A stem cell is an undifferentiated cell — a cell that has not yet specialised.
All stem cells share two key properties:
1. SELF-RENEWAL: they can divide by mitosis to produce more copies of themselves indefinitely.
2. POTENCY: they can differentiate into one or more types of specialised cell.
Stem cells are essential during development — they provide the source from which all specialised cells of the body are produced.
They also play a role in maintenance and repair throughout life — replacing worn-out or damaged cells in certain tissues.
Embryonic Stem Cells
EMBRYONIC STEM CELLS are found in the inner cell mass of a human embryo at 3–5 days old (called a blastocyst).
They are TOTIPOTENT — they can differentiate into ANY of the more than 200 cell types in the human body.
This makes them the most versatile type of stem cell.
How they are obtained: the embryo is destroyed to extract the stem cells. This is usually done using embryos left over from IVF (in vitro fertilisation) treatment that would otherwise be discarded.
Potential uses:
• Replacing insulin-producing beta cells in the pancreas → treating Type 1 diabetes
• Growing new neurones → treating Parkinson's disease or spinal cord injuries
• Producing heart muscle cells → repairing damage after a heart attack
• Growing skin for burns victims
Challenges: immune rejection (the patient's immune system may attack the transplanted cells), and the risk of tumour formation if undifferentiated cells remain.
Adult Stem Cells
ADULT STEM CELLS are found in specific tissues throughout the body — even in fully-grown adults.
They are MULTIPOTENT — they can only differentiate into the cell types found in the tissue where they live.
Bone marrow stem cells are the most well-known and well-used:
• Haematopoietic (blood-forming) stem cells in bone marrow produce ALL types of blood cell: red blood cells, white blood cells, platelets.
Clinical use — bone marrow transplant:
• Used to treat leukaemia (cancer of blood cells).
• The patient's cancerous bone marrow is destroyed by high-dose chemotherapy/radiotherapy.
• Healthy donor bone marrow (containing stem cells) is transplanted → new, healthy blood cells are produced.
Advantages of adult over embryonic stem cells: no ethical controversy, the patient can be their own donor (if using their own stem cells — autologous transplant).
Disadvantages: more limited range of cell types, harder to grow in large numbers.
Plant Meristem Cells
Plants have their own form of stem cells — MERISTEM CELLS, found in regions of the plant called MERISTEMS.
Meristems are located:
• At the tips of roots and shoots (apical meristems) — responsible for lengthening
• Along the sides of stems (lateral meristems) — responsible for thickening
Unlike most animal adult stem cells, plant meristem cells are essentially TOTIPOTENT — they can differentiate into ANY type of plant cell.
This is why plants can keep growing throughout their entire lives — a new leaf, root or flower can always be produced from meristem cells.
Applications:
• CLONING: a single meristem cell or small section of tissue can be grown into an entirely new, genetically identical plant (tissue culture / micropropagation).
• Conserving rare or endangered plant species.
• Mass-producing disease-resistant crop plants.
• Creating large numbers of identical, high-quality plants quickly.
Ethical Issues around Stem Cell Research
Stem cell research — particularly using embryonic stem cells — raises significant ethical questions:
ARGUMENTS FOR:
• Could cure debilitating diseases that currently have no treatment
• Embryos used are from IVF treatment and would be destroyed anyway
• Early embryos (blastocysts) are just a ball of ~100 cells — not yet a person
ARGUMENTS AGAINST:
• Destroying an embryo destroys a potential human life — many religious groups hold this view
• The embryo cannot consent to being used for research
• Concerns that this could lead to cloning of humans ('slippery slope' argument)
• Risk of exploitation — women may be pressured to donate eggs
In the UK, embryonic stem cell research is strictly regulated by the Human Fertilisation and Embryology Authority (HFEA) — research is permitted up to 14 days of embryo development.
⚠️ Common Mistake
Students often say stem cells 'replace organs' — they don't. They differentiate into SPECIFIC CELL TYPES that can be used to repair or replace damaged cells within an organ. Also: embryonic stem cells are TOTIPOTENT (any cell type). Adult stem cells are MULTIPOTENT (limited range). Don't mix these terms up.
📌 Key Note
Embryonic stem cells: totipotent (any cell type), ethically controversial, from IVF embryos. Adult stem cells: multipotent (limited types), bone marrow → all blood cells, used for leukaemia. Plant meristems: totipotent for plants, found at root/shoot tips, used for cloning.
🎯 Matching Activity — Match the Stem Cell Type to its Features
Match each description to the correct type of stem cell. — drag the symbols on the right to match the component names on the left.
Embryonic stem cell
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Adult stem cell (bone marrow)
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Plant meristem cell
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Embryonic stem cell
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Adult stem cell
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Totipotent — can become any of the 200+ cell types in the body
Multipotent — more limited range of cell types than embryonic
Produces all types of blood cell — used in bone marrow transplants for leukaemia
Obtained from early embryos — raises ethical concerns about destroying potential life
Found at root and shoot tips — can differentiate into any plant cell type
⭐ Higher Tier Only
In therapeutic cloning, a patient's nucleus is inserted into an enucleated egg cell — the embryo produced is genetically identical to the patient. Stem cells from this embryo will not be rejected by the patient's immune system. Risks include viral transfer. Ethical issues: some object to embryo use. Plant meristems provide a source of totipotent stem cells for rapid cloning of rare or disease-resistant varieties.
🎯 Test Yourself
Question 1 of 4
1. What does 'totipotent' mean when describing stem cells?
2. How are stem cells used to treat leukaemia?
3. Why are embryonic stem cells ethically controversial?
4. What is special about plant meristem cells compared to adult animal stem cells?
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