Stem Cells and Differentiation
All the specialised cells of a multicellular organism arise from unspecialised precursor cells through the process of differentiation. Stem cells are undifferentiated cells that retain the ability to divide and give rise to specialised cell types. Understanding stem cell biology — the different categories of potency, the mechanism of differentiation through gene expression, the therapeutic potential, and the ethical considerations surrounding their use — is required by the AQA A-Level Biology specification and is one of the most rapidly developing areas of contemporary medicine.
Spec mapping: This lesson sits across AQA 7402 Section 3.8.2 — Most of a cell's DNA is not translated (the gene-expression-control mechanism of differentiation) and Section 3.2.2 — All cells arise from other cells (the cell-cycle / mitosis foundation). Stem cells appear in the spec primarily as a worked example of how differential gene expression generates specialised cell types from a common genome. (Refer to the official AQA specification document for exact wording.)
What Are Stem Cells?
Key Definition: A stem cell is an undifferentiated cell that is capable of two related properties: self-renewal (dividing to produce more stem cells) and differentiation (giving rise to one or more specialised cell types). Differentiation is the process by which a cell becomes specialised for a particular function through the expression of specific genes.
Two functional features define a stem cell:
- Self-renewal. When a stem cell divides, at least one daughter cell remains a stem cell. This is sometimes achieved by asymmetric division (one daughter retains stem character; the other commits to differentiation) and sometimes by population-level dynamics in which divisions are statistically balanced.
- Multilineage potential. A stem cell can give rise, through subsequent divisions, to one or more types of differentiated cell. The number and identity of those cell types depend on the cell's potency (see below).
Without stem cells, multicellular organisms could not develop from a zygote, could not replace short-lived differentiated cells (red blood cells, gut epithelium, skin), and could not heal wounds.
How Differentiation Occurs
All cells in a multicellular organism contain the same DNA (the same genome) — they are genetically identical (they all arose from the same zygote by mitosis). However, different cells express different genes. Differentiation is therefore a problem of differential gene expression, not of differential DNA content.
The mechanism of differentiation
- During differentiation, specific genes are switched on (expressed) and others are switched off (silenced).
- The genes that are expressed determine which proteins are made, which in turn determines the cell's structure and function.
- Gene expression is controlled by transcription factors — proteins that bind to specific regions of DNA and either activate or repress transcription of target genes (covered in detail in Lesson 0).
- Epigenetic modifications also play a role: chemical changes to DNA (e.g., methylation of cytosine bases at CpG sites) or to histone proteins (e.g., acetylation, methylation) can alter gene expression without changing the DNA sequence. These modifications can be inherited during cell division and provide the molecular basis of cellular memory (covered in Lesson 1).
Example: A red blood cell precursor (erythroblast) expresses the gene for haemoglobin at very high levels (~10⁵ mRNA molecules per cell), while a neurone does not express this gene at all but instead expresses genes for neurotransmitter receptors, ion channels, and cytoskeletal proteins for axonal transport. Both cells contain the haemoglobin gene, but it is only active in the erythroblast. The gene is not deleted from the neurone — it is silenced, and the silencing is propagated through every neuronal division.
A cascade of master transcription factors
Differentiation proceeds through a hierarchical cascade. Early signals (often morphogens — diffusible signalling molecules) establish broad regional identities. These signals activate master transcription factors that in turn activate further transcription factors and target genes. Each step narrows the cell's developmental options.
The system has the form of a regulatory tree: at each branching point, alternative transcription factors compete; the one that wins commits the cell to one lineage and represses the alternatives. The pluripotent state at the root is maintained by a small set of master regulators (Oct4, Nanog, Sox2 in mammalian embryonic stem cells).
Irreversibility of differentiation (in most cases)
- In most animal cells, differentiation is effectively irreversible — once a cell has become specialised, it typically cannot revert to an unspecialised state under normal conditions. This irreversibility is enforced by stable epigenetic marks (DNA methylation of pluripotency genes such as Oct4) that propagate through mitosis.
- In plants, differentiation is often reversible — many differentiated plant cells can dedifferentiate and form new tissues or even entire organisms (totipotency is retained). This is the basis of cloning plants from cuttings and tissue culture.
- The discovery that adult animal cells can be experimentally reprogrammed to a pluripotent state (induced pluripotent stem cells — see below) shows that irreversibility in animals is practical, not absolute: the epigenetic locks can be picked given the right combination of factors.
Categories of Stem Cell Potency
Stem cells are classified according to their potency — the range of cell types they can give rise to.
| Potency | Definition | Examples |
|---|
| Totipotent | Can differentiate into any cell type, including extraembryonic tissues (e.g., placenta) | Zygote and cells of the early embryo (up to about the 8–16 cell stage, the morula) |
| Pluripotent | Can differentiate into any cell type of the body (all three germ layers: ectoderm, mesoderm, endoderm) but not extraembryonic tissues | Cells of the inner cell mass of the blastocyst (embryonic stem cells); induced pluripotent stem cells (iPSCs) |
| Multipotent | Can differentiate into a limited range of cell types within a particular tissue or lineage | Haematopoietic stem cells in bone marrow (give rise to all blood cell types: red blood cells, white blood cells, platelets). Neural stem cells (give rise to neurones and glial cells) |
| Unipotent | Can differentiate into only one cell type (but retain self-renewal) | Epithelial stem cells at the base of villi in the small intestine (produce only epithelial cells). Muscle satellite cells (produce only muscle cells) |
Exam Tip: You must be able to define each level of potency precisely. 'Totipotent' and 'pluripotent' are often confused — the key difference is that only totipotent cells can form the placenta and other extraembryonic tissues. Pluripotent cells can form any cell of the body proper but not the supporting tissues of the conceptus.
The mermaid: lineage tree from zygote to adult
flowchart TD
A["Zygote (totipotent)"] --> B["Morula 8-16 cells (totipotent)"]
B --> C["Blastocyst: ICM (pluripotent) + trophoblast"]
C --> D["Three germ layers: ectoderm / mesoderm / endoderm"]
D --> E["Multipotent tissue-resident stem cells"]
E --> F["Haematopoietic stem cells (bone marrow)"]
E --> G["Neural stem cells (brain)"]
E --> H["Intestinal crypt stem cells (unipotent)"]
F --> I["Red blood cells / white blood cells / platelets"]
G --> J["Neurones / glial cells"]
H --> K["Gut epithelial cells"]
Embryonic Stem Cells (ESCs)
Source
- Derived from the inner cell mass (ICM) of the blastocyst — an early-stage embryo (approximately 5–7 days after fertilisation in humans).
- The blastocyst consists of an outer layer of cells (trophoblast, which forms the placenta) and the ICM (which forms the embryo proper).
- The ICM is typically harvested from "surplus" embryos donated from IVF treatment that would otherwise be discarded.
Properties
- Pluripotent: can differentiate into virtually any cell type of the body.
- Can be grown in culture for extended periods while maintaining their undifferentiated state, provided the correct growth factors (e.g. LIF in mouse, FGF2/Activin in human) are present.
- Can be directed to differentiate into specific cell types by exposure to particular combinations of growth factors and signalling molecules — for example, BMP4 and activin A direct ESCs towards mesoderm and then cardiomyocytes.
- Maintain a characteristic transcriptional signature dominated by the master regulators Oct4, Nanog, Sox2.
Potential Therapeutic Uses
- Regenerative medicine: replacing damaged or diseased cells and tissues. Examples include:
- Growing new insulin-producing β-cells for type 1 diabetes (the diseased β-cells are autoimmune-destroyed; ESC-derived β-cells could in principle restore endogenous insulin secretion).
- Producing dopamine-secreting neurones for Parkinson's disease, in which the substantia nigra dopaminergic neurones are progressively lost.
- Generating cardiomyocytes (heart muscle cells) to repair damage after myocardial infarction.
- Growing new retinal cells for macular degeneration.
- Drug testing: differentiated cells can be used to test new drugs for efficacy and toxicity in vitro, reducing the need for animal testing.
- Disease modelling: ESCs from embryos diagnosed with genetic diseases (via pre-implantation genetic diagnosis) can be used to study disease mechanisms in human cells.
Challenges
- Tumourigenicity. Pluripotent cells can form teratomas (tumours containing all three germ layers) if injected undifferentiated. Pre-differentiation to a committed lineage before transplantation is essential.
- Immune rejection. ESC-derived tissues carry the HLA type of the donor embryo, not the recipient — they are allogeneic and may be rejected.
- Ethics. See below.
Adult Stem Cells
Source
- Found in specific locations (niches) within differentiated tissues throughout the body. The niche provides the signalling environment that maintains the stem cell in an undifferentiated state.
- Examples: bone marrow (haematopoietic stem cells and mesenchymal stem cells), brain (neural stem cells in the subventricular zone and hippocampal dentate gyrus), skin (epidermal stem cells in the basal layer; hair follicle bulge stem cells), gut (intestinal crypt stem cells), liver (hepatic progenitors), muscle (satellite cells beneath the basal lamina of muscle fibres).
Properties
- Generally multipotent (or in some cases unipotent) — they can produce only a limited range of cell types related to the tissue they are found in.
- More difficult to isolate and grow in culture than ESCs.
- Divide more slowly than ESCs in vivo.
- Present in very small numbers in each tissue (e.g. one haematopoietic stem cell per ~10⁵ bone marrow cells).
- Critical for tissue homeostasis — they replace differentiated cells that are continually lost (red blood cells turn over every ~120 days; intestinal epithelium every ~5 days; skin every ~28 days).
Therapeutic Uses
- Bone marrow transplants (haematopoietic stem cell transplants) are well-established treatments for leukaemia, lymphoma, and other blood disorders. The patient's diseased bone marrow is destroyed by chemotherapy or radiotherapy, and donor haematopoietic stem cells are infused to repopulate the marrow and produce healthy blood cells.
- Skin grafts using epidermal stem cells for burn victims.
- Corneal repair using limbal stem cells (limbal stem cell transplantation for chemical-burn-induced corneal injury is an established procedure).
- Mesenchymal stem cell therapies are being trialled for graft-versus-host disease and various inflammatory conditions.
Advantages over ESCs
- Can be obtained from the patient's own body (autologous transplant), eliminating the risk of immune rejection.
- No destruction of embryos is involved, so there are fewer ethical objections.
Disadvantages
- Limited potency (multipotent or unipotent) restricts the range of conditions that can be treated.
- Difficult to isolate in sufficient numbers.
- May have accumulated DNA damage (mutations) with age — older donors may yield stem cells with reduced regenerative capacity.
Induced Pluripotent Stem Cells (iPS Cells)
What they are
- Differentiated adult cells (e.g., skin fibroblasts) that have been reprogrammed to become pluripotent by the introduction of specific transcription factors.
- Pioneered by Shinya Yamanaka (Nobel Prize, 2012), who showed that introducing four transcription factors (Oct4, Sox2, Klf4, and c-Myc — the "Yamanaka factors") into adult mouse fibroblasts could revert them to a pluripotent state. The same approach was rapidly extended to human cells.
The reprogramming process
- Adult somatic cells (typically skin fibroblasts or peripheral blood mononuclear cells) are harvested.
- The four Yamanaka factor genes are introduced — historically via retroviral vectors, now more commonly via non-integrating methods (Sendai virus, mRNA, episomal plasmids).
- Over ~2–4 weeks, a small fraction of cells (~0.01–0.1%) undergo a dramatic epigenetic resetting: pluripotency genes are demethylated and reactivated, differentiation genes are silenced.
- Colonies of pluripotent cells emerge; they express Oct4, Nanog and Sox2, form teratomas when injected, and can differentiate into all three germ layers — the operational definition of pluripotency.
Advantages