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Show The Science, Ethics, and Politics of Stem Cell Research Bradley Curtis cells can differentiate into different types of specialized cells (National Institute of Health (NIH), 2005b). This first property is rather remarkable because most cells in the body cannot divide indefinitely. The DNA in these cells has a pre-programmed limit that controls how many divisions a cell will make. This limit gives each organ its specific size. For example, a human heart will not continue to grow to the size of a watermelon, because its genetic code does not program it to do so. Undifferentiated cells do not have these limits programmed yet, allowing them to divide for long periods in a process known as proliferation (NIH, 2005b). This property allows scientists to create stem cell lines, or cells that have been copied from the original stem cell. These stem cell lines are genetically identical to the cell from which they were derived. Stem cells have not yet differentiated, allowing the possibility that they can develop into different types of specialized cells. Scientists are just beginning to understand the interior and exterior signals that control differentiation. The cell's genetic code controls the interior signals. The code is located in the DNA of the cell and carries instructions for all the structures of the cell and their functions. The external signals that control differentiation include chemicals that other cells release, physical contact with other cells, as well as certain molecules in the environment (NIH, 2005b). The hope is that through stem cell research, scientists will one day be able to control the differentiation of the stem cells, allowing them to create whatever type of cell or organ a person needs. The time frame for this depends greatly on the availability of funding for stem cell research. Different Categories of Stem Cells A crucial task is to identify and understand the different types of stem cells that are involved in ethical and political discussions. Each type of stem cell has different characteristics and capabilities, and researchers harvest them at different times in the developmental process, thus each type raises different ethical issues. The different types are organized from most flexible to least flexible: totipotent stem cells, pluripotent stem cells, multipotent stem cells, and progenitor stem cells. Totipotent stem cells are the most flexible type of cell. They contain all the genetic information necessary to create all the cells in the human body, including the placenta. Human cells are totipotent for only the first three or four divisions of a fertilized egg. After this stage, the cells begin to specialize. It is during this specialization that these cells become pluripotent (Stem Cell Research Foundation (SCRF) Section 13, 2005). Pluripotent stem cells can become any type of cell in the human body. These cells differ from totipotent cells in that they do not contain the genetic information necessary to make a placenta. These cells are mostly found in the earliest developmental stages of the human embryo. This is the cell type that most people refer to when discussing stem cell research (SCRF Section 13, 2005). Researchers extract these pluripotent stem cells from within the embryo. Multipotent cells can produce cell types found in the tissue from which they were drawn; blood stem cells can only produce red blood cells, white blood cells and platelets. A multi-potent skin stem cell can only divide and grow into a hair follicle cell or a sweat gland cell; it could not become a lung cell, blood cell, or any other kind of cell. Multipotent stem cells are found in the adult human body (SCRF Section 13, 2005). The least flexible type of stem cells are progenitor cells, or precursor cells. Like multipotent cells, progenitor or pre-specialized cells are found in adult human beings. These immature cells are pre-coded to specialize into specific cell types that exist in human adult tissues and organs. They have a very limited ability to differentiate, but are considered stem cells because they have not yet differentiated (University of Minnesota Center for Bioethics (UMCB), 2002, p.7). Stem cells are categorized not only according to their flexibility, but also according to where and when they are derived. Embryonic stem (ES) cells are of the pluripotent class. They are derived from the inner cell mass of a blastocyst-stage embryo. A blastocyst-stage embryo forms about five days after they have undergone in vitro fertilization (IVF). Inside the outer-cell ring of the blastocyst is a mass of about thirty cells known as the inner cell mass. The inner cell mass is a group of pluripotent cells that researchers can extract and isolate. Scientist James Thomson and his team first extracted and isolated an inner cell mass at the University of Wisconsin in 1998 (CNN, 1998). In order to isolate the stem cells, researchers utilized IVF to create the blastocyst. They then took the inner cell mass and placed it in a medium that encouraged the production of stem cells. This, in turn, produces moral and ethical concerns because the scientists created the blastocyst for the sole purpose of deriving stem cells. Consequently, the extraction of ES cells destroys the embryo from which they are taken. The type of stem cell research that has received the most attention from the public, as well as from Congress, involves using excess embryos from in-vitro fertilization. When a woman undergoes IVF, multiple embryos are produced. The purpose is to give a woman a better chance at having one of them fertilized. After the woman decides how many embryos she would like to have implanted, excess embryos remain that are not implanted. These embryos are either discarded, or they are frozen for use at a later time. This store of excess embryos provides a source that scientists could use to derive additional stem cell lines. Scientists can utilize IVF to create embryos, but this is not the only method. Another possibility is to create embryos by way of somatic cell nuclear transfer (SCNT), which is more commonly known as therapeutic cloning. The benefit of SCNT is that the cells produced would be genetically identical to the person who requires the transplant. This would considerably reduce the risk of transplant rejection. 16 |
