Stem cells are cells which are not terminally differentiated and are therefore able to produce cells of other types. Medical researchers hope they can be used to grow specific tissues or organs. There are three types of stem cells: totipotent, pluripotent, and multipotent. A single totipotent stem cell can grow into an entire organism. Pluripotent stem cells cannot grow into a whole organism, but they can become any other type of cell in the body. Multipotent stem cells can only become particular types of cells: e.g. blood cells, or bone cells.
Adult stem cells
Stem cells can be found in adult human beings. Adult stem cells reproduce daily to provide certain specialized cells - for example 200 billion red blood cells are created each day in the body. Until recently it was thought that each of these cells could produce just one particular type of cell - this is called differentiation (see Morphogenesis). However in the past few years, evidence has been gathering of stem cells that can form in to several different forms. Bone marrow stem cells are known to be able to transform in to liver, nerve, muscle and kidney cells.
Adult stem cells may be even more versatile than this. Researchers at the New York University School of Medicine have extracted stem cells from the bone-marrow of mice which are they say are pluripotent. Turning one type of stem cell in to another is called transdifferentiation.
Embryonic stem cells
Stem cells which originate from embryos are seen to have the most potential becuase of their totipotent properties - they are able to grow in to any of the 200 cell types in the body. Embryonic stem cells can be obtained from a cloned embryo, created by fusing a denucleated egg-cell with a patient's cell. The embryo produced is allowed to grow, and stem cells are then extracted. Because they are obtained from a clone, they are genetically compatible with the patient.
As well as having the largest medical potential, they are also the most controversial type of stem cell because their utilization involves the destruction of human embryos. Some people believe that these embryos are human beings, and therefore destroying them for any reason is effectively murder. This belief is also the basis for 'Pro-life' opposition to abortion. Many scientists defend the destruction of embryos citing all the medical benefits that it is possible to achieve with them, and saying that many would have been destroyed anyway. 'Pro-life' groups respond that it would be possible to achieve the same benefits from the use of adult stem cells - although most scientists agree that we are further from using these in the same way hoped for embryonic stem cells.
Another controversy in the use of embryonic stem cells is the use of theraputic cloning. This involves the cloning of early embryos from which stem cells are harvested, providing a larger source of the cells. Many see this as encouraging human cloning, which they think could be dangerous or unethical.
For over 30 years, bone marrow stem cells have been used to treat cancer patients with conditions such as leukemia and lymphoma. These are detroyed in some chemotherapy treatments, but if they are removed before the process and then reinjected, the cells produce large amounts of red and white blood cells, to keep the body healthy and help to fight infection.
Since the 1980s stem cells have been taken from the blood instead of the bone-marrow, making the procedure safer for older people. Although normally scarce, the number of 'Peripheral blood cells' can be increased by a course of drugs, which release the stem cells from the bone-marrow. These are removed before chemotherapy, which kills most of them, and then re-injected.
Research injecting neural (adult) stem cells in to the brains of rats can be astonishingly succesful in treating cancerous tumours. With traditional techniques brain cancer can be almost impossible to treat because it spreads so rapidly. Researchers at the Harvard Medical School injected cells genetically engineered to convert a separately injected non-toxic substance in to a cancer-killing agent. Within days the cells had migrated in to the cancerous area end the injected substance was able to reduce tumour mass by 80 per cent.
Stem cells are also apparently able to repair muscle damaged after heart attacks. Heart attacks are due to the coronary artery being blocked, starving tissue of oxygen and nutrients. Days after the attack is over, the cells try to 'remodel' themselves so they can pump harder. However, because of the decreased blood flow this attempt is futile and results even more muscle cells to weaken and die. Researchers at Columbia-Presbyterian found that injecting bone-marrow stem cells in to mice which had had heart attacks induced in them resulted in an improvement of 33 per cent in the functioning of the heart. The damaged tissue had regrown by 68 per cent. Clinical trials in humans are hoped for by 2003.
Sources of stem cells
Blood from the placenta and unbilical cord of new-born babies is a useful source of adult stem cells. Since 1988 these 'cord blood' stem cells have been used to treat Gunther's disease, Hunter syndrome, Hurler syndrome, acute lymphocytic leukemia and many more problems mostly in children. It is collected by removing the umbilical cord, cleansing it and withdrawing blood from the umbilical vein. This blood is then immediately analysed for infectious agents and the tissue-type is determined. Cord blood is stored in liquid nitrogen for later use, when it is thawed and injected through a vein in to the patient. This kind of treatment where the stem cells are collected from another donor is called allogenic treatment. When the cells are collected from the same patient they will be used on it is called autologous.
In fact, useful sources of adult stem cells are being found in organs all over the body. Research at McGill University in Montreal have extracted stem cells from skin able to differentiate in to many types of tissue including neurones, smooth muscle cells and fat-cells. These were found in 'dermis' - a layer of tissue beneath the skin.
In the same way that organs can be transplanted from cadavers researchers at the Salk Insitute in California have found that these could be used as a source of stem cells as well. Taking stem cells from the brains of corpses they were able to coax them in to dividing in to valuable neurons. However whether they will function correctly when used in treatment has not yet been determined.
Due to the controversy surrounding embryonic stem cells, in November 2001 the US National Institutes of Health announced a list of 72 approved human embryonic cell lines which researchers are to be allowed to work with. However some scientists declared problems they had with the list - that some cell lines are less useful to work with than others. Also other scientists are not convinced that the list actually contains 72 different lines - they think some are derivations of a single line.
- http://www.newscientist.com/hottopics/cloning/cloning.jsp - New Scientist's list of stem-cell and cloning related articles it has covered.