These are cells that have the ability to replicate almost indefinitely, in the body and in culture, and which can give rise to more specialised cells when exposed to the right biochemical cues.

Embryonic stem cells, found in the earliest developmental stages of an embryo, can give rise to nearly all types of cells found in the body.

The single cell formed from fusion of a sperm and an egg is called a totipotent cell, because it has the capability to create all of the cells that make up an entire new human.

The first few divisions produce more totipotent cells. But once there are about a hundred cells in total, the cells begin to specialise.

One set — the outer layer of the hollow sphere called the blastocyst — become cells which will go on to form only the placenta and other tissue needed to support the development of the foetus in the uterus.

The inner cells of the blastocyst are known as pluripotent cells.

They are still highly unspecialised, and each one could go on to form almost any of the cells found in the body, except the placenta and other tissues described in the paragraph above. These cells then undergo further specialisation until they eventually become multipotent stem cells that can make only one specific group of cell types, for example blood cells or skin cells. These multipotent (or adult) cells are essential to the replenishment of body cells throughout life — for example, creating new skin cells or new blood cells to replenish those that are damaged or naturally die.

Adult humans have about 20 types of multipotent stem cell. It is now emerging that they are not as cell-specific as was originally thought, and that it is possible, under certain conditions, to make them produce a different type of specialised cell.

Potential Uses of Embryonic and Adult Stem Cells

Study of embryonic stem cells may lead to:

• A much fuller understanding of the complex cellular and biochemical processes by which a single, fertilised cell turns into a fully fledged human being.
• A better understanding of the mechanisms that give rise to birth defects and to conditions that arise in later life.
• New ways of developing drugs and testing them for safety by doing initial tests on cloned human cells. Only drugs that succeed at this preliminary stage would be passed for testing in animals and human volunteers.
• The ‘manufacture’ of cells and tissue for therapeutic purposes. Many companies are actively seeking stem cell techniques that will produce treatments — or even cures — for diseases ranging from diabetes to heart disease.

Adult stem cells have the ethical advantage that they are not derived from the microscopic balls of cells that many people consider to be the first stages of individual life. But one problem is that adult stem cells are present in minute quantities, and are difficult to find in the body.

Another problem is that they are multipotent rather than pluripotent; are unsuitable for some of the applications listed above.

But researchers are having some success in using them to create new cells and tissue. They do have the advantage that tissue rejection can be avoided by taking stem cells from a particular person, encouraging the cells to divide and manufacture new cells, then transplanting the new cells back into the same person.

There is also the additional problem that stem cells from diseased patients may contain the source of the genetic defect and hence only produce cells containing that defect.

For a more detailed background to stem cells and stem cell research, teachers are recommended to access two of the websites listed in the links section:

http://www.nih.gov/news/stemcell/primer.htm
The US National Institute of Health Primer of Stem Cells.

http://www.newscientist.com/nsplus/insight/clone/stem/highlycultured.html
The ‘New Scientist’ outline of current stem cell research projects.