What you need to know
- Define a stem cell and differentiation.
- Compare embryonic stem cells, adult stem cells and plant meristems.
- Describe potential medical and agricultural uses.
- Explain the principle of therapeutic cloning.
- Evaluate benefits, risks and ethical concerns using evidence.
What stem cells do
An unspecialised cell that can divide by mitosis and differentiate into one or more specialised cell types.
Stem cells first make more cells by mitosis. Chemical signals then activate different genes, causing some cells to produce particular proteins and develop specialised structures. This is differentiation.
Types of stem cell
| Source | Potential | Example or limitation |
|---|---|---|
| Embryonic stem cells | Can differentiate into almost any type of body cell. | Obtained from an early embryo; use raises ethical concerns. |
| Adult stem cells | Usually form a more limited range of cells. | Bone-marrow stem cells produce different blood cells. |
| Plant meristem cells | Can differentiate into any plant-cell type throughout the plant's life. | Found in growing tips of roots and shoots. |
The term adult stem cell means a stem cell found in developed tissue; it does not mean that it exists only in adults. For example, bone-marrow stem cells are present in children.
Potential medical uses
Scientists aim to direct stem cells to form replacement tissues. Potential applications include:
- replacing insulin-producing cells in people with type 1 diabetes;
- replacing nerve cells damaged by spinal injury;
- replacing dopamine-producing brain cells in Parkinson's disease;
- producing skin or other tissues after severe damage;
- bone-marrow transplants to replace blood-forming cells.
Some uses, such as bone-marrow transplantation, are established treatments. Others remain experimental. A potential use is not automatically a safe or effective treatment.
Therapeutic cloning
Therapeutic cloning can produce embryonic stem cells genetically matched to a patient.
Because the stem cells contain the patient's nuclear DNA, transplanted cells are less likely to be rejected by the immune system. However, mitochondrial DNA comes from the egg donor.
Therapeutic cloning aims to produce cells or tissues for treatment. Reproductive cloning aims to produce a whole cloned organism.
Plant meristems and tissue culture
Meristems contain stem cells that keep dividing and differentiating throughout a plant's life. Small groups of meristem cells can be grown on sterile nutrient media with plant hormones to produce genetically identical plants. This is tissue culture or micropropagation.
Applications include:
- rapidly producing many plants with desirable characteristics;
- producing disease-free stock from clean tissue;
- conserving rare plant species;
- cloning crop plants with reliable yield or quality.
Genetic uniformity can also be a disadvantage: if all plants share the same susceptibility, one disease or environmental change may affect the whole crop.
Evaluating stem-cell use
Potential benefits
- Replace damaged cells and treat serious disease.
- Reduce suffering and dependence on donor organs.
- Patient-matched cells may reduce rejection.
- Allow researchers to study development and test medicines.
Risks and concerns
- Cells may divide uncontrollably and form tumours.
- Infection or contamination is possible.
- Immune rejection may occur if cells are not matched.
- Embryos are destroyed when embryonic stem cells are collected.
- Long-term effects may be uncertain.
Ethical arguments depend on values as well as evidence. Some people consider the early embryo to have a right to life. Others argue that using donated embryos, which would otherwise be discarded, can be justified by the possibility of treating serious disease. Adult stem cells and induced pluripotent cells may avoid some concerns but do not remove every scientific limitation.
State a benefit, state a risk or limitation, compare their importance in the given context, and finish with a justified conclusion. Do not present a list without a judgement.
Quick retrieval check
1. What two abilities define a stem cell?
2. Why are embryonic stem cells more versatile than most adult stem cells?
3. Why might patient-matched therapeutic-cloning cells be less likely to be rejected?
4. Give one advantage and one disadvantage of cloning crop plants.
Exam connection
Evaluate the use of embryonic stem cells to treat a person with a spinal-cord injury.
Show a model line of reasoning
Embryonic stem cells may differentiate into replacement nerve cells and could restore some lost function, giving a major benefit where current treatments are limited. However, transplanted cells could be rejected or form tumours, and collecting the cells destroys an embryo. Use may be justified if the embryo was donated with informed consent, the treatment has strong safety evidence and the likely benefit outweighs the risk, but the decision depends partly on ethical views about embryo status.