Few recent scientific issues have stimulated so much media attention, public debate and government involvement as that of stem cell research. Stem cells offer people hope by promising to greatly extend the number and range of patients who could benefit from transplants, and to provide novel therapies to treat debilitating diseases such as diabetes, Parkinson's, Huntington's, heart disease and stroke, as well as accidental damage such as spinal cord injury. So why would anyone object to research in this area? The problem is simply that a particular type of stem cell, which potentially could provide many cell types for a wide range of therapeutic uses, is obtained from the very early embryo. To make matters even more contentious, the same cloning technology that gave Dolly the sheep could in theory be used to tailor stem cells to the patient. Some people worry that we are taking research too far down paths that make them feel uncomfortable, others think it is downright immoral and against their deep-held, often religious, beliefs. But what are the scientific issues and why do many of us feel equally passionate that the research should be allowed?
What are meant by stem cells and how might they be used? There are many types of stem cell, but they share several interesting properties that set them aside from other cell types. The adult body contains hundreds of specialised or "differentiated" cell types, each playing a particular role. Some of these are long lived and do not divide, such as nerve cells; others are short lived and need to be replaced through cell division. Usually, when cells divide, their daughter cells are identical and of the same type as the parent cell. In other words they divide symmetrically. Additionally, their fate and their properties are fixed once a liver cell, always a liver cell.
In contrast, stem cells undergo "asymmetric" divisions, producing both another stem cell, in a process called self-renewal, and a cell that will become differentiated. The differentiated cell may still be able to divide, but it cannot normally go back to form the original type of cell. In some circumstances stem cells can increase their numbers, giving rise only to more stem cells. However, stem cells in the adult are usually in tune with the tissue to which they belong. They divide at the appropriate rate to self renew and to give rise to just sufficient differentiated cells to replenish those that have been lost. However, with accidental trauma or disease the normal rate of regeneration is often too slow to allow for repair. This is particularly true within the nervous system, but also in other tissues where turnover is low, such as the pancreas.
How can we harness stem cells to cure diseases? We have become very used to the idea of organ transplants in medicine, to treat a wide range of problems from cataracts to kidney or heart disease. However, we are also all aware of the frequency with which they fail, often through immune rejection, and there is also a serious shortage of organ donors. Both problems could be solved if tissue could be taken from one part of the body to repair another part of the same individual. There are relatively few cases where this is done at present, e.g. using valves from leg veins to repair heart valves.
Rather than using whole donor organs or tissues, an alternative would be to use the stem cells able to form those tissues. In fact this is already done with bone marrow transplants, where the stem cells in the graft can regenerate all the different types of cell in the blood. Other types of stem cell could be used in a similar way. For example, the correct stem cell type might allow specific cell types to be replaced in the nervous system, where it is impossible to transplant whole structures. The idea is to identify and remove the stem cells from a particular tissue, multiply them outside of the body and then use them to replace damaged tissue. This is already done to some...
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