A Stem cell is a `beginner' or undifferentiated cell, which retains the ability both to keep dividing numerous times while remaining in the undifferentiated state for self-renewal of its population, and to become specialised in certain conditions into any type of cell found in the human body; this means that they are pluripotent (they have more than one potential outcome), and totipotent (the ability of a single cell to divide and produce all the differentiated cells in an organism.) Stem cells are the `master' cells of the human body, and are found in most multi-cellular organisms. They are found in various parts of the human body at every stage of development from embryo to adult.
There are two broad types of mammalian stem cells: Embryonic stem cells, and Adult stem cells
1. Adult Stem Cells or somatic stem cells are found in certain tissues throughout the human body (for example, in the umbilical cord and the bone marrow) after embryonic development, although they are rare and small in number. Their primary use in the human body is to repair damaged tissues and replenish dying cells, they are the main source of regeneration in the human body, and can multiply rapidly and indefinitely as part of ongoing maintenance.
Their properties that make them good at this are somewhat the same as those of embryonic stem cells, they are self-renewing and pluripotent. Somatic stem cells, however, can only differentiate into the types of cells of the organ from which they were originated from. This can slightly limit their potential in cell culture. On the other hand, there is no background controversy surrounding the use of adult stem cells, unlike that of embryonic stem cells. For some people, the use of stem cells derived from human embryos will never be acceptable as it involves the destruction of an embryo, and adult stem cells offer an alternative that is comparatively free from moral and ethical complications. However, whilst recent developments in adult stem cell research are promising, we don't yet know how versatile they will prove to be.
Research in the 1960's was conducted in the bone marrow. In the bone marrow, scientists have discovered two types of stem cells: one population, called hematopoietic stem cells, forms all the types of blood cells in the body. A second population, called bone marrow stromal cells. Stromal cells are a mixed cell population that generates bone, cartilage, fat, and fibrous connective tissue.
Scientists also have discovered two regions of the brain that contained dividing cells, which become nerve cells. Although, even after this discovery, many scientists believed that the adult brain could not produce stem cells. It was not until the 1990s that scientists agreed that the adult brain does contain stem cells that are able to generate the brain's three major cell types—astrocytes and oligodendrocytes, which are non-neuronal cells, and neurons, or nerve cells.
2. Embryonic stem cells are found in the `blastocyst' which is the structure formed in early embryogenesis. A blastocyst is basically a hollow ball of around 50-150 cells which is formed 4-5 days after fertilization.
The blastocyst consists of two chief cell lines, the trophoblast, which forms the outer lining of the blastocyst, and the inner cell mass or embryoblast found in the blastocyst cavity.
The inner cell mass or embryoblast cells are the cells that will eventually form stem cells, and create the ultimate structures of the foetus. They will do this by firstly, differentiating into epiblast, and hypoblast cells. The epiblast cells will then differentiate into all three germ layers of the definitive embryo: ectoderm, endoderm, and mesoderm. These germ layers eventually give rise to all of an animal's tissues and organs through the process of organogenesis, this includes over 200 different types of specialised cells in the eventual body.
So we can see that due to their immense plasticity, and unlimited capacity of self-renewal, embryonic stem cells could have outstanding use in regenerative medicine or tissue replacement. However due to ethical issues, human embryonic stem cells have not been used, as they are taken from spare human embryos left over from fertility treatments, or from cloned human embryos developed in the laboratory. An alternative method for making cloned embryos is called nuclear transfer. During this process, genetic material or DNA from a donor is inserted into an empty egg cell. After the resulting hybrid cell has been 'activated' (normally using an electrical pulse), it begins to divide, creating new cells and forming a cloned embryo, this of course raises many ethical issues that will be covered later on in the paper.
Because stem cells are so versatile, and because of the properties mentioned above, they could potentially be used to repair and replace damaged human tissue. In future it is hoped that stem cells could be used to treat and cure a variety of diseases and injuries.
Stem cells can now be grown and transformed into specialized cells with characteristics consistent with cells of various tissues such as muscles or nerves through cell culture. Highly plastic adult stem cells from a variety of sources are routinely used in medical therapies. Embryonic cell lines and autologous embryonic stem cells generated through therapeutic cloning have also been proposed as promising candidates for future therapies.
Scientists are now researching stem cells, having discovered their potential, and are studying how they develop, and how cells from the patient could be taken and potentially influenced in a laboratory into becoming specific types of cells which could then be cultured into replacement tissue. This is called therapeutic cloning, and this is very beneficial as the replacement tissue that has been cultured would have exactly the correct genetic information if the patient, as the tissues were derived from the patient themselves. The patient's immune system would not reject the replacement tissues, as is the case of many transplants using donor tissues. This would also discard any problems with ethical and controversial issues in using embryonic stem cells.
As we said previously, the use of adult stem cells in stem cell therapy has already occurred and has proved widely successful. These treatments include bone marrow transplants to treat leukaemia, which transplants hematopoietic stem cells or blood stem cells taken from the bone marrow. These cells taken from the bone marrow contain adult stem cells, which prove vital in the treatment of many cancers. Scientists hope to use hematopoietic stem cell therapy as a stepping stone to be able to discover a wide range of technologies derived from this type of stem cell research, and this could then in turn, allow the treatment of many other diseases including Parkinson's disease, spinal cord injuries, Amyotrophic lateral sclerosis, multiple sclerosis and muscle damage, amongst a number of other impairments and conditions. Nevertheless, there still lies a great deal of uncertainty, both scientific and social.
Another point to be made is the recent discovery of versatile stem cells in the amniotic fluid surrounding the baby, which could possibly be kept as `spares' for the baby, and then later be recovered and grown in a laboratory without the problem of tissue rejection by the immune system. New research has revealed that these stem cells can be extracted relatively easily and grown into many different types of differentiated cells. Amniotic stem cells have been described as `halfway houses' between embryonic stem cell and adult stem cells. This in beneficial as they have almost the same versatility and pluripotence of embryonic stem cells, without the controversy surrounding it. They also, unlike embryonic stem cells, do not form tumours.
There is a large ethical debate surrounding stem cell research, largely centred on the creation, usage and destruction of human embryonic stem cells and due to current technology, the extraction of these embryonic stem cells inevitably results in the destruction of the embryo itself.
On one hand, there is the argument that in creating these embryos artificially and then simply discarding them when they have the potential to become a human, fundamentally devalues the worth of a human being. This argument however, is tested, by researchers, who say that the necessity of the pursuit of embryonic stem cell research is far greater than the ethical issues brought up in destroying the resulting embryo. They believe that stem cell research must continue in order to save lives with its incredible medical potential. Also, they believe that with the consent of the donors, they have the right to use the embryonic stem cells for `the greater good'. This point controversially disagrees with the pro-life movement, who advocate for the protection of pre-born human life.
Very recently though, a breakthrough was made in stem cell research in America. President Obama, who supports the medical potential of stem cell research, has lifted the restrictions imposed by President Bush on federal funding for research on human embryonic stem cell. Although, he will not be sending much more money to the stem cell researchers, due to the current global financial crisis, his lift on the ban has allowed scientists to flourish in their research, and also could result in a drop in research costs.