What Is a Stem Cell? A Plain-Language Guide

Nearly every article on this site eventually mentions a "stem cell" of one kind or another — mesenchymal stem cells for joint injections, hematopoietic stem cells for a bone marrow transplant, induced pluripotent stem cells in a research pipeline. But these are not interchangeable things, and the differences between them affect everything from how a therapy is regulated to whether it has actually been proven to work. This guide starts from zero: what a stem cell actually is at the biological level, the handful of major categories you'll encounter, and why the type of stem cell involved matters enormously when you're trying to evaluate a specific treatment claim.

The Two Traits That Actually Define a Stem Cell

Not every "young" or "undifferentiated" cell qualifies as a stem cell. By the definition used across the field — including the National Institutes of Health's Stem Cell Basics resource — a true stem cell must have two properties working together. First, self-renewal: the cell can divide and produce more stem cells, essentially copying itself, for long periods without exhausting its supply. Second, potency (also called differentiation capacity): the cell can, under the right signals, turn into one or more specialized cell types — a muscle cell, a neuron, a blood cell — that carry out a specific job in the body. A cell that can only do one of these two things isn't a stem cell in the strict biological sense. It's this combination that makes stem cells useful as a kind of internal renewal system, and it's also why scientists and clinicians care so much about which stem cells they're working with.

The Potency Spectrum: Not All Stem Cells Can Become "Anything"

One of the most common misconceptions is that "stem cell" means a cell that can turn into any tissue in the body. In reality, potency exists on a spectrum, and where a given stem cell sits on that spectrum changes what it can realistically be used for. At the very top is totipotent, a state reserved for the fertilized egg and the cells produced in the first few divisions after fertilization — these can form every cell type in the body plus the placental and other supporting tissues needed to build an entire organism, according to the International Society for Stem Cell Research (ISSCR). A step down is pluripotent, meaning the cell can become virtually any cell type in the body itself, but not the extra-embryonic structures like the placenta. Embryonic stem cells and induced pluripotent stem cells both fall into this category. Further down the spectrum are multipotent stem cells, which can differentiate into a limited family of related cell types — for example, a blood-forming stem cell that can become red blood cells, white blood cells, and platelets, but not a liver cell or a neuron. Most of the adult stem cells found in your body today are multipotent, not pluripotent — a distinction worth remembering any time a clinic markets an adult-derived cell product as capable of regenerating "any tissue."

Embryonic Stem Cells

Embryonic stem cells are pluripotent cells derived from the inner cell mass of a blastocyst, an early-stage embryo roughly three to five days old and made up of about 150 cells, per Mayo Clinic and NIH descriptions. Because they can, in principle, be coaxed into forming almost any cell type in the body, they've been central to decades of developmental biology research. Human embryonic stem cells were first successfully isolated and cultured in a lab in 1998, a milestone that opened an entirely new field of study — and also one that has long been tied up in ethical and regulatory debate, since deriving them requires the destruction of the embryo. Very few approved therapies today use embryonic stem cells directly; their scientific importance has mostly been in what they taught researchers about pluripotency itself.

Adult (Somatic) Stem Cells: The Body's Ongoing Repair Crew

Unlike embryonic stem cells, adult stem cells (also called somatic stem cells) are present in small numbers throughout many of your organs and tissues right now, and they've been there since before you were born. NIH describes them as functioning like an "internal repair system," sitting relatively dormant until injury, disease, or normal cell turnover calls them into action to replenish a specific tissue. They are generally multipotent — limited to producing the cell types of the tissue they live in — rather than pluripotent. Two examples show up constantly in stem cell therapy discussions. Hematopoietic stem cells (HSCs) reside mainly in bone marrow and are responsible for continuously producing all the cell types found in blood: red blood cells, white blood cells, and platelets. They are also the best-established stem cell therapy in existence — bone marrow and umbilical cord blood transplants using HSCs have been used clinically for decades to treat leukemia, lymphoma, and certain immune and blood disorders. Mesenchymal stem cells (MSCs), by contrast, are typically isolated from bone marrow, adipose (fat) tissue, umbilical cord tissue, or cord blood, and in the body they normally contribute to maintaining and repairing connective tissues such as bone, cartilage, and fat. MSCs are the cell type most often used in the joint and orthopedic injections marketed at private stem cell clinics — a use that sits on much less settled regulatory and evidentiary ground than HSC transplants do.

Induced Pluripotent Stem Cells (iPSCs)

In 2006, Shinya Yamanaka's lab discovered that ordinary adult cells — skin cells, for instance — could be genetically reprogrammed in the lab to revert back to an embryonic-like, pluripotent state, without ever involving an embryo. These reprogrammed cells are called induced pluripotent stem cells, or iPSCs. The discovery earned Yamanaka the 2012 Nobel Prize in Physiology or Medicine, shared with John Gurdon, and it remains one of the most consequential findings in modern biology: it means a pluripotent cell line can, in theory, be generated from any person's own tissue, sidestepping both the embryo-sourcing and (potentially) the immune-rejection issues that come with embryonic stem cells. iPSCs are heavily used in research today — to model diseases in a dish, screen drugs, and study development — and are moving into early clinical trials for a growing number of conditions, though they remain, for now, largely an investigational tool rather than an approved treatment category.

The Stem Cells You Already Have vs. a Lab-Grown "Stem Cell" Product

It's worth pausing on a distinction that gets blurred in a lot of marketing material: the multipotent stem cells naturally coursing through your bone marrow, fat, and other tissues right now are not the same thing as a vial labeled "stem cells" that arrives from a lab or clinic. A therapeutic stem cell product has typically been removed from a donor or from your own body, then processed — isolated, concentrated, expanded in culture, sometimes combined with other cell types or growth factors — before being reintroduced. The FDA's regulatory framework hinges substantially on concepts like "minimal manipulation" and "homologous use" — essentially, how much a cell product was altered from its natural state and whether it's being used to do the same basic job it does in the body. In other words, "it's just your own cells" does not automatically mean a given injection is low-risk, well-studied, or lightly regulated — the details of processing and intended use are what actually determine that.

Why This Matters for Everything Else on This Site

"Stem cell therapy" is not one thing with one safety and evidence profile. Hematopoietic stem cell transplantation is a mature, FDA-approved standard of care for specific blood cancers and disorders. According to the FDA, the only stem cell products currently approved for use in the United States are blood-forming stem cells derived from cord blood, approved specifically for disorders affecting the blood-forming system — a narrow, well-defined use case. Meanwhile, many of the MSC-based injections offered at private clinics for orthopedic, neurological, or other conditions have not gone through that same FDA approval process, and the FDA has repeatedly warned that unapproved regenerative medicine products have been linked to serious adverse events, including infections, tumor formation, and blindness, when marketed for uses lacking adequate safety and effectiveness data. Knowing whether a therapy you're reading about involves HSCs, MSCs, embryonic cells, or iPSCs — and how heavily that specific product was processed — is the first step toward understanding its actual regulatory and evidence status.

Bottom Line

A stem cell is defined by two properties — the ability to copy itself and the ability to become one or more specialized cell types — but that simple definition covers a wide range of very different biological realities, from the totipotent cells of a days-old embryo to the multipotent cells quietly maintaining your bone marrow. Some categories, like hematopoietic stem cells, underpin a decades-old, FDA-approved standard of care. Others, like many MSC-based clinic offerings, involve real and interesting biology but have not been proven safe or effective for most of the conditions they're marketed for, and are not FDA-approved for those uses. Understanding which type of stem cell a given therapy actually uses — and how it was processed — is essential before evaluating any specific claim made about it.