What are stem cells and why are they important?
Stem cells have the unique property of being able to reproduce indefinitely, creating more of themselves, and also giving give rise to specialised cells, tissues, and even organs. As one would expect, early embryos consist mainly of stem cells, and even in adults stem cells probably exist in every tissue and organ of the body, though some are easier than others to find. They survive for several years when frozen and in theory they could be collected and stored to repair damaged organs and provide spare parts in later life.
Thus stem cells set the stage for a revolution in medicine and biology. More than a decade of research on the biology of mouse stem cells has helped to pave the way for developing human stem cell lines and using them to treat disease. However, because human trials have given equivocal results, more animal research is still needed.
Stem cells can also be used to study how cells become specialised, a process called differentiation, and to study what causes cancer cells to form and divide. One day it may be possible to treat developmental defects by adjusting the switches that control differentiation.
Nuclear transfer (cloning), the technology used to produce Dolly the sheep, could also be used to produce stem cells with enhanced therapeutic potential. Stem cells could also replace some animal testing, by generating human cell types for drug and toxicity evaluation.
Stem cell research has huge therapeutic potential:
• Nerve cells could treat stroke, Parkinson's, Alzheimer's, multiple sclerosis, spinal cord damage, and brain damage, even when it is congenital. This has particular potential as the brain is 'immune privileged' and does not reject tissue from another individual
• Heart muscles cells could treat heart attacks and congestive heart failure
• Skeletal muscle cells could treat muscular dystrophy
• Insulin-producing cells could treat diabetes. Although many cases of diabetes are caused by an immune response, transplant experts are trying to prevent this
• Cartilage cells could treat osteoarthritis
• Blood cells could treat cancer, immune deficiencies, inherited blood diseases, and leukaemia
• Liver cells could treat hepatitis and cirrhosis
• Skin cells could treat burns and other skin wounds, and replace scars
• Bone matrix cells could treat osteoporosis
• Retinal cells could treat several forms of blindness
• Animal research is looking at the possibility of regrowing missing teeth.
However, although it is now routine to get mouse stem cells to specialise into different types in the laboratory, scientists do not yet know how to control this process and generate cells for transplantation therapy. Instead, they mainly rely on bone marrow stem cells. These are easily harvested from blood after treating the animal (or human) with growth factors. We also need to know about the long-term function of laboratory-generated cells, and this will involve animal research.
Where are they collected from?
Stem cells are found at all stage in life, but for practical purposes are regarded as being from three stages of development: early embryos, fetuses, and adults.
Scientists collect embryonic stem cells from the inner layer of early embryos, usually of rodents, when they are about five days old and consist of a double-layered ball of cells. These embryos can be created using test tube (in-vitro) fertilisation. The cells can be implanted into other animals, even of a different species, without being rejected by the body's immune system.
Fetal stem cells are collected from blood taken from the placental end of the umbilical cord. There is thus a plentiful supply of cells that are normally discarded. Although each sample is relatively small, it is possible to make the stem cells contained therein multiply.
Adult stem cells are harder to find, with the notable exception of bone marrow cells that give rise to the different kinds of blood cells. Bone marrow stem cells can be induced to proliferate by injecting a growth factor. This makes them spill over into the bloodstream, from where they are easily collected. Skin stem cells have been isolated in mice, and have been used to grow new skin in recipient mice.
Embryonic stem cells give rise to any of the 216 kinds of cell in the body, but adult stem cells are more specialised—for example, bone marrow stem cells normally only form blood cells. However, they seem to 'change direction' (transdifferentiate) when injected into other tissues such as heart and brain, and some research is aimed at growing stem cells in the laboratory and 'taking back' (dedifferentiating) adult stem cells to become more versatile and less able to provoke an immune reaction. We know that stem cells from adult mice, when grown in mouse or chick embryos, revert to an unspecialised state and become identical to the surrounding tissue, depending on which cell layer they are injected into.
The extent to which stem cells transdifferentiate is uncertain; some of them also seem to fuse with host cells, forming pluripotent cells, and recent (2004) debate and experiment has aimed at finding out the extent of these two factors.
In October 2004 a third quality of stem cells was discovered. Scientists at the University of California in San Diego found, amazingly, that when embryonic stem cells — just 15 cells, taken from normal mice — are injected into the abdomen of mice pregnant with fetuses carrying a genetic heart defect, the pups are born with normal hearts. The stem cells do not cross the placenta into the fetuses, but secrete hormone-like chemicals that play an essential role of normal growth and development. to their chemical action. Two of these chemicals have been identified — they are called IGF1 and WNT5a.
Stem cell transplants today
Following successful animal research, stem cell transplants are now routine in the treatment of several types of cancer. They enable patients to undergo high-dose chemotherapy despite its toxic effect on bone marrow, as the patient can afterwards be given their own or a donor’s bone marrow stem cells, which engraft back into the bone.
A further development, pioneered in animals and now entering clinical practice, is to use donor stem cells to treat certain serious and often fatal inherited blood disorders such as Fanconi anaemia, severe combined immune deficiency, and, potentially, sickle cell disease and thalassaemia. The patient’s own bone marrow, which produces diseased blood cells, is destroyed using high-dose chemotherapy and replaced by donor stem cells.
Bone marrow stem cells for tissues other than blood
Research has aimed to replace various missing tissues using bone marrow stem cells, and there seems to be evidence that this happens at least some of the time. The problem researchers have faced is knowing whether, some months later, any of the cells in the tissue being studied (usually heart or brain) is derived from the injected stem cells. The presence of apparently newly-generated tissue could also be due to the body’s existing capacity to repair itself or to environmental influences, which include oxygen lack and convulsions. Even learning and exercise can stimulate new nerve cell production.
One way round this problem is to inject male cells into female bodies, wait a few months, and see if the brain or other organ contains Y-chromosomes, which are only normally found in males. It is also essential to check whether the Y-chromosomes are in new nerve cells and are not —as sometimes happens—merely the product of fusion of the injected stem cells with existing cells. Scientists have developed techniques to do this. A further problem is that a wait of six months between giving the transplant and killing a rodent for study represents a quarter of its lifespan.
The heart, blood vessels, and blood
During heart attacks — myocardial infarction — there is cell death in the area deprived of oxygen. Early (2001) research in rats and mice suggested that when bone marrow stem cells were injected into the heart, new heart muscle cells grew. It was then tried in humans, and clinicians thought that heart muscle was regenerated: the patients' heart seemed stronger and it was presumed that this was due to new muscle formation.
In April 2004, two papers in the prestigious science journal Nature challenged these observations and the science underpinning them. Using state-of-the-art genetic tools, they discovered that bone marrow stem cells showed little or no capacity to turn into heart muscle. Instead, they turned into blood cells. Any functional improvement, they said, seems to have been caused by growth of new blood vessels, which has proved an unexpected side effect of treatment. In view of the potential hazards of mistaken cellular identity, reports of bone marrow stem cell versatility are now undergoing rigorous scrutiny.
Better, but mixed, results came from research using muscle stem cells. When patients’ hearts were injects with their own muscle stem cells, there was an over-riding improvement in heart function in most patients, but some developed an irregular heartbeat, probably because the transplanted cells failed to become electrically linked to the rest of the heart. The best animal results yet (May 2004) came when embryonic stem cells were transplanted within a man-made 3D structure into damaged hearts of rodents which had suffered heart attacks. In the same month there were reports of successful human grafts, where it was shown that new heart cells were derived from injected umbilical cord stem cells.
In animals, fetal heart cells graft easily into the heart, adopt the identity of adult heart cells, and become electrically coupled. However, their use in humans is ethically problematic. The search is now on for heart precursor cells and to discover the signals that guide the way they migrate, renew themselves, and become adult heart cells.
Making new muscle
New muscle cells have been formed in mouse muscle in the test-tube, and in living mice. In both cases the cells were reprogrammed by signals from surrounding cells, suggesting that adult tissues may be able to instruct transplanted cells to adopt the fates appropriate to their new location.
Muscle regeneration has also resulted from injecting several types of stem cells — from bone marrow, from human knee joints, and from newborn mouse muscles — repairing damage similar to muscular dystrophy. Injured muscle seems to send distress signals around the body that summon the cells. But there are still doubts about whether adult stem cells can be used to provide effective treatments for diseases such as muscular dystrophy.
The brain and nervous system
The brain is immune privileged, which means that it does not reject cells from other animals or even other species, and this makes it a prime target for stem cell therapy. Fetal brain stem cells are relatively easy to collect and grow in the laboratory, and can divide indefinitely with no tendency to tumour growth.
Research to date shows similar problems to that of heart research. Stem cells transplants have improved (but not cured) most (but not all) mice with Parkinson's, stroke, EAE (an induced form of multiple sclerosis) spinal cord injury, motor neurone disease and other conditions. Some of the injected stem cells create an environment that protects and aids the survival of the host nerve cells. In the spinal cord, only a tiny proportion of injected cells become new motor nerves that penetrate muscle. In normal brains after injury nerve stem cells cluster round the injury site and generate new stem cells, though it is self-evident that they do not produce sufficient to fully repair the damage done.
In the eye, injected stem cells attach to a type of retinal cell called astrocytes. However, unless they are genetically modified, they form new blood vessels, which can obstruct clear vision.
In May 2004 it was reported that three cancer patients, all female, who had received stem cell transplants from their brothers formed new brain support cells. The new cells contained Y-chromosomes, which proves they were of male origin, but the findings do not rule out fusion between bone marrow and other stem cells. Purkinje nerve cells in the cerebellum can fuse with bone marrow cells in both mice and humans.
Digestive organ diseases; diabetes
Research in rats and mice shows that pancreatic, liver, and bone marrow stem cells can transform into insulin-producing cells. Stem cell research may also cure Hirschprung disease, a genetic condition wherein a defect stops nerve stem cells forming nerves that control the small intestine. With diabetes, some researchers have found that bone marrow stem cells transdifferentiated into pancreatic beta cells, whereas other research teams have failed to see this.
In July 2004 The Lancet commented: "As in every emerging field in biology, early reports seem conflicting and confusing. Embryonic and adult stem cells are potential sources for [pancreatic] beta-cell replacement and merit further scientific investigation. Discrepancies between different results need to be reconciled. Fundamental processes in determining the differentiation pathways of stem cells need to be elucidated, so that rigorous and reliable differentiation protocols can be established. Encouraging studies in rodent models may ultimately set the stage of large-animal studies and translational investigation."
Further information
The Royal Society policy pages on cloning and stem cells, includes official statements, comprehensive information and links.
Edinburgh University Institute for Stem Cell Research answers questions such as What are stem cells? and Why are scientists so interested in them?
The Cambridge Stem Cell Research Institute opened in June 2004.
The Medical Research Council's booklet titled Stem cells focuses on MRC-funded research into stem cells, including the exciting potential these have for repairing damaged body tissues and replacing them with healthy new cells.
The National Institute for Medical Research has a 2001 "Mill Hill Essay" on stem cell therapy and research.
Use of human stem cells in research raises ethical, legal, religious, and policy questions which were addressed in a 1999 report of the American Association for the Advancement of Science and the Institute for Civil Society.
Mice in the world of stem cell biology, Geraldine Guasch & Elaine Fuchs, 2005, Nature Genetics supplement, 37, 1201.
Stem Cells. Stem Cell Science in the UK (2008) Human Tissue Authority.
Hope and hype, an analysis of stem cells in the media, November 2011, NHS Choices.
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