What is a stem cell?
Stem cells are special cells that can make more of themselves and give rise to specialized cells that make up tissues and organs. That dual ability—self-renewal and differentiation—is why they sit at the center of developmental biology, regenerative medicine research, and decades of scientific and policy debate. This page explains what stem cells are, how scientists classify them, and how key discoveries shaped the field. For short Q&As, see our FAQ. For dates and milestones, see the Stem Cell Research Timeline.
The two defining properties
Stem cells are typically defined by two properties:
- Self-renewal — they can divide to produce more stem cells over long periods.
- Differentiation — they can generate specialized cell types (for example, blood, nerve, or muscle cells) under the right signals.
Not every dividing cell in the body is a stem cell. Many specialized cells can divide a limited number of times but cannot both renew a stem-cell pool and produce multiple mature lineages the way true stem cells can.
Potency: how broad is a stem cell’s potential?
Scientists often describe stem cells by potency—how many cell types they can become:
- Totipotent — can form an entire organism (including extra-embryonic tissues). In mammals, the fertilized egg and earliest cleavage stages are totipotent.
- Pluripotent — can form essentially all cell types of the body, but not a full organism on their own. Embryonic stem cells and induced pluripotent stem cells are pluripotent.
- Multipotent — can form a limited family of related cell types (for example, hematopoietic stem cells that generate blood lineages).
- Unipotent / progenitors — more restricted; often renew one lineage.
These categories are research tools. Real biology is nuanced, and laboratory definitions depend on assays, markers, and culture conditions.
Embryonic, adult, and iPS cells compared
| Feature | Embryonic stem cells (ESCs) | Adult (tissue) stem cells | Induced pluripotent stem cells (iPSCs) |
|---|---|---|---|
| Typical potency | Pluripotent | Usually multipotent | Pluripotent (after reprogramming) |
| Usual source | Inner cell mass of blastocyst-stage embryos | Tissues such as bone marrow, skin, gut, etc. | Adult cells (often skin or blood) reprogrammed in the lab |
| Self-renewal in culture | Can be expanded extensively as cell lines | Often harder; varies by tissue | Can be expanded extensively as cell lines |
| Ethical / policy flashpoint | Embryo-derived starting material | Generally fewer embryo-related concerns | Avoids embryo destruction; other issues remain (consent, clinical translation) |
| Landmark history | Human ESC lines, James Thomson, 1998 | Bone marrow / HSC transplantation (clinical tradition); Till & McCulloch 1961 proof of stem cells in mice | Yamanaka factors, mouse 2006; human iPS soon after (Thomson/Yamanaka era 2007) |
Embryonic stem cells helped prove that pluripotent human cells could be maintained in culture—an enabling step for developmental biology and disease modeling.
Adult stem cells already underpin established medicine in some areas, especially hematopoietic stem cell transplantation for certain blood cancers and disorders.
iPS cells showed that pluripotency can be restored in differentiated cells by reprogramming—changing what counts as “starting material” for pluripotent research.
A short history of the idea (with deeper reading)
- 1961 — Till & McCulloch provided key experimental evidence for blood-forming stem cells in mice: the 1961 stem cell proof.
- 1998 — Thomson and colleagues derived human embryonic stem cell lines, a watershed for human pluripotent cell culture; see the Stem Cell Research Timeline.
- 2006–2007 — Yamanaka’s reprogramming work (and parallel human advances) launched the iPS era: what iPS cells changed.
- Policy context in the U.S. — Funding limits, Dickey-Wicker, and changing administrations shaped the field: U.S. embryonic stem cell policy.
For definitions and further context, visit the Glossary and our overview of stem cell legislation.
Why stem cell research matters
- Understanding development and disease
- Modeling illnesses in a dish
- Screening drugs
- Exploring regenerative strategies under regulated clinical research
Potential is not the same as an approved therapy. Results from a laboratory model or an early clinical study do not automatically establish safety or effectiveness. For “Are there any approved stem cell therapies?”, see our FAQ.
Educational note
Stem Cell History is an educational history resource, not a clinic. Nothing here is medical advice. See About to learn more about this site.
Last reviewed: September 18, 2026.