in around four days, following several cell division cycles, the cells start to specialize and form a blastocyst. This is a hollow sphere of cells, with an outer layer and an inner cell mass. The outer layer of cells form placenta and supporting tissues. The inner cell mass forms the human body tissues. The inner cell mass cells are pluripotent since they give rise to many cell types. They are not totipotent or embryos however, since they can not give rise to all types of cells. Multipotent cells form when pluripotent cells undergo further specialization; these give rise to particular functions. Multipotent stem cells continue to be found in children and adults; blood stem cells in the bone marrow and blood stream replenish blood cells (Borror, 2000, pp. 54-55).
Although stem cells are found in the adult body, they are in minute quantities, are hard to isolate and purify, and they have limited potential. Stem cells for all cells and tissues have not been found. The process of changing from totipotent to pluripotent to multipotent is irreversible, and multipotent stem cells in adults are too far along the specialization path to be as useful as embryo stem cells (Borror, p. 59).
Ethical issues are identified with regard to stem cell research. These ethical issues include what is considered appropriate levels of respect for human embryos and fetal tissue when used as sources of stem cells. Within this context, some fear that stem cell research will slip into human cloning practices (Lee, 2000, p. 81).
Ethical questions include whether the embryo is a human life or a collection of cells. Next comes the question regarding how the embryo is being used and whether the benefits matter more. Where the embryo comes from is a further concern. Whether it comes from the intent of creating life as with infertility clinics with a supply of embryos, or whether is it created for research purposes, are ethical factors (Kahn, 1999)....