Medical potential of stem cells
Stem cell therapy has actually been used for decades - in the form of bone marrow transplantation. Stem cells in the transplanted bone marrow produce a permanent supply of new blood cells for the recipient.
If cells are damaged or dying, due to injury, disease or ageing, the hope is that introduced stem cells can come in and take over their function, repairing tissues. There is an impressively wide range of applications for this regenerative medicine approach - from brains to bones. Interestingly, many stem cells seem to have a built-in 'homing capacity', and tend to migrate automatically to sites of damage.
An additional step is to create artificial scaffolds for stem cells, so they are able to form three-dimensional structures, including organs - prototype artificial bladders have been grown in this way. It is also possible to include signalling molecules in these scaffolds, such as factors that stimulate the growth of neurons for use in nerve repair.
Recently, stem cell therapy has been combined with gene therapy. Stem cells can be genetically manipulated outside the body to make them produce therapeutically useful products and then reintroduced.
This kind of approach is also useful in inherited conditions, where stem cells taken from a patient will have the same genetic defect as the rest of the cells in the patient's body. This defect could be corrected in stem cells by gene therapy outside the body.
Stem cells have great potential for treatment of autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus, where the body's defences turn on its own cells. One promising approach is to eliminate a patient's immune cells, ridding the body of self-reactive cells, and to replace them with haematopoietic stem cells grown in culture.
As well as providing a source of new cells, stem cells can also support the body's own powers of regeneration. They may secrete factors that encourage local repair processes.
There are also some signs that ageing may be related to a depletion of stem cells. It is possible that reviving the body's stem cell stocks may one day be able to delay the deterioration seen in ageing.
As well as tissue repair, stem cells can be used in the opposite way - to remove unwanted cancer cells. This approach uses the homing ability of stem cells, with engineered stem cells delivering toxic compounds specifically to tumours.
There are challenges in all these areas, however. The properties of stem cells are still incompletely understood, and how their differentiation is controlled is often unclear. Growing them in conditions suitable for clinical use remains a challenge and there is a fear that their ability to proliferate could lead to cancer following transplantation. Similarly, genetic modification raises the risk that genes controlling cell growth could be disrupted, again leading to uncontrolled cell proliferation and cancer.
So prospects are bright, but considerable research is needed to ensure treatments are feasible, effective and safe.
Understanding disease and drug development
As well as direct therapeutic uses, stem cells are advancing medicine in other ways. They are increasingly being used in drug screening processes, providing cell-based systems to test the effects of potential new therapeutics. Pluripotent cells are particularly useful, as they can be induced to form all the cells found in adults, including those that are the targets of drugs in development.
A similar approach may also help to identify drugs that enhance regeneration - by driving stem cells to develop into the types of differentiated cell needed by the body.
Stem cell approaches can also help reveal more about disease processes. Cellular differentiation and tissue development can be studied much more easily in culture than in a living person.
Image: Researcher picking colonies of cultured human embryonic stem cells; Wellcome Library, London
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