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Although scientists have high hopes regarding the use of stem cells - it could lead to therapies for a variety of diseases - the research on embryonic stem cells is one of the most controversially debated areas of medical science. Moreover there are still problems regarding the usage of especially totipotent stem cells and more research is needed in this area.

A good knowledge of the current utilization of stem cells and of pros and cons of stem cell research is necessary to understand the ongoing debates about the use of stem cells.

In the following, stem cells will be explained in an in-depth way, looking at what stem cells are and what differences exist between embryonic and adult stem cells. In the second step, the possible benefits of stem cells on the one hand and the ethical issues in their usage on the other hand will be regarded. Thereafter it will be shown how adult stem cells are already being used for medical treatment on the example of the myocardial infarction. To fully understand this process, further information will be given regarding the heart and heart attacks. Problems in the use of stem cells will also be explained, before focusing on the newer research that concentrates on rejuvenating adult cells to create embryonic-like stem cells and on the differentiation process.

This document concentrates on presenting information about the different types and uses of stem cells. As a specific example is given the use of adult stem cells in the treatment of heart attacks (myocardial infarction).

The body of an adult is made of innumerable cells, whereby most of these cells have specific attributes and functions: some cells control the movement of muscles, others detect light and others transport oxygen in the body.

But every person starts as one cell, the fertilized egg cell. This cell and also its daughter cells divide again and again, forming the embryo - and in this process differences appear between the cells, as some assume special forms and functions (=differentiation). It is due to this differentiation process that you can find a huge variety of specialised cells and also some partly undifferentiated cells in children and adults.

Undifferentiated cells that can produce specialised cells are called stem cells. There are many different types of stem cells but they all have some features in common:

Depending on the age of the organism that the stem cells are taken out of, the cells have the ability to either change into all possible kinds of cells or only into cells that belong to one closely related family of cells. The two most known kinds of stem cells are embryonic and adult stem cells.

Embryonic stem cells: Embryonic stem cells are undifferentiated cells that have the ability to create a lot of different cell types. It is thought that these cells are totipotent (omnipotent) until the eight-cell-stadium. The totipotency is lost thereafter, but the ability of differentiating into different cells stays very high (pluripotency).

Adult stem cells: The body does also contain stem cells after the birth, these are called "adult stem cells". You can find these for example in the bone marrow and in the brain. Adult stem cells can only differentiate into some specific cell types (multipotency). It is the task of these stem cells to produce new specialized cells during the whole lifetime of an organism which is very important, bearing in mind that cells are continually dying.

While bone marrow stem cells have the ability of duplicating or producing blood cells, the skin cells can only create new skin cells. Other cells, for example nerve cells, do not have the ability of dividing themselves at all.

Researchers have high hopes in the use of stem cells: Paralysed people could maybe walk again, Parkinson and Alzheimer could be treated and therapies against diabetes or heart diseases could be found - as these diseases are due to particular types of cells not functioning correctly.

Through the use of stem cells it could be possible to grow specific cells, tissues and even entire organs in the laboratory, so that broken organs or defect cells could be replaced. However, before these procedures can be used in day to day medical practice, much research must be carried through.

Many scientists are especially interested in embryonic stem cells, as those can differentiate into almost any kind of cell. But the research on embryonic stem cells has lead to hot-tempered discussions. The opponents say that human life gets erased for science, as researchers have to destroy the embryo in a very early cell stadium to win the stem cells. Supporters on the other hand argue that the cells are taken out of pre-embryos that were produced as part of in-vitro fertilisation and that are not needed anymore anyway - as doctors usually incubate 5 to 20 egg cells of a women with male semen, but only some (e.g. in Germany up to three) fertilised eggs get implanted into the woman.

Ethical thoughts lie behind many of the rules and limitations regarding the use and research on embryonic stem cells that exist in most countries.

Currently, only bone marrow stem cells, which are a kind of adult stem cell, are used for routine therapy. They are applied in the treatment of various blood diseases (e.g. leukemia). In addition, various studies have shown that new forms of therapy, some using other types of adult stem cells, have been proven effective. For instance, stem cell treatment of heart and liver diseases, diabetes and various types of cancer was shown to have a positive effect on test patients. However, these methods are not being used routinely in every day medicine so far.

Therapeutic cloning, also called embryo cloning or biomedical cloning, is the procedure to harvest stem cells that can be used to study human development and to treat disease. Stem cells are pluripotent, which means that they can be used to generate virtually any type of specialized cell in the human body. This property is important for the understanding of how cells develop from foetus to adult and for the development of cell therapies.

Stem cells could thus, at least in theory, be used to treat diseases in any body organs or tissues by replacing damaged and dysfunctional cells. If the stem cells originate from the patient itself, then the cells, the cultured tissue or organ would have the sick person's original DNA. It would mean that there is no danger of rejection and there would be no need to take immunosuppressant drugs. This technique would be vastly superior to relying on current organ transplants from other people.

It may seem far fetched that reprogrammed stem cells to become nerve or brain cells could cure Parkinson's or Alzheimer's disease. However current research already shows promising results with stem cells, that were harvested from the bone marrow of patients with heart failure, replacing damaged blood vessel tissue in the heart of the same patients. (see EUSEM film "Beat of the Heart"). Another interesting possibility is to generate tissue cells from individuals with genetic defects and test in laboratory dishes (in vitro) whether chemicals or biological substances could correct the faulty behaviour of the cells.

It should be noted that there is a long way to go. Much more research is needed to obtain more knowledge about the nature of differentiation factors that determine what type of cell a given stem cell may become. The next step will be to obtain sufficient tissue of proper quality. Artificial skin, cartilage and blood vessels can already be cultured from stem cells and are cartilage cells have even been grown in the shape of an ear or nose. In the distant future lies the production of full organs. It is thought that stem cells could differentiate and grow on moulds to reach certain shapes, but organs are often built from many different cell types and have a relatively complicated three-dimensional structure. For instance, in order to make a kidney it would be necessary to arrange all the different cell types correctly in relation to one another for the organ to be able to perform its function.

During the initial phases when a fertilised egg cell starts dividing and forms an embryo, all cells are still identical. They can develop into all types of future body cells as well as the foetal membranes, placenta, umbilical cord. These cells are therefore called totipotent stem cells. As soon as the foetus gets a distinctive shape (the blastocyst stage), the inner cells differentiate into pluripotent embryonic stem cells, which still can form all of the body cells but not the foetal membranes, placenta, umbilical cord.

The embryonic stem cells can make any one of the 220 different cells in the human body and they can reproduce themselves many times over. These are the stem cells of choice for therapeutic cloning activities and they can be obtained from surplus in vitro fertilized (IVF) eggs. While it is certainly possible to produce early embryos in test tubes for research only, this is ethically not accepted. However, each year, hundreds of thousands of poor-quality embryos are discarded during the course of IVF, and these could provide an ethically acceptable source of stem cells for research.

Embryonic cells from the blastocyst can be cultured to produce different kids of tissue cells. http://theblackcordelias.files.wordpress.com/2009/03/stem-cells.jpg

Aborted foetuses also carry pluripotent stem cells in their genital tissue, which can be isolated. However, these cells could not be cultured with the same success as IVF derived cells in the laboratory.

Stem cells can also be harvested from the umbilical cord blood, which remains in the cord and the placenta after the baby is born. Most cord blood stem cells are hematopoietic cells that only differentiate into different types of blood cells, such as red blood cells, white blood cells and platelets. Since this is still a valuable source of cells when treating blood related diseases such as leukaemia etc., initiatives have been taken to collect and store the material in cord blood banks for possible future use. It is not yet clear what curative potential of these stem cells will be. Commercial firms have nonetheless made unsubstantiated claims promising insurance of infants or family members against serious illnesses in the future.

Adults also have stem cells present in the bone marrow, muscle, brain, lungs and other organs and tissues. Adult stem cells, also called somatic stem cells, may remain non-dividing for long periods of time. But when the need arises to maintain or repair the tissue, cell division is activated. Scientists have thought that adult stem cells would only be capable of giving rise to the same type of tissue from which they originated. Recent research has shown, however, that the cells may be induced to behave more like the embryonic stem cells and may have the potential to generate other types of cells as well. To achieve such reprogramming of adult stem cells it may be needed to fuse with cells from the target tissue, or to introduce specific genes (see the section 'epigenetics' on the EUSEM server for a discussion of the principles of differentiation and programming).

Scientists in many laboratories are therefore trying to find better ways to grow large quantities of adult stem cells in cell culture and to manipulate them to generate induced pluripotent stem cells (iPSC), so they can be used to treat injury or disease. This would circumvent the ethical problems of using embryos for research and medical treatments. One such treatment has been used for 40 years already: adult hematopoietic, or blood-forming, stem cells from bone marrow have been used in hematopoietic stem cell transplantation (HSCT) for hematological malignancies (leukaemia patients). Examples of future treatments include regenerating bone using cells derived from bone marrow stroma, developing insulin-producing cells for type 1 diabetes and repairing damaged heart muscle following a heart attack with cardiac muscle cells.

The induced pluripotent stem (iPS) cell technology could even brought a step further if normal, fully differentiated adult cells could routinely be reprogrammed to act like pluripotent embryonic cells. Several studies have now reported successes using regular skin cells or cells from connective tissue. In these experiments the cells underwent genetic modification by the introduction of specific genes that are active in tumours. While the iPS cells display the necessary properties of embryonic stem cells, their genetic makeup is not considered as safe enough to use in treating diseases in patients.

Future stem cell therapies probably would in majority rely on cells that are donated by another person. This raises the possibility of donor cell rejection by the patient's immune system. Some stem cells are more genetically compatible with the patient's own tissue than others and there are indications that some embryonic stem cells can be transplanted without any major rejection problems. It may also be possible to use a person's own source of stem cells to regenerate tissue, which would eliminate the danger of rejection. There are a number of options:

http://www.stemcelltherapy21.com/stemcelltherapy21/stemcells/images/potential%20of%20stem%20cell%20therapy.jpg

Any stem cell therapy should be proven safe for the patient. Researchers have recognised that many common cancers may be caused by adult stem cells with damaged DNA, leading to a change in behaviour like uncontrolled growth, cell type change and migration. Just sampling stem cells from an individual and reintroduction in a tissue with defects may therefore turn out to be counterproductive (if helpful in the first place). Embryonic germ stem cells seem to be the safest source of stem cells in this respect, but these could in turn pose problems with immune rejection since these originate, by definition, from another individual.

Myocardial infarctions (also know as heart attacks) typically occur in older people, in most cases men. That said, women catch up once they reach the 50-to-60 age range and in the over 65s, heart attacks are as common in women as in men. One current cause for concern, however, is the increasing number of young people - both men and women - who are suffering heart attacks. It seems that there is a clear correlation between this increase and the number of young people who smoke. Based on this evidence, the conclusion was made that smoking is a contributing factor in the formation of blood clots, which can block blood vessels.

Coronary vessels, that provide the heart with oxygen, can be constricted by plaques or fatty substances that build up inside the walls of the vessels. The plaques also attract blood components, which stick to the wall lining. This process is called atherosclerosis and develops gradually over many years. In a myocardial infarction, such a vessel (sometimes more than just one) is blocked by a blood clot. The subsequent cardiac muscle tissue is not supplied with oxygen anymore and dies, if this condition persists for a longer period of time. If large parts of the heart are affected (> 30 %), the heart is not capable of pumping blood to the vital organs anymore, which leads to the patient's death.

In many cases a myocardial infarction is accompanied by life threatening heart rhythm disturbances. If this condition is not treated (e.g. by means of defibrillation) this can lead to a cardiac arrest, and ultimately to death. Even today, 40 per cent of all people who suffer this type of a heart attack will experience sudden cardiac death before they arrive in a hospital.

If the patient survives the heart attack, it is vital that the resulting damage is kept to a minimum, so that there is no progressive weakening of the heart in the years following the attack. It is important to know that the heart muscle cells don't only die in the acute situation of an infarction, this effect is also seen afterwards - a process that can lead to a chronic cardiac insufficiency (Ärzte Zeitung, 5.03.2007). Those who suffer a myocardial infarction at a relatively young age are at a great disadvantage, because the weakened heart will continue to pose a health risk.

Some common therapies that doctors undertake after a heart attack:

Coronary artery bypass surgery: Doctors take a segment of a healthy blood vessel from another body part and make an alternative routing around the blocked section of the coronary artery.

A new method of myocardial therapy that uses the adult stem cells of respective patients is currently being researched in a placebo-controlled study in hospitals. The stem cells are extracted from the bone marrow, purified and subsequently injected in the coronary vessels. There, they stimulate the generation of new vessels and prevent more cardiac muscle cells from dying.

A detailed display of the procedure can be seen below:

Firstly, bone marrow is extracted using a special device.

In the second step the bone marrow is processed. This happens immediately after the extraction. In a clean-room, the bone marrow is filtered under 100 % sterile conditions.

Next, the purified bone marrow is divided into sterilized test tubes.

In order to produce the appropriate preparation for the patient, the different cell types in the extracted bone marrow must be separated. A special solution is added and the mixture is given into a centrifuge. By means of density gradient centrifugation a gradient is generated: The different cell types separate themselves into layered bands.

The red blood cells settle at the very bottom of the tube. The marked milky white band contains the leukocytes and also the stem cells.

This layer of leukocytes and stem cells is removed using a pipette and transferred into new test tubes.

These new test tubes also undergo density gradient centrifugation, whereby the cells settle at the bottom. They are then purified and brought into solution again - the stem cell preparation is finished.

The preparation is then injected into the patient's coronary vessels by means of a cardiac catheter.

The aims of this stem cell therapy are that less heart muscle cells die after the heart attack and that the inserted stem cells create new vessels, which lead to a better perfusion and thereby to a stronger pumping of the heart. To reach these aims, injected stem cells have to stay in the area of the heart attack. This doesn't happen for all the cells, but about 10 percent stick to the vascular wall.

This treatment could be performed within the daily routine of a heart clinic and is already being used in some hospitals in a trial phase with a stem cell group and a placebo control group. Complications have not been observed so far, but in a study with 200 patients it could be seen that six re-infarctions happened in the placebo group, but not a single one in the stem cell group. The therapy includes a catheter examination, something that is currently undertaken e.g. in Germany on around 1.2 million people a year.

The modification that is needed in order to place cells in the heart is minimal. Without additional technology, time or effort it can be carried out by anyone who is able to undertake catheter examinations and who is trained in this field. The preparation of the cells is also simple and relatively inexpensive, with the whole treatment costing around EURO 1500. Further research needs to be conducted, but if this method proves to be effective for a large number of patients - by increasing their chances of survival and preventing cardiac insufficiency - it is a treatment which can easily be integrated into an everyday clinic routine.

Adult stem cells can only differentiate into a certain family of cells and there are a lot of human tissues and organs that could not be replaced by only using adult stem cells. But a problem regarding the use of embryonic stem cells is how to make the cell differentiate into the needed cell. Embryonic stem cells require specific signals for correct differentiation. In the laboratory, researchers add specific nutrition to the stem cells and give certain temperatures to start the differentiation process.

Embryonic stem cells can for example be trained to grow into heart muscle cells that clump together and pulse even in a laboratory tray. When the "new" heart cells get injected in animals with heart disease, the cells fill in for dead cells and help the animal to recover quickly. But in some animal research studies it was shown that the pluripotent embryonic stem cells sometimes grow into tumours or change into unwanted cells - they could form, for example, bits of bone in the hearts that they are supposed to be repairing. Up to now, animal experiments, at least those on the heart, have shown that more than 70 per cent of applications contain cancerous cells. This certainly indicates that the cells are very powerful, but also shows how dangerous it would be to treat patients with this method.

Another problem in the use of adult stem cells occurs, when cells of another person, a donor, are being used. This problem exists for example in the utilization of donor's bone marrow stem cells for the treatment of leukaemia: The given cells can be rejected by the patient's immune system. These problems were not encountered in the above described myocardial infarction therapy, where patient's own adult stem cells are being used.

In newer research, scientists try to take other adult cells, for example skin or fat cells, and turn those into cells that are undifferentiated and can create most kinds of cells. These cells are called induced pluripotent stem cells (iPS) and are very similar to embryonic stem cells.

A skin cell can be altered and rejuvenated by genetic manipulation where four genes are inserted into the cell via viruses, but this method bears several risks. The main problems are that the used genes are linked to the development of cancerous tumors and that the inserted DNA stays in the cell permanently, but gets implemented in any place of the gene-strand.

A breakthrough was achieved in 2009 when scientists found a new method of converting adult cells into embryonic-like stem cells. The researchers used recombinant proteins (these are proteins that are made from the recombination of DNA-fragments of different organisms), purified the engineered proteins and experimented with chemically defined conditions until they managed to find the right mix to reprogram the adult cells - without adding genes into the mix. The altered cells are called protein-induced pluripotent stem cells (piPS) and seem to behave in exactly the same way as normal embryonic stem cells. But more research needs to be conducted, for example regarding the potential of contamination from the different proteins that are used in the process.

Using piPS could have several advantages: on the one hand there are no ethical issues as stem cells are not taken from embryos, on the other hand there is no need for concerns regarding immune rejection as the stem cells are taken from the same patient that they are needed for.

The research on embryonic stem cells will still be needed in future years though, for example to attain a better understanding of the differentiation process that embryonic stem cells undergo when they create specific organ cells and how to control this process or to further investigate possible differences between embryonic stem cells and (p)iPS.