons:

• They are accessible. They can be drawn from your own bone marrow or fat without the ethical and legal complexity of embryonic cells or the laboratory reprogramming of iPSCs.
• They are reasonably safe in research. Being autologous and multipotent (rather than pluripotent), they do not carry the tumour-formation concerns that limit pluripotent cells.
• They are biologically busy. Beyond becoming bone, cartilage, or fat cells, they are active signallers — and that signalling, rather than the cell-into-tissue conversion, is now seen as their main contribution.

None of this means MSC treatments are proven for any given condition — only that, among the options, MSCs are the practical and best-studied cells to work with.

How are they thought to work?

Here is the part that has changed the most — and that the marketing has been slowest to catch up with. The old, intuitive picture was that injected stem cells travel to a damaged area, turn into new tissue, and physically rebuild it. The modern scientific understanding is mostly different: MSCs appear to work largely by signalling — releasing a "secretome" of growth factors and messengers that calm inflammation and nudge the body's own cells to repair — rather than by becoming much new tissue themselves ². In other words, they act more like a coordinator than a building block. We unpack exactly what that means, and what it can and cannot achieve, in our companion guide: what can stem cells actually do?

What this means honestly

The biology of stem cells is real and remarkable. But "stem cells are powerful" is not the same as "this stem cell treatment will fix your condition" — the leap from established science to a specific clinical promise is exactly where the field gets oversold. Patient-education resources from bodies like the ISSCR exist precisely to help people tell credible science from marketing ¹. We take the same stance: explain the science honestly, and keep any clinical use physician-led and offered only after assessment.

What we see at the clinic

People often arrive with a fixed idea — that stem cells will "regrow" something — picked up from an advert. The conversation we have is usually about resetting that picture toward the real biology: these are signalling cells with genuine promise in some areas and a lot of unproven hype in others. Understanding what they are makes it much easier to judge what is realistic.

Common questions

Are the stem cells in clinics the same as embryonic stem cells? No. Regenerative clinics work with adult mesenchymal stem cells (from marrow or fat), not pluripotent embryonic cells, which are a research tool.

Do stem cells turn into new tissue when injected? Far less than people assume. The leading understanding is that they mostly signal — supporting your own repair — rather than becoming significant amounts of new tissue.

Where do the cells come from? Usually your own bone marrow or fat, which makes them autologous and contributes to their relatively favourable safety profile in research.

Are stem cells and exosomes the same thing? Not quite — exosomes are tiny vesicles released by cells as part of that signalling secretome. We cover them in our guide to exosome therapy.

Why are mesenchymal stem cells (MSCs) the ones clinics use? Because they're relatively easy to obtain — from bone marrow, fat, or donated umbilical-cord tissue — they avoid the ethical questions around embryonic cells, and they're versatile signallers. Popularity isn't proof, though: for most conditions, MSC treatments remain investigational however common the marketing.

How are the cells collected and prepared? Typically from a bone-marrow draw or a small fat (liposuction) sample — or from donated cord tissue for donor (allogeneic) cells — then processed and concentrated in a lab before use. We explain the own-cells-versus-donor distinction in autologous vs allogeneic stem cells.

Does it matter whether cells go in by IV drip or injection into the area? Yes, quite a lot — the route changes where the cells actually end up, and most cells given by IV are caught in the lungs rather than reaching a distant target. We cover this in IV vs local stem cell therapy.

Key takeaway

Stem cells are real, fascinating cells defined by self-renewal and the ability to specialise — and the ones used in regenerative clinics are adult mesenchymal stem cells, which are now thought to work mostly by signalling rather than by rebuilding tissue directly. Knowing that is the best defence against the hype. For the practical question of what they can genuinely achieve, read on in what can stem cells actually do?