How do unspecialised cells become specialised, and how might stem cells be used in medicine?
The nature and types of stem cells, how cells become specialised and organised into tissues and organs, and the potential uses and ethical issues of stem cells.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on stem cells and differentiation, covering the types of stem cells, how cells differentiate and organise into tissues and organs, and the potential uses and ethical issues of stem cells.
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What this dot point is asking
Edexcel wants you to describe the nature and types of stem cells, explain how cells become specialised and organise into tissues and organs, and discuss the potential uses and ethical issues of stem cells. Examiners reward linking differentiation to gene expression and giving a balanced argument on ethics.
What stem cells are
Stem cells are classified by their potency:
Differentiation and organisation
Cells differentiate because different genes are switched on or off (controlled by transcription factors and epigenetic modifications), so different proteins are made and the cell gains a specialised structure and function. A red blood cell loses its nucleus and fills with haemoglobin; a root hair cell elongates to increase surface area for absorption. Specialised cells of the same type form a tissue, tissues form an organ, and organs work together as an organ system. This hierarchy (cells, tissues, organs, systems) lets a multicellular organism carry out functions no single cell could.
Differentiation is normally a one-way process: once a cell has specialised, the genes for other fates stay silenced. iPS technology reverses this artificially by forcing key transcription factor genes back on.
Uses and ethics
Stem cells could be used to replace damaged or lost cells, for example to treat spinal cord injury, type 1 diabetes (replacing insulin-producing beta cells), Parkinson disease or heart muscle damage after a heart attack. Adult (multipotent) stem cells, already used in bone-marrow transplants for leukaemia, raise few ethical issues. Embryonic (pluripotent) stem cells require the destruction of an embryo, which many people object to on the grounds that an embryo is a potential human life; others argue the medical benefit to existing patients outweighs this. Induced pluripotent stem cells (made by reprogramming a patient own adult cells) offer a way to avoid both embryo destruction and immune rejection, though their safety is still being established.
Examples in context
Example 1. Bone-marrow transplants. Multipotent stem cells in donated bone marrow can form all the blood cell types. In a transplant for leukaemia, the patient diseased marrow is destroyed and replaced with donor stem cells, which then self-renew and differentiate to rebuild a healthy blood system. This is the longest-established clinical use of stem cells and raises few ethical objections.
Example 2. Type 1 diabetes trials. Researchers have differentiated pluripotent stem cells into insulin-producing beta cells and implanted them in patients whose own beta cells were destroyed by autoimmune attack. Early trials show some patients producing their own insulin again. This illustrates the medical promise that drives the ethical debate over the source of the cells.
Try this
Q1. Explain the difference between a pluripotent and a multipotent stem cell. [2 marks]
- Cue. Pluripotent cells can form any body cell type; multipotent cells can form only a limited range.
Q2. Explain how two cells with identical DNA can become different specialised cells. [2 marks]
- Cue. Different genes are switched on or off in each cell, so different proteins are made.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksExplain how a stem cell and a mature neurone can contain identical DNA yet have very different structures and functions.Show worked answer →
The key idea is differential gene expression.
Both cells contain the same complete set of genes because they descend by mitosis from the same zygote. They differ because different genes are switched on or off in each cell. In the neurone, genes coding for proteins needed for transmitting impulses (such as channel proteins) are expressed, while in the stem cell those genes are silenced and genes for self-renewal are active. The proteins made determine the cell structure and function, so the same genome gives different specialised cells.
Markers reward: same DNA or genes from mitosis; different genes expressed or switched on/off; different proteins made; leading to different structure and function.
Edexcel 20236 marksEvaluate the use of embryonic stem cells compared with induced pluripotent stem cells (iPS cells) for treating disease.Show worked answer →
An evaluate question needs benefits, drawbacks and a judgement for both.
Embryonic stem cells are genuinely pluripotent and well understood, but obtaining them destroys an embryo, which raises strong ethical objections, and cells from a donor embryo may be rejected by the patient immune system. Induced pluripotent stem cells are made by reprogramming a patient own adult cells, so they avoid embryo destruction and avoid rejection because they are genetically matched, but the reprogramming can introduce mutations and there is a risk the cells form tumours, and the technique is newer and less proven. Judgement: iPS cells are ethically preferable and reduce rejection, so they are increasingly favoured, though embryonic cells remain a useful research benchmark while safety of iPS cells is established.
Markers reward at least two points each side and a reasoned judgement, not just a list.
Related dot points
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Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)