Stem Cells and Differentiation — AQA A-level Biology

SPEC 3.8.2.1 - GENE EXPRESSION — STEM CELLS

Stem Cells

Totipotent, pluripotent, multipotent and unipotent cells; iPS cells; therapeutic uses and ethical evaluation

The Central Question: Why Do Cells Differ?

Every cell in your body — from a neuron to a liver cell to a red blood cell — contains exactly the same genome. Yet these cells are profoundly different in structure and function.

Cell Differentiation

The process by which a cell develops a more specialised function. During differentiation, cells express only a subset of their genes — the genes relevant to their specialised role.

The Key Principle

All cells contain the same DNA — the full genome

Cells differ because they express different subsets of genes

Early embryonic cells (totipotent) can translate all their genes

As development proceeds, cells progressively restrict which genes they express

This restriction is controlled by transcription factors and epigenetic modifications (covered in Lesson 8.03)

⚠️ EXAMINER ALERT: Do not say "cells have different DNA" or "cells lose genes during differentiation." All body cells retain the complete genome. The difference is in which genes are expressed, not which genes are present.

The Four Levels of Potency

Potency describes how many different cell types a stem cell can produce. There are four levels, ranging from greatest to most restricted:

LevelCan produceFound where?Example

TotipotentAny cell type, including extra-embryonic cells (e.g. placenta)Very early embryo only (up to ~8-cell stage)Zygote; first few blastomeres

PluripotentAlmost any body cell type — but NOT extra-embryonic cellsInner cell mass of blastocyst (embryo)Embryonic stem cells (ESCs)

MultipotentA limited range of related cell types within one tissueAdult tissuesHaematopoietic stem cells → all blood cell types

UnipotentOne specific cell type onlyAdult tissues (specific progenitor cells)Cardiac progenitor cells → cardiomyocytes

Memory Tip

Total → Pluri → Multi → Uni: each level is more restricted than the last. "Totipotent" = totally able; "pluripotent" = many possible; "multipotent" = multiple but limited; "unipotent" = one only.

Totipotent Cells

Totipotent Cell

A cell that can divide and differentiate to produce any cell type, including extra-embryonic cells such as those forming the placenta and umbilical cord. Totipotent cells occur only for a limited time in the early mammalian embryo.

Key Facts — Totipotent

Found only in the very early embryo — from the fertilised egg (zygote) up to approximately the 8-cell stage in humans

The zygote itself is totipotent — it can produce the entire organism including the placenta

After the 8-cell stage, cells begin to specialise and lose totipotency

Totipotent cells have no genes permanently silenced — they retain the capacity to express any gene in the genome

They divide by mitosis to produce the blastocyst

⚠️ EXAMINER ALERT: The AQA specification states totipotent cells occur "for a limited time in early mammalian embryos." Do not say they persist throughout development. Once cells commit to the inner cell mass or trophoblast they are no longer totipotent.

Pluripotent Cells & the Blastocyst

Pluripotent Cell

A cell that can differentiate into almost any body cell type, but cannot form extra-embryonic cells (placenta, umbilical cord). Pluripotent cells are found in the inner cell mass of the blastocyst.

Spec Point: Pluripotent Stem Cells

Pluripotent stem cells can divide in unlimited numbers in culture and can be used in treating human disorders. They come from the inner cell mass of the blastocyst — their isolation requires destruction of the embryo, which is ethically contentious.

Multipotent & Unipotent Cells

Multipotent Cell

A cell that can differentiate into a limited range of related cell types within a particular tissue. Found in adult tissues. Example: haematopoietic (blood) stem cells in bone marrow can produce all blood cell types — red blood cells, white blood cells, platelets — but cannot produce liver or nerve cells.

Unipotent Cell

A cell that can produce only one specific cell type. Example specified by AQA: cardiac progenitor cells differentiate into cardiomyocytes (heart muscle cells) only.

Cardiomyocytes — The AQA Unipotent Example

The spec explicitly names cardiomyocyte formation as the example of unipotent cell differentiation

Cardiac progenitor cells (unipotent) → cardiomyocytes only

Cardiomyocytes are highly specialised: many mitochondria, organised sarcomeres, intercalated discs for electrical coupling

Adult heart muscle has very limited regenerative ability — hence interest in stem cell therapies for heart disease

Induced Pluripotent Stem (iPS) Cells

Induced Pluripotent Stem Cell (iPS Cell)

An adult somatic cell that has been reprogrammed to a pluripotent state by introducing specific transcription factor genes. The resulting cell behaves like an embryonic stem cell but was derived from an adult cell, not an embryo.

How iPS Cells Are Made — The Key Steps

Take an adult somatic cell (e.g. skin fibroblast or blood cell)

Introduce four key transcription factor genes (originally Oct4, Sox2, Klf4, c-Myc) into the cell using a viral vector

The transcription factors are expressed → they switch on embryonic pluripotency genes and switch off adult differentiation genes

The cell is reprogrammed to a pluripotent state — it can now differentiate into (almost) any cell type

Advantages Over Embryonic Stem Cells

No embryo destroyed — avoids the main ethical objection to ESCs

Patient-matched — made from the patient's own cells, so transplanted tissues will not be rejected by the immune system

Can be made in large quantities in the laboratory

Therapeutic Uses of Stem Cells

The AQA specification requires you to evaluate the use of stem cells in treating human disorders. You need both the potential benefits and the specific limitations for each type.

Stem Cell TypeCurrent / Potential UseStatus

Adult stem cells (multipotent) Haematopoietic stem cells from bone marrowBone marrow transplant to treat leukaemia: patient receives donor haematopoietic stem cells that repopulate the bone marrow with healthy blood cellsCurrent clinical use ✓

Embryonic stem cells (pluripotent)Could replace damaged tissue in Parkinson's disease (dopamine-producing neurons), type 1 diabetes (beta cells), spinal cord injury (neurons), macular degeneration (retinal cells)Mainly research / early trials

iPS cells (pluripotent — induced)Patient-matched cell replacement in the conditions above, plus drug testing on patient-specific cells; disease modellingResearch / early clinical trials

Exam Tip

When asked for a "current" use, specify bone marrow transplant using haematopoietic stem cells for leukaemia. This is the only stem cell therapy in widespread routine clinical use. Embryonic and iPS cell therapies are largely still in research or early trial phases.

Evaluating Stem Cell Research: The Ethical Debate

AQA requires you to evaluate — giving both sides with reasoning. Evaluation questions expect you to reach a justified conclusion.

Arguments FOR Embryonic Stem Cell Research

Potential to treat currently incurable conditions (Parkinson's, spinal cord injury, type 1 diabetes)

Embryos used are typically spare embryos from IVF that would otherwise be discarded — not created solely for research

Early blastocyst (4–5 days) has no nervous system, no capacity for pain or sentience

The scientific benefit to many patients could outweigh the use of a small cluster of cells

Arguments AGAINST Embryonic Stem Cell Research

Requires destruction of a human embryo — ethical objection if life is considered to begin at fertilisation

Concern about "instrumentalising" human life — using embryos as a means to an end

iPS cells may make ESC research unnecessary in the future

Scientific / Practical Limitations of ESCs

Risk of immune rejection — donor ESCs carry different HLA antigens from the recipient; immunosuppressive drugs may be needed

Undifferentiated pluripotent cells can form teratomas (tumours containing multiple tissue types) if not fully guided before transplant

Ethical controversy may restrict funding and regulatory approval

iPS Cells: Limitations Despite Advantages

Risk of cancer: reprogramming uses viral vectors; the transcription factor c-Myc is also an oncogene — may trigger uncontrolled division if not properly controlled

Reprogramming is not fully efficient — not all cells successfully convert

Regulatory hurdles before clinical use

The Potency Gradient: From Fertilisation to Differentiation

DEEP Framework: Evaluating iPS Cells as Therapy

Exam question: "Evaluate the use of induced pluripotent stem (iPS) cells in treating human disorders." [6 marks]

DDefine: iPS cells are adult somatic cells reprogrammed to a pluripotent state by introducing transcription factor genes. They can differentiate into almost any cell type, making them potentially useful for replacing damaged tissue.

EEvidence FOR: iPS cells avoid the ethical problems of embryonic stem cells as no embryo is destroyed. They can be produced from the patient's own cells, so transplanted tissues are less likely to be rejected by the immune system. They can divide in unlimited numbers in culture, providing a large supply.

EEvidence AGAINST / limitations: Reprogramming involves viral vectors and uses the transcription factor c-Myc, which is an oncogene — there is a risk of triggering uncontrolled cell division and tumour formation. Reprogramming is not fully efficient. Clinical trials are still ongoing; long-term safety has not yet been established.

PConclusion/Practical: iPS cells represent a significant advance — they combine the therapeutic potential of pluripotent cells with patient-specific compatibility and no embryo destruction. However, safety concerns around cancer risk must be resolved before widespread clinical use. The technique is most promising for conditions where cell replacement is well-defined, such as leukaemia or retinal disease.

Synoptic Links & Examiner Alerts

Synoptic Link — Topic 8.03

Stem cell differentiation is controlled by transcription factors — proteins that bind regulatory DNA sequences to switch genes on or off. The loss of totipotency as cells specialise reflects the progressive silencing of genes by transcription factors and epigenetic mechanisms (methylation, histone modification). This is covered in full in Lesson 8.03.

Synoptic Link — Topic 2.04

Stem cells divide by mitosis to self-renew and also to produce differentiating daughter cells. The ability of pluripotent stem cells to "divide in unlimited numbers" (as AQA states) connects to cell cycle control — and also to why loss of cell cycle regulation is dangerous in the context of iPS cell reprogramming.

⚠️ ERROR 1: "Totipotent cells are found throughout the embryo." Wrong — they occur only for a limited time in the very early embryo (up to ~8-cell stage). After that, cells commit and become pluripotent at most.

⚠️ ERROR 2: "Stem cells change their DNA during differentiation." Wrong — differentiation involves changes in gene expression, not changes to the DNA sequence itself.

⚠️ ERROR 3: "iPS cells are identical to embryonic stem cells." Not quite — they are functionally similar (pluripotent) but may carry epigenetic differences and have a different risk profile (cancer risk from reprogramming).

Practice Question

(a) Explain why totipotent cells occur only for a limited time in the early mammalian embryo. [2 marks]

(b) A patient with leukaemia receives a bone marrow transplant. Explain why the donor haematopoietic stem cells are described as multipotent rather than pluripotent. [2 marks]

(c) A scientist proposes using iPS cells derived from a patient's own skin cells to treat their Parkinson's disease. Evaluate this proposal, including one advantage over using embryonic stem cells and one concern about the use of iPS cells. [4 marks]

a)

Totipotent cells can produce any cell type including extra-embryonic cells. As development proceeds, cells commit to becoming either trophoblast (will form placenta) or inner cell mass. Once this commitment is made, their gene expression is restricted by transcription factors and epigenetic modifications — they can no longer express all genes and so lose totipotency. [2 marks]

b)

Haematopoietic stem cells can differentiate into a range of blood cell types (red blood cells, white blood cells, platelets) — i.e. a limited range of related cell types within one tissue. They cannot produce unrelated cell types such as neurons or liver cells. Pluripotent cells can produce almost any body cell type. Haematopoietic stem cells are therefore restricted to blood cell lineages only — multipotent, not pluripotent. [2 marks]

c+:

Advantage over ESCs: iPS cells are derived from the patient's own adult somatic cells — no embryo is destroyed, avoiding the ethical objection to ESC use. The cells are also patient-matched, so the transplanted neurons are less likely to be rejected by the immune system. [2 marks]

c-:

Concern: Reprogramming uses viral vectors carrying the transcription factor c-Myc, which is an oncogene. There is a risk that the reprogramming process could trigger uncontrolled cell division in the transplanted cells, potentially leading to tumour formation. Clinical safety has not yet been fully established. [2 marks]

Summary: Stem Cells

Key Points

ConceptKey Fact

Cell differentiationSame genome in all cells; cells differ because different genes are expressed

TotipotentAny cell type including extra-embryonic; only in very early embryo (up to ~8 cells)

PluripotentAny body cell type but NOT extra-embryonic; inner cell mass of blastocyst

MultipotentLimited range of related cell types; adult tissues (e.g. haematopoietic → blood cells)

UnipotentOne cell type only; e.g. cardiac progenitor → cardiomyocytes

iPS cellsAdult somatic cells reprogrammed by transcription factors; pluripotent; avoids embryo destruction; cancer risk

Current clinical useBone marrow transplant using haematopoietic stem cells (leukaemia)

Evaluation Checklist

For ESCs: therapeutic potential vs embryo destruction

For iPS: no embryo, patient-matched vs cancer risk from reprogramming

Always give both sides + a justified conclusion

Must-Know Distinctions

Totipotent ≠ Pluripotent: only totipotent can form placenta

Multipotent ≠ Pluripotent: multipotent is limited to one tissue lineage

iPS ≠ ESC: same potency, different source and risk profile

Exam Tip

For any "evaluate" question on stem cells: state the potential therapeutic benefit, then the specific limitation or ethical concern for that cell type, then a justified conclusion. Never just list pros and cons without a conclusion — Level 3 marks require judgement.