Medical uses of stem cells
Some of the earliest trials of stem cells have been in heart disease, generally by transplantation of a patient’s own stem cells (e.g. from bone marrow, though there is also interest in stimulating the heart’s own stem cells). Results have been mixed, though recent trials have shown a small but significant improvement in heart function.
Fetal brain stem cells have been trialled as a treatment for Parkinson’s disease, with mixed results. The ethical issues associated with such cells, as well as their scarcity, suggest that other sources may have better long-term prospects.
A slightly different but promising approach is to see stem cells as promoting regeneration rather than being the source of new building blocks themselves. Factors released by transplanted stem cells may encourage the growth and differentiation of stem cells already present in the brain, helping the brain to mend itself.
This rationale has underpinned small-scale trials of mesenchymal stem cells in amyotrophic lateral sclerosis (ALS, or motor neuron disease). There are encouraging signs that these cells can form a protective environment for motor neurons and slow down the loss of cells and development of symptoms.
Stem cell therapy is seen as a highly promising area for type 1 diabetes, caused by autoimmune destruction of insulin-producing beta cells in the pancreas. Various approaches are being tested, including stored cord blood cells, human embryonic stem cells and mesenchymal stem cells. The latter are highly promising as they also seem to modulate the immune response to the beta cells. The hope is that mesenchymal stem cells will be grown outside the body, stimulated to develop into beta cells, then returned to the patient.
The eye has been the site of considerable clinical interest. In one area - repair of the cornea - stem cell therapy has been highly successful. Adult stem cells from the eye have proved to be a good treatment for corneal damage. On the other hand, stem cell repair of damaged retinas is proving more challenging. It may be possible to generate retinal cells from stem cells within the eye or from mesenchymal stem cells, but it may be difficult to integrate them into the light-detection and signalling pathways of the retina itself.
The liver is one of the few organs that can regenerate of its own accord. A subpopulation of cells in adults can divide and provide a new population of liver cells. Perhaps surprisingly, though, progress towards artificial regeneration, either by stimulation of these liver cells or by transplantation of adult- or embryonic stem cell-derived liver cells, has been slow and remains highly experimental.
Loss of bone mineral density, leading to osteoporosis, is a common problem in old age. It could be countered by increasing the numbers of bone-forming osteoblasts. Interestingly, osteoblasts are formed from mesenchymal stem cells in the bone marrow that have the choice of becoming fat cells or osteoblasts. There is great interest in pharmacological manipulations that tip the scales in favour of osteoblasts (in fact, statins may have this effect).
There is also interest in using stem cells in veterinary medicine. Stem cells have already been used to repair injuries in racehorses. Knowhere, winner of a Gold Cup trial in February 2008, had stem cell treatment for tendon damage in 2005.
A major dilemma in stem cell research is whether to continue research to find out more about their properties or to go straight to patients. Many researchers feel that not enough is yet known for stem cell therapies to be used widely - and that failures could set the field back. Many doctors, though, argue that new therapies are urgently needed in many conditions and should be pushed through as rapidly as possible.
Image: Neurons derived from human neural stem cells. Cells such as these could form the basis of future treatments for degenerative brain diseases such as Parkinson's and Alzheimer's. Yirui Sun, Wellcome Images.
References
Amyotrophic Lateral Sclerosis (ALS)
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Bones
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Pharmacologic targeting of a stem/progenitor population in vivo is associated with enhanced bone regeneration in mice. J Clin Invest. 2008 Feb;118(2):491-504. PMID: 18219387
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Cardiovascular disorders
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Cardiovascular/autoimmune disorders
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Clinical applications of blood-derived and marrow-derived stem cells for nonmalignant diseases. JAMA. 2008 Feb 27;299(8):925-36. Review.PMID: 18314435
Diabetes
Gangaram-Panday ST et al.
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Eyes
Pellegrini G, et al.
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Neurodegenerative conditions
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Racehorses
Richardson LE et al.
Stem cells in veterinary medicine--attempts at regenerating equine tendon after injury. Trends Biotechnol. 2007 Sep;25(9):409-16. Review. PMID: 17692415
Trophic support
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