What you need to know
- 3.17(i)stem cellAn undifferentiated cell that can divide and differentiate into specialised cell types., totipotentDescribes a stem cell that can differentiate into any cell type, including placenta cells. Early embryo cells are totipotent., pluripotentDescribes a stem cell that can differentiate into almost any cell type in the body, but not placenta cells., morula and blastocyst.
By the end, you should be able to…
- Define self-renewal and differentiationThe process by which a cell becomes specialised, because some genes are switched on and others are switched off..
- Distinguish totipotency from pluripotency.
- Identify the key features of a morula and blastocyst.
Build your understanding
Two abilities define a stem cell
A stem cellAn undifferentiated cell that can divide and differentiate into specialised cell types. is an unspecialised cell able to self-renew through division and to produce cells that differentiate. Self-renewal maintains a stem-cell population; differentiationThe process by which a cell becomes specialised, because some genes are switched on and others are switched off. produces specialised descendants. A cell that divides is not automatically a stem cell with broad developmental potential.
Potency describes the range of cell types a cell can generate under suitable conditions. It does not measure how rapidly it divides or guarantee that it will form every possible type without appropriate signals.
Totipotent and pluripotent
A totipotentDescribes a stem cell that can differentiate into any cell type, including placenta cells. Early embryo cells are totipotent. cell can give rise to embryonic and extraembryonic cell types and has the potential to support development of a whole organism under suitable conditions. The zygoteThe diploid cell formed when two gametes fuse at fertilisation. and very early embryonic cells illustrate this developmental potential.
A pluripotentDescribes a stem cell that can differentiate into almost any cell type in the body, but not placenta cells. cell can give rise to the body’s cell types but cannot by itself form all the extraembryonic tissues needed for a complete organism. Embryonic stem cells derived from the inner cell mass are pluripotent. Tissue stem cells generally have a more restricted range; blood-forming stem cells produce several blood-cell lineages, not every body cell.
A solid ball becomes a structure with a cavity
Following fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote., successive mitotic cleavage divisions increase cell number without a comparable increase in the embryo’s overall size. The morula is a solid ball of cells. As development continues, a fluid-filled cavity forms and the blastocyst develops.
The blastocyst has an outer cell layer, a cavity and an inner cell mass. The inner cell mass contributes to the embryo proper; the outer trophoblast contributes to supporting tissues, including the fetal contribution to the placenta. The entire placenta is not made exclusively by the embryo.
Potency changes during development. Do not assume that every cell of an entire morula or blastocyst has identical developmental potential. For the specification, recognise the structural stages and distinguish whole-organism potential from the ability to make body cell types.
Adult, embryonic and induced cells
Embryonic stem cells are obtained from early embryos and can be maintained in culture. Adult or tissue stem cells maintain and repair particular tissues, with more limited potency. Induced pluripotent stem cells are specialised cells reprogrammed to a pluripotent state.
These categories describe origin and potential, not a ranking from safe to unsafe. Reprogramming does not turn a cell into a complete embryo, and laboratory pluripotency does not by itself establish clinical usefulness.
NOT: pluripotent is not totipotent, and a specialised cell is not a stem cell.
- A stem cell can self-renew and produce a differentiated daughter
- Totipotent includes placenta-type tissues
- Multipotent does not mean “any tissue”
- Using stem cells in medicine is a separate lesson
Two traps this diagram blocks
- Do not call an adult stem cell totipotent.
- Do not say embryonic stem cells can already form a placenta as their defining potency.
From the diagram alone
Question
Distinguish totipotent, pluripotent and multipotent stem cells. [3]
Show a model answer
Totipotent cells, such as the zygote, can produce every cell type including extra-embryonic tissues. Pluripotent embryonic stem cells can produce many cell types of the body. Multipotent adult stem cells can produce only a limited range of related cell types.
| Term | Defining feature |
|---|---|
| Totipotent | Embryonic and extraembryonic developmental potential |
| Pluripotent | Potential to generate body cell types |
| Morula | Solid ball of early embryonic cells |
| Blastocyst | Cavity, outer layer and inner cell mass |
Make your reasoning visible
Include both defining abilities
A stem-cell definition needs self-renewal and the potential to differentiate.
Use extraembryonic tissues to distinguish potency
“Can make many cell types” does not distinguish totipotentDescribes a stem cell that can differentiate into any cell type, including placenta cells. Early embryo cells are totipotent. from pluripotentDescribes a stem cell that can differentiate into almost any cell type in the body, but not placenta cells..
Separate the embryo stages
Morula: solid ball. Blastocyst: cavity, outer layer and inner cell mass.
Key explanations
- A stem cellAn undifferentiated cell that can divide and differentiate into specialised cell types. can divide and produce a daughter that remains undifferentiated, and daughters that specialise.
- Adult stem cells are multipotent. They replace cells within a limited family, such as blood cells.
- As cells specialise, the pattern of gene expression becomes restricted.
- Potency names that restriction. It is not a measure of how fast the cell divides.
Avoid this misconception
The claim: Pluripotent cells can form a whole organism by themselves.
The correction: They can generate body cell types but lack the full developmental potential of a totipotent cell.
Order early development and potency
As an embryo develops, its cells keep less of their potential. Order the stages from the zygote to specialised cells.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the stages in order, from fertilisation to specialised cells.
0 of 6 in the right place
Potency does not change at one sharp moment, and not every cell of a morula or blastocyst has the same potential. Adult stem cells are not ordered here.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
State two defining properties of a stem cell.
Show suggested answer
Self-renewal through division and the ability to give rise to differentiated cells.
Practice question
Retrieval and explanation · suggested answer
Explain why pluripotent and totipotent are not interchangeable.
Show suggested answer
Totipotent potential includes embryonic and extraembryonic lineages needed for whole-organism development. Pluripotent cells can produce body cell types but not all those supporting lineages by themselves.
Practice question
Retrieval and explanation · suggested answer
Describe how a blastocyst differs from a morula.
Show suggested answer
A morula is a solid ball of cells. A blastocyst contains a cavity, an outer cell layer and an inner cell mass.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A cultured cell forms neurons, muscle and liver cells. Is this enough to demonstrate totipotency?
Show a hint
What ability has not been tested?
Explore the reasoning
No. These are body cell types, consistent with broad potency but not proof of whole-organism potential. Extraembryonic developmental capacity and more extensive evidence are missing.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare totipotent and pluripotent stem cells.
Stem cells and therapies
Practice question
Retrieval and explanation · suggested answer
Compare totipotent and pluripotent stem cells.
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Induced pluripotency
3C · Stem CellsThe 2012 Nobel Prize recognised reprogramming mature cells into a pluripotent state. These cells give researchers a way to study development using cells whose starting state was specialised.…Specification 3.17(i)Read & use the scienceClose article
Why this matters
The 2012 Nobel Prize recognised reprogramming mature cells into a pluripotent state. These cells give researchers a way to study development using cells whose starting state was specialised. Pluripotent cells can form many body-cell types; totipotent cells have wider developmental potential. Those terms describe tested potential, not how impressive a cell looks under a microscope. An induced pluripotent stem cell can be made from a mature cell, creating a model for studying disease in a laboratory dish. Before therapy, scientists must also show that cells differentiate as intended and do not grow uncontrollably. The Nobel experiment illustrates what reprogramming can achieve, while the specification still expects precise definitions of morula, blastocyst and potency.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.17(i) understand what is meant by the terms stem cell, pluripotent and totipotent, morula and blastocyst
Use the science
Why should a reprogrammed cell not automatically be described as totipotent?
Explore a possible response
The demonstrated state is pluripotent: broad body-cell potential does not establish the full embryonic and extraembryonic potential of totipotency.
Explore the source
- Nobel Prize 2012: cellular reprogrammingNobel Prize Outreach
A stem-cell claim needs a safety question
3C · Stem CellsAdvertisements sometimes treat the words stem cell as proof of a cure. The FDA warns that many regenerative-medicine products marketed for a wide range of conditions have not…Specification 3.17(i) · 3.17(ii)Read & use the scienceClose article
Why this matters
Advertisements sometimes treat the words stem cell as proof of a cure. The FDA warns that many regenerative-medicine products marketed for a wide range of conditions have not been shown to be safe and effective. A cell’s ability to divide and differentiate is biologically interesting, but it does not establish where it will go after treatment or whether it will grow in the right way. This is a direct application of stem-cell potency and societal decision-making in the specification. Students can ask what cell type was used, what outcome was measured, and whether a controlled trial supports the claim.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.17(i) understand what is meant by the terms stem cell, pluripotent and totipotent, morula and blastocyst
- 3.17(ii) be able to discuss the ways in which society uses scientific knowledge to make decisions about the use of stem cells in medical therapies
Use the science
Why does a pluripotent cell’s broad potential not by itself prove that an injection will repair a damaged joint?
Explore a possible response
Potential in suitable conditions is not controlled differentiation at the target site. Safety, delivery, function and clinical benefit all need evidence.
Explore the source
- Important patient information about regenerative medicineUS Food and Drug Administration