Stem cells in animals are primal undifferentiated cells that retain the ability to divide and differentiate into other cell types. In higher plants this function is the defining property of the meristematic cells. Stem cells have the ability to act as a repair system for the body, because they can divide and differentiate, replenishing other cells as long as the host organism is alive.
Medical researchers believe stem cell research has the potential to change the face of human disease by being used to repair specific tissues or to grow organs. Yet there is general agreement that, "significant technical hurdles remain that will only be overcome through years of intensive research."()
The study of stem cells is attributed as beginning in the 1960s after research by Canadian scientists Ernest A. McCulloch and James E. Till.
Stem Cell Types
Stem cells are categorized according to their source, as either adult, embryonic, cancer or cord blood stem cells.
Adult stem cells
Adult stem cells are undifferentiated cells found among differentiated cells of a specific tissue and are mostly multipotent cells.
They are more accurately called somatic (Greek σωμα sōma = body) stem cells, because they need not come from adults but can also come from children or umbilical cords. Particularly interesting are adult stem cells termed "spore-like cells". They are present in all tissues () and seem to survive long time periods and harsh conditions.
Embryonic stem cells
Embryonic stem cells are cultured cells obtained from the undifferentiated inner mass cells of an early stage human embryo (sometimes called a blastocyst, which is an embryo that is between 50 to 150 cells). Embryonic stem cell research is "thought to have much greater developmental potential than adult stem cells," according to the National Institutes of Health.()Research using embryonic stem cells remains at the zenith of stem cell science because, unlike somatic cells, embryonic stem cells are totipotent. However, research using stem cells derived from the human embryo is still in the basic research phase, as these stem cells were first isolated in 1998 (at least for humans), whereas adult stem cells have been studied since the 1960s. ()
Research with embryonic stem cells derived from humans is controversial because, in order to start a stem cell 'line' or lineage, the destruction of a human embryo and/or therapeutic cloning is usually required. Some believe this to be a slippery slope to reproductive cloning and tantamount to the objectification of a potential human being. In an attempt to overcome these moral, political and ethical hurdles, medical researchers have been experimenting with alternative techniques of obtaining embryonic stem cells by extraction, which does not involve cloning and/or the destruction of a human embryo.
Cord blood stem cells
Cord blood stem cells are derived from the blood of the placenta and umbilical cord after birth. Since 1988 these cord blood stem cells have been used to treat Gunther's disease, Hunter syndrome, Hurler syndrome, Acute lymphocytic leukemia and many more problems occurring mostly in children.
Umbilical cord blood use has become so common that there are now umbilical cord blood banks that accept donations from parents. It is collected by removing the umbilical cord, cleansing it and withdrawing blood from the umbilical vein. This blood is then immediately analyzed for infectious agents and the tissue-type is determined. The cord blood is processed and depleted of red blood cells before being stored in liquid nitrogen for later use, at which point it is thawed, washed of the cryoprotectant, and injected through a vein of the patient. This kind of treatment, where the stem cells are collected from another donor, is called allogeneic treatment. When the cells are collected from the same patient on whom they will be used, it is called autologous and when collected from identical individuals (i.e. homozygous twin), it is referred to as syngeneic.
Cancer stem cells
Cancer stem cells arising through malignant transformation of adult stem cells are proposed to be the source of some or all tumors and cause metastasis and relapse of the disease. ()The stem cell origin of leukemias is well established (). The role of stem cells in other tumors is under intensive investigation.
Stem cells are also categorized by potency, which specifies the ameliorative potential of the cell type.
- Totipotent stem cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg cell are also totipotent. These cells can grow into any type of cell without exception.
- Pluripotent stem cells are the descendants of totipotent cells and can grow into any cell type except for totipotent stem cells.
- Multipotent stem cells can produce only cells of a closely related family of cells (e.g. blood cells such as red blood cells, white blood cells and platelets).
- Unipotent cells can produce only one cell type, but have the property of self-renewal which distinguishes them from non-stem cells.
Medical researchers believe that stem cell research has the potential to change the face of human disease and alleviate suffering. A number of current treatments already exist, although the majority of them are not commonly used because they tend to be experimental and not very cost-effective. Medical researchers anticipate being able to use technologies derived from stem cell research to treat cancer, spinal cord injuries, and muscle damage, amongst a number of other diseases, impairments and conditions. However, there still exists a great deal of social and scientific uncertainty surrounding stem cell research, which will only be overcome by gaining the acceptance of the public and through years of intensive research.
Controversy surrounding stem cell research
There exists a widespread controversy over stem cell research, which emanates from the techniques used in the creation and usage of stem cells. In particular, research using embryonic stem cells has proven to be an immensely sensitive topic. This is because, with the present state of technology, starting a stem cell 'line' requires the destruction of a human embryo and/or therapeutic cloning. Opponents of the research argue that this practice is a slippery slope to reproductive cloning and tantamount to the objectification of a potential human being. Contrarily, medical researchers in the field argue that it is necessary to pursue embryonic stem cell research because the resultant technologies are expected to have significant medical potential. The ensuing debate has prompted national and international authorities to seek suitable regulatory frameworks and highlighted the fact that stem cell research represents a moral, social and ethical minefield.