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Stem cells in humans

An embryo develops from a fertilised egg. Cells in the early stages of the development of an embryo are known as stem cells. These are cells that have not undergone differentiation. A cell which has not yet become specialised is undifferentiated.

If cells are removed from the embryo they will differentiate into any cell type. These are called embryonic stem cells. Some stem cells remain in the bodies of adults as adult stem cells.

Adult stem cells can be found in limited numbers in several regions of the body, eg:

• brain
• eyes
• blood
• heart
• liver
• bone marrow
• skin
• muscle

Adult stem cells can differentiate into related cell types only, for instance, bone marrow cells can differentiate into blood cells and cells of the immune system, but not other cell types.

Using human stem cells

Stem cells can divide to produce new cells, which can then divide into different cell types. They therefore have the potential to be transplanted into patients to treat medical conditions and disease. They could be used to replace cells that have been damaged or destroyed, for example:

• in type 1 diabetes
• in cases of multiple sclerosis, which can lead to paralysis
• in cases of spinal cord or brain injury, which have led to paralysis

The stem cells used could be:

• embryonic stem cells
• adult stem cells

Embryonic stem cells can differentiate into a wider range of cell types, but are difficult to obtain. The best source is the five-day-old embryo.

Adult stem cells will differentiate into a narrower range of cell types. Bone marrow transplants are an example of adult stem cell transplant. Bone marrow cells will differentiate into different types of blood cells. Bone marrow transplants are carried out:

• in cases of blood cell cancer, eg leukaemia and lymphoma
• when blood cells have been destroyed, eg during cancer treatment

Benefits and risks associated with the use of stem cells in medicine

Stem cells have great potential, eg in treating patients with currently untreatable conditions, growing organs for transplants, and research. The benefits of using your own stem cells include:

• no rejection
• no need to find a donor
• no need for tissue typing

But there are clinical, ethical and social issues associated with their use. These issues will be different for the growth and transplant of adult, embryonic, or therapeutically-cloned stem cells. They will also depend on whether the stem cells are to be used for therapy or research.

Clinical issues

• There is no guarantee how successful these therapies will be, for example, the use of stem cells in replacing nerve cells lost in Parkinson’s disease patients.
• The current difficulty in finding suitable stem cell donors.
• The difficulty in obtaining and storing a patient’s embryonic stem cells. These would have to be collected before birth – some clinics offer to store blood from the umbilical cord when a person is born.
• Mutations have been observed in stem cells cultured for a number of generations, and some mutated stem cells have been observed to behave like cancer cells.
• Cultured stem cells could be contaminated with viruses which would be transferred to a patient.

Ethical issues

• One source of embryonic stem cells is unused embryos produced by in vitro fertilisation (IVF).
• For therapeutic cloning, is it right to create embryos for therapy, and destroy them in the process?
• Embryos could come to be viewed as a commodity, and not as an embryo that could develop into a person.
• At what stage of its development should an embryo be regarded as, and treated as, a person?

Social issues

• Educating the public about what stem cells can and cannot do is important.
• Whether the benefits of stem cell use outweigh the risks.
• Much of the research is being carried out by commercial clinics, so reported successes are not subject to peer review.
• Patients could be exploited by paying for expensive treatments and being given false hope of a cure as stem cell therapies are only in their developmental stages.