Every cell in your body came from one fertilised egg. Somewhere along the way, cells had to choose what to become — and some cells never made that choice at all, so they could keep supplying replacements for the rest of your life. Those are stem cells, and they are defined by two abilities, not one.
📚 What you need to know
A stem cell has two properties: it can self-renew (divide by mitosis an unlimited number of times) and it can differentiate into specialised cells.
Once a cell has differentiated, it is no longer a stem cell.
Some stem cells stay in specific locations called stem cell niches, which give tissues the capacity to regenerate and repair. Bone marrow and hair follicles are the standard examples.
A niche must be able to keep stem cells inactive and also to stimulate proliferation and differentiation when needed.
Potency is the range of cell types a stem cell can become. Four levels: totipotent, pluripotent, multipotent, unipotent.
The zygote is totipotent, and so are the embryonic cells up to the 16-cell stage.
Most cells in an animal body are unipotent.
What makes a cell a stem cell
Two properties, both requiredSelf-renewal (divide by mitosis without limit) + Differentiation (become a specialised cell)
When a stem cell divides, each daughter cell has a choice. It can remain a stem cell, keeping the supply topped up, or it can differentiate into a specialised cell such as a red blood cell or a muscle cell.
That second route is one-way. After differentiation the cell is no longer counted as a stem cell, because it has lost the ability to become anything else.
Watch the wording in exam questions. A cell that divides endlessly but cannot differentiate is not a stem cell, and neither is a cell that could differentiate but cannot keep dividing. You need both, so say both.
Potency: how many options a stem cell has
Potency is the range of specialised cell types a stem cell can differentiate into. There are four levels, and they get narrower as development goes on.
Extra-embryonic cells are the ones that build the placenta rather than the embryo itself. Only totipotent cells can make those, which is the single fact separating the top two rows.
Potency
Can differentiate into
Example
Totipotent
Any cell type found in an embryo, plus extra-embryonic cells (the placenta)
The zygote formed when a sperm fertilises an egg, and embryonic cells up to the 16-cell stage
Pluripotent
Any cell type found in an embryo, but not extra-embryonic cells
Embryonic stem cells
Multipotent
A limited range of closely related cell types
Bone marrow stem cells, which produce the different types of blood cell
Unipotent
Only cells of their own lineage
Cardiomyocytes, which make new heart muscle cells. Most cells in animal bodies are unipotent
Adult stem cells are still stem cells. Multipotent and unipotent cells can divide any number of times, so they meet the definition. They have simply already partially differentiated, which narrows the range of things they can become. Bone marrow stem cells make blood cells — they cannot make a neurone.
Stem cell niches
Not all stem cells disappear after the embryo. Some remain in specific locations in the adult body, called stem cell niches. Their presence is what gives a tissue the capacity to regenerate and repair itself.
Blood cells have short lives and must be replaced continuously, which is exactly why a niche is needed. A one-off batch of stem cells would run out within weeks.
The two niches to learn
Bone marrow. Provides a niche for the stem cells that replace red blood cells, white blood cells and platelets. These are needed indefinitely, so continual production matters.
Hair follicles. The niche sits at the root of the hair, where the hair is anchored into the skin. Stem cells here drive continual hair growth.
🤔 Why a niche needs two opposite abilities
The environment of a niche must be able to keep the stem cells inactive, and also to stimulate proliferation and differentiation. Those sound contradictory, and that is the point. If the niche only stimulated division, the stem cell pool would be used up and the tissue could never be repaired again. If it only kept cells dormant, no replacements would ever be made. A niche has to be able to hold the cells in reserve and release them on demand, which is why its signalling environment is so tightly controlled.
Worked examples
WORKED EXAMPLE
State the two properties that define a stem cell. [2]
Property 1It can self-renew, dividing by mitosis an unlimited number of times.Property 2It can differentiate into one or more types of specialised cellTwo marks, two properties. Do not spend the answer on examples.
WORKED EXAMPLE
A researcher takes a cell from an early human embryo and finds it can produce every cell type of the embryo, but not placental tissue. Identify its potency and justify your answer. [2]
Step 1: check against the definitionsTotipotent cells make embryonic and extra-embryonic cells. Pluripotent cells make embryonic cells only.Step 2: decidePluripotent — it can form any embryonic cell type but cannot form extra-embryonic cells such as the placentaThe placenta is the deciding detail in almost every question of this type.
WORKED EXAMPLE
Explain why the presence of a stem cell niche in bone marrow is important. [3]
Point 1: what the niche holdsIt keeps a population of multipotent stem cells that can both self-renew and differentiate.Point 2: what they produceThey give rise to red blood cells, white blood cells and platelets, which are needed continuously.Point 3: the consequenceThe niche gives the tissue the capacity to regenerate and repair, so blood cell production never stops
💡 Exam tip
Always give both defining properties: self-renewal and differentiation.
Learn the four potency levels in order, with one example each. Examples are usually where the marks are.
The totipotent versus pluripotent distinction is extra-embryonic cells. Say “placenta” and the examiner knows you understand it.
Quote the 16-cell stage as the limit of totipotency in human embryos.
For niches, name the location and what it produces — bone marrow gives blood cells, hair follicles give hair growth.
Remember a niche must both maintain an inactive state and stimulate proliferation and differentiation.
⚠ Common mix-up
Thinking a stem cell just means “a cell that divides a lot”. Cancer cells divide a lot and are not stem cells in this sense.
Calling a differentiated cell a stem cell. Once it has specialised, it is no longer one.
Swapping totipotent and pluripotent. Toti = total, including the placenta.
Assuming adult stem cells are not real stem cells. They are — they simply have lower potency.
Confusing multipotent with unipotent. Multipotent gives several closely related types; unipotent gives one.
Describing a niche as just a place. It is an environment that actively controls whether the stem cells rest or divide.
Up next: Cell Specialisation — what happens after a stem cell commits, and why that decision puts a hard limit on how big a cell can get.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
