Stem Cells: Types, Potency and Medical Uses

Figure 0. One cell that can become many: a stem cell divides, and its daughters become a red blood cell, a nerve cell and a muscle cell.

Level: GCSE Biology (AQA, OCR, Edexcel), A-level Biology (AQA, OCR A, Edexcel), AP Biology and IB Biology. The words are simple. The depth is full.

Your body holds about 200 different types of cell — skin cells, nerve cells, muscle cells, blood cells and many more. Every one of them started from a single cell: the fertilised egg. The cells that make that possible are stem cells (unspecialised cells that can divide to make more stem cells and can change into specialised cells). This post teaches the whole story from the start: what a stem cell is, the four levels of potency, where stem cells live, how one becomes a blood cell or a nerve cell, and what stem cells can — and cannot — do in medicine.

What is a stem cell?

AQA's definition is the one to learn word for word: a stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation. Undifferentiated (not yet specialised for a particular job) is the key word — a stem cell has no fixed job yet. Differentiation (the process by which an unspecialised cell becomes a specialised cell with a particular structure and function) is what gives it one.

Every stem cell has two defining properties, and examiners ask for both. First, self-renewal (the ability to divide and make more stem cells, so the supply never runs out). Second, potency (the range of different cell types the stem cell can become). A cell with only one of the two is not a stem cell: a skin cell can divide but cannot become a nerve cell, and a fertilised egg can become anything but is not a stem cell pool. The cell division lesson explains the mitosis that self-renewal uses, and the cell theory lesson explains why every cell in your body comes from an earlier cell.

One fact ties the whole topic together: every body cell carries the same DNA. A skin cell and a nerve cell have the same genes. What differs is which genes each cell reads — and that is the mechanism of differentiation, covered in the section below. The inheritance lesson explains how genes are passed on unchanged, and the prokaryotic versus eukaryotic cells lesson shows the cell types that carry them.

How does a stem cell divide?

A stem cell has two ways to divide, and the body uses each for a different job. In symmetric division (one stem cell dividing into two identical stem cells) the pool grows — this is how the embryo builds up its stem cells, and how the body rebuilds the pool after injury. In asymmetric division (one stem cell dividing into one stem cell and one different daughter cell) the body gets new specialised cells without losing its stem cells. The different daughter is called a progenitor cell (a young cell that has started down the road to specialisation but can still divide a few more times). The progenitor's daughters divide again, and each round takes them one step closer to a finished, specialised cell.

Stem cells do not divide whenever they like. Each population lives in a niche (the local environment around a stem cell — its neighbouring cells, the signalling molecules they release, and the surrounding matrix — that controls how the stem cell behaves). The niche tells the stem cell when to stay quiet, when to divide and when to differentiate. The two niches the IB syllabus names are the bone marrow niche (where blood stem cells live) and the hair follicle niche (where the stem cells that regrow hair live). Take a stem cell out of its niche and it behaves differently — the niche is part of the stem cell's identity, not just its address. The cytoskeleton lesson explains the structures that let a dividing cell split into two different daughters.

What is the potency ladder?

Potency is a ladder with four rungs (five if you count oligopotent, which sits between multipotent and unipotent). The higher the rung, the more cell types the stem cell can become — and the earlier in life it is found. Learn the ladder top to bottom:

Potency	What it can become	Where it is found
Totipotent	Any cell type, including the cells of the placenta — it can form a whole organism	The zygote (fertilised egg) and the cells of the very early embryo, for the first few divisions
Pluripotent	Almost any body cell type, but not the placenta	The inner cell mass of the blastocyst (a hollow ball of cells, about five days after fertilisation)
Multipotent	A limited range of related cell types, inside one tissue	Adult tissues — for example, blood stem cells in the bone marrow
Oligopotent	Two or three closely related cell types	Adult tissues — for example, the myeloid progenitor that makes a few blood cell types
Unipotent	One cell type only	Adult tissues — for example, skin basal cells, and the cardiac progenitor cells that make only cardiomyocytes (heart muscle cells)

Figure 1 shows the same ladder as numbers. The human body has about 200 cell types, and the top two rungs can make nearly all of them. The drop from pluripotent to multipotent is the steep one: a blood stem cell makes about 10 types, all of them blood. That is the whole point of potency — it is a measure of how many doors are still open.

Figure 1. The potency ladder. Totipotent cells can make every cell type plus the placenta; pluripotent cells can make every body cell type; multipotent, oligopotent and unipotent cells are restricted to one tissue, a few types, or a single type.

Pluripotent cells can make cells of all three germ layers (the three sheets of cells that form in the early embryo and give rise to every tissue). The ectoderm (outer layer) makes skin and nerve. The mesoderm (middle layer) makes muscle, blood and bone. The endoderm (inner layer) makes the gut, liver and lungs. That three-layer test is how scientists prove a cell is pluripotent: if it can make ectoderm, mesoderm and endoderm cells, it passes. The cellular totipotency lesson goes deeper into totipotency itself, and the evolution lesson explains why the same three germ layers appear in every vertebrate animal.

Where do stem cells live in the body?

Stem cells are found in three places, and the three sources differ in potency, in use and in ethics. Embryonic stem cells come from the inner cell mass of a blastocyst about five days after fertilisation. They are pluripotent, they can divide in unlimited numbers in a dish, and the first human embryonic stem cell lines were grown by James Thomson in 1998. Getting them destroys the embryo — the fact behind the ethical debate in the ethics section below. Adult stem cells (also called tissue stem cells) live in small numbers in many tissues of children and adults: blood stem cells in the bone marrow, neural stem cells in the brain, stem cells in the lining of the gut, basal cells in the skin, and limbal stem cells at the edge of the cornea. They are multipotent or unipotent, and their job is repair and replacement. The third source is perinatal (around the time of birth): the blood of the umbilical cord and placenta is rich in blood stem cells, and it is collected and stored after birth for possible transplants.

Each adult tissue renews itself at its own speed. The lining of the gut is replaced every few days — its stem cells sit in pockets called crypts (small pits in the gut wall) and divide constantly, because food, acid and enzymes wear the lining away. The skin (epidermis) renews in about a month, driven by its basal stem cells. The bone marrow makes about 2 million red blood cells every second. The brain renews very slowly: its neural stem cells make new neurons and glial cells (supporting cells of the nervous system, such as astrocytes and oligodendrocytes), but only in small numbers. The cell organelles lesson explains the structures these renewing cells need to divide, and the cell membrane lesson explains how the signals that control them cross into the cell.

Plants have stem cells too, and they keep them for life. Meristems (the growing tips of shoots and roots) hold stem cells that can still make any plant cell type, which is why a plant keeps growing all its life while most animal cells differentiated early and lost that power. The GCSE syllabus asks for meristems by name, and the plant stem-culture lesson shows how those cells are used to clone plants quickly and cheaply.

How does a stem cell become a specialised cell?

Differentiation is not a change in DNA — it is a change in which genes are read. The process runs in five steps, and the animation below walks through them. Step 1: a signal arrives — a chemical from a neighbouring cell, or a hormone travelling in the blood. Step 2: the signal switches on a transcription factor (a protein that binds to DNA and switches a gene on or off). Step 3: the transcription factor switches on a whole set of genes and switches others off — this is differential gene expression (different cells reading different sets of the same genes). Step 4: the cell makes new proteins, and the proteins change its shape and its job. Step 5: epigenetic changes (chemical marks on DNA and its packaging proteins that lock genes on or off without changing the DNA sequence) make the decision stick — the genes that are not needed are packed away and silenced, often for the life of the cell.

The red blood cell is the clearest example. A blood stem cell's daughter switches on the genes for haemoglobin (the red protein that carries oxygen) and switches off the genes for nerve proteins, muscle proteins and everything else it does not need. As it matures it ejects its nucleus, leaving more room for haemoglobin — and losing the ability to divide. A nerve cell does the opposite: it switches on nerve genes and switches off the haemoglobin genes. Same DNA, different reading, different cell. The membrane transport lesson explains how the signals in step 1 reach the cell, and the microscopy lesson explains how scientists watch differentiation happen under the microscope.

Watch it move — one stem cell becomes specialised cells in 5 steps

One stem cell in its niche — a quiet, unspecialised cell that can divide and can change.

Follow the animation's logic and you can answer any differentiation question: the signal comes first, the transcription factor switches genes, the proteins change the cell, and epigenetics locks the choice in. The common error is saying the DNA changes — it does not. Only the reading of the DNA changes, which is why a differentiated cell still carries the full genome and why reprogramming (the next-but-one section) is possible at all.

Blood — the worked example

Blood is the example every syllabus uses, because one stem cell makes the whole system. The haematopoietic stem cell (the blood-forming stem cell, from the Greek haima, blood, and poiesis, making) sits in the bone marrow. It first makes two progenitor cells: the common myeloid progenitor and the common lymphoid progenitor. Each then makes its own set of finished cells, as Figure 2 shows.

Figure 2. Blood formation. One haematopoietic stem cell makes every blood cell: the myeloid line makes red cells, platelets and the white cells that engulf microbes; the lymphoid line makes the B cells, T cells and natural killer cells of specific immunity.

Read the numbers in the figure. Red blood cells carry oxygen and live about 120 days. Platelets clot blood and live about 7–10 days. Neutrophils engulf microbes. Eosinophils fight parasites and drive allergy. Basophils release histamine. Monocytes leave the blood and become macrophages. B cells make antibodies, T cells kill infected cells, and natural killer cells kill abnormal cells — the three lymphoid types are the cells of specific immunity. Because red cells live only about 120 days and platelets about a week, the marrow must replace them constantly: about 2 million red blood cells are made every second. That is why a bone marrow transplant works — give a patient healthy blood stem cells and the whole system rebuilds itself, cell type by cell type. The bioenergetics MCQs show what all that dividing costs in energy.

What are iPS cells?

In 2006 Shinya Yamanaka did something textbooks had said was impossible: he turned an adult cell back into a stem cell. He took a mouse skin cell and made it express four transcription factors — Oct4, Sox2, Klf4 and c-Myc — the proteins that switch on the genes of pluripotency in an embryo. The factors switched the skin cell's pluripotency genes back on and switched its skin genes off, and the cell became pluripotent again. The result is an induced pluripotent stem cell (an iPS cell — a pluripotent stem cell made by reprogramming an adult cell, not taken from an embryo). Human iPS cells followed in 2007, and Yamanaka shared the Nobel Prize in 2012 with John Gurdon, who had shown decades earlier that a specialised cell's nucleus still holds the full genome.

Figure 3. Making iPS cells. Four transcription factors reprogramme an adult skin cell back to pluripotency. The iPS cell can then be turned into any cell type — heart muscle, nerve or insulin-making beta cells among them — with no embryo used at any step.

iPS cells matter for three reasons. First, they avoid the embryo question entirely — no embryo is created or destroyed. Second, they can be made from the patient: skin or blood cells from a person with Parkinson's disease can be reprogrammed and then turned into dopamine neurons in a dish, giving scientists the exact diseased cells to study. That is disease modelling (growing a patient's diseased cells in the laboratory to study the disease and test drugs on them). Third, drugs can be tested on those cells before they are ever given to a person. The risk to remember is the same as for any pluripotent cell: an iPS cell that is not fully differentiated can form a tumour, so every cell must be checked before it goes near a patient. The climate change lesson shows the same idea of modelling a system in miniature before acting on the real thing.

What can stem cells do in medicine?

Three stem cell treatments are proven, routine medicine. The first and biggest is haematopoietic stem cell transplantation (HSCT — a bone marrow transplant, in which a patient's diseased marrow is replaced with healthy blood stem cells). It treats leukaemia, lymphoma, aplastic anaemia, thalassaemia and severe immune deficiencies. More than 1.5 million transplants were done worldwide between 1957 and 2019, at about 84,000 a year, and the global donor pool holds over 44 million volunteer donors plus 0.76 million stored cord blood units. The second is skin grafts: a small piece of a burn patient's own skin is grown in the laboratory into large sheets of new skin, which are grafted onto the burn without rejection. The third is limbal stem cell transplant: stem cells from the rim of the cornea can restore sight when the cornea is damaged by burns or disease.

Beyond those three, stem cells are in trials — tested, but not yet routine. In Parkinson's disease, dopamine-making neurons grown from stem cells are being transplanted into the brain. In type 1 diabetes, insulin-making beta cells (the cells of the pancreas that make insulin) grown from stem cells are being tested. In macular degeneration, the first iPSC trial ran in Japan in 2014: retinal cells grown from the patient's own iPS cells were put into the eye, so no immune-suppressing drugs were needed. Cord blood is also banked: the blood left in the umbilical cord after birth is rich in blood stem cells, and the first cord-blood transplant was done in 1988 for a child with Fanconi anaemia, a disease in which the bone marrow fails.

Matching matters in every transplant. Donor and patient are matched on HLA markers (the identity tags on the surface of cells that the immune system reads). A mismatch means the patient's immune system attacks the graft, or the donor's immune cells attack the patient — graft-versus-host disease (donor immune cells attacking the patient's body). Only about a quarter of patients have a fully matched sibling donor, which is why the 44-million-strong volunteer registry exists. The specific immunity lesson explains how the immune system tells self from foreign.

Seven milestones in stem cell medicine

Figure 4. Seven milestones. Cord blood came first (1988), then human embryonic stem cell lines (1998), then reprogramming (2006–2007), then the first trials (2010, 2014) and the Nobel Prize (2012).

Read the timeline left to right: 1988, the first cord-blood transplant; 1998, the first human embryonic stem cell lines; 2006, mouse iPS cells; 2007, human iPS cells; 2010, the first embryonic stem cell trial, for spinal cord injury; 2012, the Nobel Prize for reprogramming; 2014, the first iPSC trial, for macular degeneration. The pattern to notice is the gap: the first transplant (HSCT) was routine decades before the first embryonic stem cell trial, because blood stem cells were understood — and safe — long before pluripotent cells were.

What stem cells cannot do yet

Honesty matters here, because clinics sell hope. Most conditions that stem cell clinics advertise — autism, cerebral palsy, spinal cord injury, ageing — have no proven stem cell treatment. The clinics inject cells that have not been tested in trials, and the risks are real. Pluripotent cells that are not fully controlled can form a teratoma (a tumour that contains a jumble of different tissues, such as hair, bone and muscle). Any injection can carry infection. And the cells may simply do nothing at all. Three safety rules decide whether a stem cell treatment is real medicine: it has been tested in a proper clinical trial, the cells are fully differentiated before use, and the source is a licensed hospital, not a clinic selling injections. If a treatment fails any of the three, it is not proven medicine — whatever the advertisement says.

There is also a limit built into the biology. A multipotent stem cell cannot become any cell type — a blood stem cell cannot make a nerve cell. An adult stem cell divides a limited number of times, and its daughters age with it. And even a perfect cell cannot fix a disease whose cause is not the missing cell: replacing dopamine neurons helps Parkinson's disease because the missing cells are the problem, but it cannot help a disease caused by a faulty gene in every cell of the body unless the gene is fixed too. That is why gene therapy and stem cell therapy are often combined in current trials.

What are the ethical questions?

The ethical debate has one centre: the embryo. Embryonic stem cells come from the inner cell mass of a blastocyst, and taking them destroys the embryo. Some people regard the embryo as a human life and oppose the research; others argue that a five-day ball of cells is not a person, and that the medical benefits — treatments for diabetes, Parkinson's disease and spinal cord injury — outweigh the objection. The UK draws a legal line with the 14-day rule: a human embryo may not be grown in the laboratory beyond 14 days after fertilisation, the point at which the primitive streak (the first sign of the body's axis) appears. The rule is written into the Human Fertilisation and Embryology Act 1990 and enforced by the HFEA.

Two developments have softened the debate without ending it. The first is iPS cells: because they come from adult cells, they avoid the embryo question entirely. The second is therapeutic cloning (making an early embryo with the same genes as the patient, so that stem cells taken from it would not be rejected). Therapeutic cloning still uses an embryo, so it carries the same objection — but it solves the rejection problem that ordinary embryonic stem cells have. For the exam, the evaluation must balance three things: the medical benefit, the risk (tumours, rejection, infection), and the ethical objection — and then give a reasoned judgement, not just a list. The model paragraph in the next-but-one section shows how.

Five common mistakes

1. Saying a stem cell can become any cell type. Only totipotent cells can. A blood stem cell is multipotent — it makes blood cells only. Always name the potency level before claiming a range.
2. Saying differentiation changes the DNA. It never does. Differentiation changes which genes are read. The DNA stays the same, which is why reprogramming works.
3. Mixing up pluripotent and totipotent. Totipotent can make the placenta too and can form a whole organism; pluripotent cannot make the placenta. The placenta is the test.
4. Saying embryonic stem cells are the only useful kind. The most used stem cell treatment in the world is the bone marrow transplant, which uses adult blood stem cells — and it has been routine for decades.
5. Saying stem cells can cure anything. Only three treatments are proven (bone marrow transplant, skin grafts, corneal repair). Everything else is in trials, and unproven clinic injections carry real risks.

How to answer a 6-mark question

AQA asks students to evaluate the use of stem cells in treating human disorders. An evaluation is not a list — it is a judgement with reasons on both sides. Step 1: state the benefit with a named example (bone marrow transplant for leukaemia; insulin-making beta cells for type 1 diabetes). Step 2: state the risk with a named example (teratoma from pluripotent cells; rejection without HLA matching). Step 3: state the ethical point (embryo destruction for embryonic stem cells; iPS cells avoid it). Step 4: weigh them — the proven treatments against the unproven ones. Step 5: conclude with a judgement: stem cells are valuable where the treatment is proven and the cells are matched, but unproven uses are not justified.

Model paragraph (6 marks). Evaluate the use of stem cells in treating human disorders. "Stem cells can treat human disorders: bone marrow transplants using haematopoietic stem cells cure leukaemia, with over 1.5 million transplants done worldwide, and iPS cells can be made from a patient's own skin, avoiding rejection. However, there are risks: pluripotent cells can form teratomas if they are not fully differentiated, and donor cells can be rejected unless HLA matched. There are also ethical concerns, because obtaining embryonic stem cells destroys the embryo, although iPS cells avoid this. Overall, stem cell treatment is justified where it is proven and the cells are matched and fully differentiated, as in bone marrow transplants, but unproven clinic treatments are not justified because the benefit is untested and the risks are real." Every sentence earns its mark: benefit, risk, ethics, weighing, judgement.

FAQs — stem cells

Are stem cells the same as cancer cells? No. Both divide, but a stem cell divides under control — it stops when the niche tells it to, and its daughters differentiate. A cancer cell has lost that control: it divides without signals and does not differentiate properly. The difference is control, not division.

Can adults still make new stem cells? Adults do not make new embryonic stem cells, but they keep small pools of adult stem cells — in the bone marrow, gut, skin, brain and cornea — that divide throughout life to replace worn-out cells. The pools shrink with age, which is one reason healing slows down.

Why is a bone marrow transplant not called a stem cell transplant? It is — haematopoietic stem cell transplantation (HSCT) is its proper name. "Bone marrow transplant" is the older name, from the time the cells were taken straight from the marrow. Today the cells are often collected from the blood or from cord blood instead.

Do stem cells come from the patient or a donor? Both. An autologous (from the patient) transplant uses the patient's own cells, so there is no rejection — but the cells may carry the same disease. An allogeneic (from a donor) transplant uses a matched donor's cells, which can cure the disease but needs HLA matching and carries the risk of graft-versus-host disease.

Can stem cells grow a whole new organ? Not yet. Scientists can grow small pieces of tissue — sheets of skin, patches of retina, clusters of beta cells — but a whole organ needs blood vessels, nerves and a precise three-dimensional shape that no one can yet build. Organoids (tiny lab-grown models of organs) are used for research and drug testing, not for transplant.

Is the 14-day rule the same everywhere? No. The UK's 14-day limit is law, and several countries follow it. Other countries set different limits or have no limit at all. The rule is a legal and ethical line, not a biological one — the biology of the embryo is the same everywhere.

Sources

• AQA GCSE Biology specification 8461, §4.1.1.4 Cell differentiation and §4.1.2.3 Stem cells — aqa.org.uk
• AQA A-level Biology specification 7402, §3.8.2.1 Gene expression is controlled by a number of features — aqa.org.uk
• Edexcel GCSE Biology (9-1) 1BI0, Topic 1 Key concepts in biology, and Edexcel A-level Biology A, Topic 3 Cell differentiation — qualifications.pearson.com (GCSE) and qualifications.pearson.com (A-level)
• OCR A-level Biology A (H420), Module 2.1.6 Cell division, cell diversity and cellular organisation — ocr.org.uk
• AP Biology Course and Exam Description, Unit 6 Gene Expression and Regulation (Topics 6.5/6.6) — College Board AP Central
• IB DP Biology Guide 2023, Theme B, B2.3 Cell Specialization — ibo.org
• NIH Stem Cell Information — stemcells.nih.gov
• International Society for Stem Cell Research — isscr.org
• EuroStemCell — eurostemcell.org
• OpenStax Biology 2e, Chapter 47 Animal Development — openstax.org
• Worldwide Network for Blood and Marrow Transplantation, Haematologica 2022 (1.5 million transplants 1957–2019) — unimelb.edu.au

Continue learning

• Cell Division: Mitosis, Meiosis and the Cell Cycle — the mitosis that self-renewal uses, and the checkpoints that stop it going wrong.
• Cellular Totipotency — totipotency in depth, in plants and animals.
• Inheritance and Genetic Variation: Genes, Crosses and Variation — how the same genome is passed on to every cell.
• Specific Immunity: Humoral vs Cell-Mediated Defense — the B cells and T cells that blood stem cells make, and why HLA matching matters.
• Stem Cultures in Plant Tissue Culture: History, Advantages, Protocol & Why It Matters in 2026 — the plant side: meristem stem cells that keep their potency for life.

20 Explained MCQs — answers hidden until opened

Open each question to reveal its answer and explanation. The interactive quiz below uses a different set of questions.

1. What is a stem cell?

A. A cancer cell that divides without control   B. An undifferentiated cell that can give rise to many more cells of the same type, and from which certain other cells can arise by differentiation   C. A cell with no nucleus that carries oxygen around the body   D. A cell that has already specialised and can never divide again
Answer: B. The AQA definition has two parts: a stem cell can copy itself (self-renewal) and its daughters can become different, specialised cells (differentiation). A specialised cell cannot do the first part, and a cancer cell has lost control of division.

2. Which two properties define every stem cell?

A. Mitosis and meiosis   B. Photosynthesis and respiration   C. A nucleus and a cell wall   D. Self-renewal and potency
Answer: D. Self-renewal means the stem cell can divide to make more stem cells, so the supply never runs out. Potency means it can give rise to specialised cells. A cell with only one of the two is not a stem cell.

3. Which type of stem cell can become the widest range of cell types?

A. Pluripotent — it can become any body cell but not the placenta   B. Totipotent — it can become any cell type, including the cells of the placenta   C. Multipotent — it can become a few related cell types   D. Unipotent — it can become only one cell type
Answer: B. Totipotent is the top of the potency ladder: the zygote and the cells of the very early embryo can make every cell of the body plus the extra-embryonic tissues such as the placenta, so they can form a whole organism.

4. Where are totipotent cells found?

A. In the skin of an adult   B. In the inner cell mass of the blastocyst only   C. In the zygote and the cells of the very early embryo, for the first few divisions after fertilisation   D. In the bone marrow of an adult
Answer: C. Totipotency lasts only for a limited time in early mammalian embryos. After the first few divisions the cells become pluripotent, then more restricted. Bone marrow and skin hold multipotent and unipotent stem cells, not totipotent ones.

5. Where are pluripotent stem cells found in a human embryo?

A. In the inner cell mass of the blastocyst, about five days after fertilisation   B. In the umbilical cord blood of a newborn baby   C. In the lining of the gut   D. In the placenta
Answer: A. The blastocyst is a hollow ball of cells. Its inner cell mass is pluripotent: it can become almost any body cell type, but not the placenta. Embryonic stem cell lines are grown from this inner cell mass.

6. A haematopoietic stem cell in the bone marrow can make red blood cells, white blood cells and platelets. What kind of potency is this?

A. Multipotent — a limited range of related cell types   B. Unipotent — one cell type only   C. Totipotent — any cell type at all   D. Pluripotent — any body cell type
Answer: A. A haematopoietic stem cell makes many types, but they are all blood cells — one lineage. That limited range inside one tissue is the definition of multipotent. Totipotent and pluripotent cells can make far more types, across many tissues.

7. A stem cell in the skin can only make new skin cells. What kind of potency is this?

A. Totipotent — any cell type at all   B. Pluripotent — any body cell type   C. Unipotent — one cell type only   D. Multipotent — a limited range of related cell types
Answer: C. Unipotent means one: the cell can divide and make copies of itself and one specialised type. Skin basal cells and the cardiac progenitor cells that make only cardiomyocytes (heart muscle cells) are the textbook examples.

8. When a stem cell divides asymmetrically, what happens?

A. Two specialised cells are made and the stem cell is lost   B. Two identical stem cells are made and the pool doubles   C. The cell divides by meiosis into four gametes   D. One daughter stays a stem cell and the other becomes a progenitor cell that will specialise
Answer: D. Asymmetric division is how the body gets new specialised cells without losing its stem cells: one daughter keeps the stem-cell job, the other starts down the road to specialisation. Symmetric division, which makes two stem cells, is how the pool grows.

9. What is a stem cell niche?

A. The local environment around a stem cell — neighbouring cells, signalling molecules and the surrounding matrix — that controls how the stem cell behaves   B. The fluid that stem cells are stored in after a transplant   C. The nucleus of a stem cell   D. A disease caused by stem cells dividing too fast
Answer: A. The niche is the stem cell's home: its neighbours and their signals tell it when to stay quiet, when to divide and when to differentiate. The bone marrow niche and the hair follicle niche are the IB examples.

10. A skin cell and a nerve cell have the same DNA, but they look and work completely differently. Why?

A. The nerve cell lost half of its DNA during development   B. They have different numbers of chromosomes   C. They express different sets of genes — differentiation is differential gene expression   D. The skin cell has no genes for nerve proteins at all
Answer: C. Every body cell carries the same genome. Differentiation works by switching genes on and off: a red blood cell precursor switches on the haemoglobin genes, a nerve cell switches on nerve genes. The DNA itself does not change — which genes are read changes.

11. In a developing red blood cell, which genes are switched on?

A. The nerve genes, so the cell can carry signals   B. The haemoglobin genes, so the cell fills with haemoglobin and loses its nucleus to make room   C. The insulin genes, so the cell can control blood sugar   D. All of the cell's genes at once, so the cell can do every job
Answer: B. A red blood cell's job is to carry oxygen, so it switches on the genes for haemoglobin and switches off the genes it does not need. As it matures it even loses its nucleus, leaving more room for haemoglobin. It cannot divide after that.

12. Pluripotent cells can make cells of all three germ layers. Which three?

A. Ectoderm, mesoderm and endoderm   B. Mesoderm, endoderm and bone marrow   C. Ectoderm, endoderm and placenta   D. Epidermis, dermis and hypodermis
Answer: A. The three germ layers form in the early embryo: ectoderm makes skin and nerve, mesoderm makes muscle, blood and bone, endoderm makes the gut, liver and lungs. A pluripotent cell can become any of the three — that is the test of pluripotency.

13. How are induced pluripotent stem (iPS) cells made?

A. A skin cell is fused with an egg cell   B. An adult skin cell is made to express four transcription factors — Oct4, Sox2, Klf4 and c-Myc — which reprogramme it back to a pluripotent state   C. An embryo is cloned and its inner cell mass is removed   D. Bone marrow is taken from a donor and given to a patient
Answer: B. Yamanaka's four factors switch on the genes for pluripotency that were silenced in the adult cell, and switch off the genes of its specialised state. The cell becomes pluripotent again without any embryo being used. Mouse iPS cells were made in 2006, human iPS cells in 2007.

14. What did Shinya Yamanaka win the Nobel Prize for in 2012?

A. Performing the first bone marrow transplant   B. Discovering the first human embryonic stem cell lines   C. Showing that adult cells can be reprogrammed into pluripotent stem cells   D. Discovering DNA
Answer: C. Yamanaka shared the 2012 Nobel Prize with John Gurdon. The first human embryonic stem cell lines were grown by James Thomson in 1998, and the first bone marrow transplants were done in the 1950s.

15. A patient with leukaemia receives a bone marrow transplant. What is the transplant meant to do?

A. Replace the patient's diseased bone marrow with healthy blood stem cells that can make normal blood cells   B. Replace the patient's heart muscle with new cardiomyocytes   C. Give the patient new skin to cover burns   D. Give the patient a new immune system that attacks cancer directly
Answer: A. In leukaemia the bone marrow makes faulty white blood cells. A transplant (HSCT) first destroys the diseased marrow, then gives healthy haematopoietic stem cells — from the patient or a matched donor — which settle in the marrow and rebuild the whole blood system.

16. How many haematopoietic stem cell transplants have been done worldwide?

A. Fewer than 1,000 in total, because the treatment is still experimental   B. None — the treatment has never been used in patients   C. About 1.5 million every year   D. More than 1.5 million between 1957 and 2019, at about 84,000 per year
Answer: D. HSCT is the oldest and most proven stem cell treatment. The Worldwide Network for Blood and Marrow Transplantation counted more than 1.5 million transplants from 1957 to 2019, with about 84,000 a year. It is routine medicine, not an experiment.

17. Why is umbilical cord blood collected and stored after a baby is born?

A. It contains embryonic stem cells that can make a whole new baby   B. It is used to make vaccines against childhood diseases   C. It is rich in haematopoietic stem cells that can be used for transplants later   D. It contains only red blood cells, which are used for transfusions
Answer: C. Cord blood is the blood left in the umbilical cord and placenta after birth. It is rich in blood stem cells, and the first cord-blood transplant was done in 1988 for a child with Fanconi anaemia. Stored cord blood can be a matched source for a transplant.

18. Why do some people object to research on embryonic stem cells?

A. Obtaining the cells destroys a human embryo, which some people regard as a human life   B. Embryonic stem cells are always rejected by the patient's body   C. Embryonic stem cells can never be grown in a laboratory   D. Embryonic stem cells can only make blood cells
Answer: A. Embryonic stem cell lines come from the inner cell mass of a blastocyst, and taking the cells destroys the embryo. That is the ethical objection. iPS cells were welcomed partly because they avoid this step — no embryo is used at all.

19. What is the UK 14-day rule for human embryos?

A. Embryos may be kept for 14 years before they must be destroyed   B. Human embryos may not be grown in the laboratory beyond 14 days after fertilisation   C. Embryonic stem cells must be used within 14 days of being made   D. A stem cell transplant must be done within 14 days of diagnosis
Answer: B. Under the Human Fertilisation and Embryology Act 1990, a human embryo cannot be kept or used in research beyond 14 days after fertilisation — the point at which the primitive streak appears. The rule draws a legal line around embryo research.

20. Why do doctors warn patients against unproven stem cell clinics?

A. Stem cell treatments are illegal in every country   B. The treatments have no proven benefit, and pluripotent cells can form tumours while injections carry a risk of infection   C. Stem cells from a clinic can never divide once they are injected   D. The clinics only treat plants, not people
Answer: B. Clinics that sell stem cell injections for many diseases have not proved their treatments work in trials. The risks are real: pluripotent cells can grow into tumours (teratomas), and any injection can carry infection. Proven stem cell medicine is HSCT, skin grafts and corneal repair — the rest is mostly still in trials.

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