Many types of stem cells exist in the human body. All stem cells have the capacity to replicate, to self-renew and to differentiate in order to produce specific body parts. Stem cells can be classified into four broad types based on their origin, viz. stem cells from embryos; stem cells from the fetus; stem cells from the umbilical cord; and stem cells from the adult1. All these stem cells have different capacities to differentiate and these capacities are generally classified as totipotent, pluripotent and multipotent.Totipotent cell is capable of forming every type of body cell. Each totipotent cell may replicate, differentiate and become a human being. Pluripotent stem cells can develop into any three major tissue types: endoderm (interior gut lining), mesoderm (muscle, bone, blood) and ectoderm (epidermal tissues and nervous system). Pluripotent stem cells can eventually specialize in any bodily tissue, but they cannot themselves develop into a human being. Multipotent stem cells are tissue-specific stem cells committed to making blood, muscle, nerve, bone of other tissues2.Most of the controversies surrounding stem cell research are focused on pluripotent cells. In order for us to understand the issue, we have to trace the development of a fertilized egg.1.2 Early development of a fertilized egg
Fertilization of an oocyte (unfertilized egg) by a sperm results in a one-cell zygote (fertilized egg), which begins to divide without any increase in size (Figure 1). By 3-4 days after fertilization, cell division results in a compact ball of 16-32 cells known as a morula. By 5-6 days, a blastocyst is formed consisting of a sphere of about 200-250 cells. The sphere is made up of an outer layer of cells (the trophectoderm), a fluid-filled cavity (the blastocoel), and a cluster of cells in the interior (the inner cell mass). The cells of the inner cell mass will give rise to the embryonic disk and ultimately the fetus, but not the placenta, which arises from the trophectoderm3. Among these three different types of cells, only the inner cell mass are pluripotent as they would develop and form all the parts of the human body.
1.3.1 Human embryonic stem cells
As early as 1994, Ariff Bongso, a researcher of in-vitro fertilization (IVF) program in National University of Singapore, first described isolation and culture of cells of the inner cell mass of human blastocysts5. Techniques for deriving and culturing stable human embryonic stem (hES) cell lines were first reported by James Thomson, an associate veterinarian in the University of Wisconsin's Regional Primate Research Center, in 19986. Their techniques of isolating the hES cells have since become the standard in most stem cell research labs. First, spare fertilized eggs (embryos), which themselves are totipotent stem cells, are taken from in vitro fertilization (IVF) clinics. They are next cultured to the blastocyst stage. The trophectoderm is then removed in order to extract the ICM. The ICM are separated before placing them on a feeder tray and cultured. These separated cells known as the hES cells divide and reproduce themselves. Each hES cell is pluripotent, capable of making any bodily tissue.Other than the surplus embryos from IVF clinics, there are other ways to obtain hES cells in vitro. There are embryos created in vitro specifically for stem cell research. Somatic cell nuclear transfer (SCNT) is another source of stem cells. In SCNT, the nucleus from the oocyte is removed. The oocyte without the nucleus is called the enucleated human oocyte. Adult donor cells are extracted and de-differentiated in cell culture before fusing it with the enucleated oocyte. This forms an embryo with the DNA of the donor intact. If grown to full term, it would produce a clone and the classic example is Dolly the sheep. This process is known as reproductive cloning. However, in most cases, the embryo would be cultivated till blastocyst stage before being terminated and this is known as therapeutic cloning.1.3.2 Adult Stem Cells
Perhaps the least controversial method for producing stem cell is by cultivating adult stems cells. Adult stem cells are obtained from adult body and they can be derived in very small quantity from any one person. The most familiar example is extracting hematopoietic stem cells from bone marrow. Once these are removed from human body, they are cultured in tissue culture flasks, with addition of appropriate liquid nutrient medium.1.3.3 Human Embryonic Germ Cells
Last but not least is another method developed by John Gearhart, a professor of gynecology and obstetrics at Johns Hopkins University School of Medicine, in 1998. His procedure involves drawing human embryonic germ cells (hEG cells) from fetal gonadal tissue. These cells, when taken from an aborted fetus at about five to eight week stage, resemble in nearly all respects the pluripotent stem cells7. It is not yet clear whether or not hES cells are identical to hEG. Both are pluripotent and appear to be equivalent in function. Yet, it may be discovered that different alleles appear in different hES, because hES cells could be imprinted by either the male or female source. The blastocyst stage of embryogenesis is a stage that avoids the gender imprint. What is not yet known is whether original gender imprint will matter. For the foreseeable future the two types of stem cells will be treated the same188.8.131.52 Summary of sources of stem cellsTo summarize, stem cells may be derived from:(i) Surplus embryos from IVF;(ii) Embryos created specifically for research;(iii) Somatic cell nuclear transfer;(iv) Adult stem cells;(v) Aborted fetus at about five to eight week stage.
1.3 Sources of Stem Cells
1.1 Classification of stem cells
1. Ariff Bongso and Eng Hin Lee," Stem Cells: Their Definition, Classification and Sources," in Stem Cells: From Bench to Bedside, ed. Ariff Bongso and Eng Hin Lee (Singapore: World Scientific Publishing Co., 2005), 3.
2. Ted Peters, Science, Theology, and Ethics (Burlington, VT: Ashgate Publishing Limited, 2003), 179.
3. Committee on Guidelines for Human Embryonic Stem Cell Research, National Research Council, Guidelines for Human Embryonic Stem Cell Research, 29-30. http://www.nap.edu/catalog/11278.html
4. Ibid, 30.
5. Bongso, A., Fong, C. Y., Ng, S. C., and Ratnam, S.,"Isolation and culture of inner cell mass cells from human blastocysts,"in Human Reproduction, 1994, 9:2110-2117.
6. Thomson, J. A., Itskovitz-Eldor, J., Shapiro, S. S., Waknitz, M. A., Swiergiel, J. J., Marshall, V. S., and Jones, J. M., "Embryonic stem cell lines derived from human blastocysts," in Science, 1998, 282:1145-1147.
8. Ibid., 180.