Stem Cells and Their Use in Medicine

This wide range of possibilities also makes embryonic stem cells valuable in medical research. Cells grown in the laboratory can be stimulated to differentiate into the types needed to replace damaged tissue.

Adult stem cells maintain and repair tissues

Adult stem cells remain in particular tissues after early development. They produce new cells for growth and replace cells that are damaged or lost. Unlike embryonic stem cells, they can differentiate into only a limited range of related cell types.

For example, bone marrow inside bones contains stem cells that continually produce new blood cells throughout life, including red blood cells and white blood cells. Stem cells in the skin help replace skin cells. Their narrower range of possible cell types suits their role in maintaining particular tissues rather than building an entire body.

Meristems provide new cells for plant growth

Plants also retain unspecialised cells. These are found in regions of tissue called meristems, including the tips of roots and shoots and regions within stems and roots. Meristem cells divide by mitosis, maintaining a supply of unspecialised cells and producing cells that can differentiate into specialised plant cells.

At root and shoot tips, this supply of new cells enables continued growth in length. Cell division increases the number of cells; elongation increases their length; differentiation gives them the specialised features needed for their jobs. Meristems elsewhere in stems and roots also contribute to growth in width.

For example, cells produced by shoot meristems can form new leaves and flowers. Meristem cells also give rise to tissues such as xylem, which transports water, and phloem, which transports sugars. Meristems therefore do more than make a plant larger: they supply the specialised cells needed to build its growing structures.

Unlike adult animal stem cells, which produce a limited range of related cell types, meristem cells can give rise to any type of plant cell. Their continued activity allows plants to keep producing new growth.

How stem cells could replace damaged cells

The medical value of stem cells comes from their ability to produce specialised replacement cells. Scientists can grow stem cells in controlled laboratory conditions, a process called cell culture, and encourage them to differentiate. Suitable cells can then be transplanted into a patient.

Some uses are already established. Bone marrow transplants containing adult stem cells are used to treat some patients with leukaemia, a cancer affecting blood-forming cells. The transplanted stem cells can produce new blood cells.

Other uses have potential but should not be described as guaranteed cures:

• In type 1 diabetes, the pancreas does not produce enough insulin to control blood glucose. Stem cells could be differentiated into insulin-producing pancreatic cells and transplanted to replace the missing function.
• Some forms of paralysis result from damage to nerve cells in the brain or spinal cord, preventing signals from reaching muscles. Stem cells could potentially produce replacement nerve cells for the damaged region.

Embryonic stem cells offer a wider range of possible replacement cells than adult stem cells. However, producing cells of the required type is only part of the challenge: a treatment must also work safely in the patient.

Biological benefits and risks

Replacing damaged cells could restore tissue function, reduce disability and improve quality of life. Growing replacement tissue could also reduce reliance on donated organs. Using a patient's own stem cells can reduce the risk of immune rejection.

There are important biological risks. A patient's immune system may recognise donor cells as non-self and attack them: this is immune rejection. Cells grown in culture could become contaminated with viruses, which might be transmitted to the patient. Changes in cultured cells could also lead to uncontrolled cell division and cancer.

Adult stem cells have already demonstrated benefits in some treatments, but success with one disease does not prove that a proposed treatment for another disease will be safe or effective. Claims about new treatments need reliable clinical evidence.

Ethical choices and access to treatment

Obtaining embryonic stem cells can involve destroying an embryo. Some people object because they believe an embryo has a right to life. Others argue that the potential to relieve suffering justifies using very early embryos, including unused embryos from fertility treatment. Questions about who can give permission to use these embryos are also important.

Adult stem cells avoid the specific ethical objection of destroying an embryo, although they cannot produce as wide a range of cell types. Treatments and cell storage can also be expensive, raising questions about who can access them, while suitable donors may be limited.

A balanced judgement considers the particular treatment: how strong is the evidence that it helps, what biological risks remain, and how were the cells obtained? A promising benefit does not remove a risk or settle an ethical disagreement.

• Stem cells: divide to maintain a supply of undifferentiated cells and differentiate into specialised cells.
• Embryonic: form the different tissues and organs during development; can produce any type of body cell.
• Adult: maintain and repair particular tissues; produce a limited range of related cells. Bone marrow stem cells continually produce blood cells.
• Plant meristems: found at root and shoot tips and within stems and roots. Cells divide by mitosis and differentiate, supplying new cells for continued growth and new tissues. They can give rise to any type of plant cell.

Medical uses

• Established: adult stem cells in bone marrow transplants help treat leukaemia.
• Potential: insulin-producing cells for type 1 diabetes; replacement nerve cells for some forms of paralysis.
• Embryonic stem cells offer a wider range of possible replacement cells than adult stem cells.

Benefits, risks and ethics

• Benefits: replace damaged cells, restore function and reduce reliance on donated organs. A patient's own cells reduce rejection risk.
• Biological risks: immune rejection, infection and uncontrolled division leading to cancer.
• Ethical issue: destroying embryos to obtain stem cells; differing views about an embryo's moral status and consent.
• Access and evidence: costs, donor availability and reliable evidence of safety and effectiveness matter.