Under the Microscope:Stem-cell research offers the hope of producing treatments and cures for many serious medical conditions, writes Dr William Reville.
Human stem cells are conventionally obtained from the embryo, the foetus, umbilical cord or adult tissues. Serious ethical considerations apply to harvesting stem cells from the embryo or the foetus. As I write, the creation of human stem cells with embryonic qualities from skin cells has been announced. If this technique proves reliable and safe, it will remove the pressure to harvest stem cells from embryos.
Stem cells can grow and divide indefinitely to produce more stem cells. They are undifferentiated cells, ie not specialised cell types specific to tissues, eg muscle, but, under the right conditions can differentiate into one or many specific cell types. The flexibility of differentiation is called potency. A totipotent stem cell is one that can differentiate into any tissue cell type. The zygote (the egg fertilised by sperm at conception) is totipotent, as also effectively are the cells produced up to the embryonic blastocyst stage about four or five days after conception. Blastocyst cells can give rise to the 200-plus cell types in the adult body.
Stem cells, in decreasing order of potency, can also be isolated from the foetus, umbilical cord and adult tissue. Typically stem cells of adult tissues are either multipotent (can differentiate only into cells of a closely related family of cells) or unipotent (can differentiate only into one cell type, eg muscle cells). It is generally easier to harvest and grow stem cells from the blastocyst than from other sources.
Stem cells in adult tissue provide a reservoir that can differentiate and grow to repair any damage to the tissue. Medicine hopes to develop therapies to repair or regenerate failing organs or tissues by infusion of stem cells. In order to do research on stem cells that might produce such treatments, they are first harvested from one of the sources mentioned above and then grown on indefinitely in the laboratory as stem cell lines.
Most biologists feel that embryonic stem cells have the greatest medical potential because they are the most potent and the easiest to harvest. However, this great flexibility poses a problem because science does not yet know how to control the differentiation of these cells. If you infuse embryonic stem cells into the brain, for example, hoping they will differentiate into brain tissue, they may differentiate instead into some other tissue type (eg liver), or into a tumour. Consequently there are no clinical trials ongoing anywhere using embryonic stem cells. Such results as we have to-date on the human medical applications of stem cells come from adult stem cells. I think it is fair to say that all of the potential medical results of embryonic stem cells are also available from cord blood stem cells and adult stem cells, but in the latter cases, the potential will probably be more difficult to realise.
Embryonic stem cells are usually harvested from "spare" embryos from IVF procedures. Harvesting embryonic stem cells from the blastocyst kills the embryo which is ethically unacceptable to the many people who believe that the embryo has human moral status. Foetal stem cells are prepared from aborted foetuses, which is also unacceptable to many.
Human life begins at conception when a sperm cell from the father fuses with an egg cell from the mother to produce a unique new cell called a zygote. The zygote divides to produce two cells, which again divide to form four cells, and so the process continues. After a few cycles of cell division, the entity is called an embryo.
The cells continue to divide and differentiate to form the foetus which develops in the womb into a baby, who is born and develops through childhood into an adult who grows old and dies. The whole process from zygote to death is a continuous unfolding between two boundary conditions - conception and death.
Each of us was once a zygote. The full genetic plan that directed our development into mature adults was present in the zygote. No new genetic information is added. Human life is obviously present at all stages - zygote, embryo, foetus, child, adult, old person - because the cells are alive and of the species Homo Sapiens. But when does full human life begin?
I believe that full human life begins at conception. Each stage of subsequent development occurs on a continuum and each is the full expression of humanity appropriate to its stage. It is logical to choose conception as the instant when full human life begins because this event starts the continuum of life that ends only at death. To pick a later point as marking the beginning of full human life is, in my opinion, illogical and arbitrary.
The announcement on November 20th that human skin cells can be genetically transformed into stem cells of high potency is a great breakthrough. If this technique works efficiently it will dissolve the case for harvesting stem cells from human embryos, thereby removing a huge ethical headache. Dr Ian Wilmut, the scientist who cloned Dolly the sheep, has decided to pursue this new approach instead of working with embryos.
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie