The big idea: A stem cell is an unspecialized cell with two special abilities.
First, it can divide again and again to make more cells (called self-renewal).
Second, it can differentiate — develop into one or more specialized cell types (such as a muscle cell, a red blood cell or a nerve cell).
How many different cell types a stem cell can become is called its potency.
- Stem cell
- An unspecialized cell that can keep dividing (self-renew) and can differentiate into one or more specialized cell types.
- Self-renewal
- The ability of a stem cell to divide repeatedly to produce more stem cells, keeping a supply available in the body.
- Differentiation
- The process by which an unspecialized cell becomes a specialized cell with a particular structure and function.
- Potency
- A measure of how many different cell types a stem cell is able to differentiate into.
- Niche
- The specific location in the body where a particular type of stem cell is found (for example, blood-forming stem cells in red bone marrow).
| Defining property | What it means | Why it matters |
|---|---|---|
| Self-renewal | A stem cell can divide repeatedly to make more stem cells | Keeps a lasting supply of stem cells in the body |
| Differentiation (potency) | A stem cell can develop into one or more specialized cell types | Replaces and builds the specialized cells the body needs |
Two properties, always together: Both properties must be present for a cell to count as a stem cell:
it must be able to self-renew (keep dividing) and able to differentiate (turn into specialized cells).
A cell that can only divide, or only differentiate, is not a stem cell.
Not all stem cells are equal. Some can become any cell type, while others are limited to just a few.
We rank them by potency — the number of different cell types they can differentiate into. There are four named tiers, from most powerful to least.
The four potency terms: Totipotent — can become any cell type plus the placenta. Example: the zygote (fertilized egg) and the very early embryo.
Pluripotent — can become any cell type of the body, but not the placenta. Example: embryonic stem cells.
Multipotent — can become a limited family of related cell types. Example: adult stem cells, such as the blood-forming cells in red bone marrow.
Unipotent — can become only one cell type.
The potency hierarchy: totipotent cells can become any cell type plus the placenta, pluripotent any body cell, multipotent a limited family of related cells, and unipotent just one. Potency falls as you go down.
Interactive diagram
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Potency falls as cells specialize: Read the hierarchy top to bottom: potency decreases and specialization increases as you go down.
Early in development a cell can become almost anything (totipotent). As the embryo develops and cells start to differentiate, their options narrow — most stem cells in the adult body are only multipotent or unipotent.
Key contrast for the exam: totipotent includes the placenta; pluripotent does not. That single difference is how the two top tiers are told apart.
| Potency term | Can become… | Example (and where it is found) |
|---|---|---|
| Totipotent | ANY cell type PLUS the placenta | The zygote and the very early embryo |
| Pluripotent | Any cell type OF THE BODY (but not the placenta) | Embryonic stem cells (in the early embryo) |
| Multipotent | A limited family of related cell types | Adult stem cells — e.g. blood-forming cells in red bone marrow |
| Unipotent | Only ONE cell type | Some adult tissue stem cells (e.g. skin) |
Potency AND niche — name both: An exam can ask you to classify a stem cell two ways at once: its potency type and its niche (where it is found in the body).
The classic example is a blood-forming (haematopoietic) stem cell: it is multipotent (it makes the family of blood cells) and its niche is the red bone marrow.
So 'what kind of stem cell' often needs both answers — the potency tier and the location.
| Stem cell | Potency type | Niche (where it is found) |
|---|---|---|
| Zygote / very early embryo | Totipotent | The fertilized egg, before the embryo specializes |
| Embryonic stem cell | Pluripotent | The inner cell mass of the early embryo |
| Blood-forming (haematopoietic) stem cell | Multipotent | Red bone marrow |
Totipotent vs pluripotent
- Totipotent: any cell type + the placenta
- Example: the zygote / very early embryo
- Pluripotent: any cell of the body, but not the placenta
- Example: embryonic stem cells
Multipotent vs unipotent
- Multipotent: a limited family of related cell types
- Example: bone-marrow (blood-forming) stem cells
- Unipotent: only one cell type
- Both are typical of the adult body
A memory hook: Toti = total (any cell + the placenta).
Pluri = plural / many (any cell of the body).
Multi = multiple but limited (a related family, e.g. blood cells).
Uni = one (a single cell type).
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How this is tested: On Paper 1A a 1-mark question often asks you to name the potency term for a described cell — for example, a very early embryo / zygote is totipotent.
Another common format asks you to choose the correct description of potency for a given tissue — match the tier to how many cell types it can form (pluripotent = any body cell; multipotent = a limited family).
A trickier version asks you to classify a stem cell by both its potency type and its location — e.g. a blood-forming stem cell is multipotent and is found in the red bone marrow.
IB-style question — classify a blood-forming stem cell
A stem cell taken from red bone marrow gives rise to red blood cells, white blood cells and platelets, but no other cell types. State the potency type of this stem cell and identify its niche. [2]
How to score both marks
- Decide the potency. The cell makes several related cell types (the blood cells) but not every cell type in the body — so it is multipotent.
- Name the niche. The question tells you it was taken from red bone marrow — that is its niche (the location where this stem cell is found). (Mark 1: multipotent. Mark 2: red bone marrow.)
Final answer
Multipotent — it can form a limited family of related cell types (the blood cells); its niche is the red bone marrow.
✓ Why this scores full marks: The giveaway is 'a limited family of related cell types' — that is the definition of multipotent, not pluripotent.
A common slip is calling it pluripotent. Pluripotent cells can become any body cell; this one only makes blood cells, so it is multipotent.
| Potency term | Can become… | Example (and where it is found) |
|---|---|---|
| Totipotent | ANY cell type PLUS the placenta | The zygote and the very early embryo |
| Pluripotent | Any cell type OF THE BODY (but not the placenta) | Embryonic stem cells (in the early embryo) |
| Multipotent | A limited family of related cell types | Adult stem cells — e.g. blood-forming cells in red bone marrow |
| Unipotent | Only ONE cell type | Some adult tissue stem cells (e.g. skin) |