Introduction 
3 
Stem cells first arise during embryonic development and ex- 
ist at all developmental stages and in many systems of the 
body throughout life. The best described to date are the blood- 
forming (hematopoietic) stem cells of the bone marrow, the 
progeny of which differentiate (throughout life) into the various 
types of red, white, and other cells of the blood. It appears that 
some stem cells travel through the circulatory system, from 
their tissue of origin, to take up residence in other locations 
within the body, from which they may be isolated. Other stem 
cells may be obtained at birth, from blood contained in the 
newborn’s umbilical cord. Once isolated and cultured outside 
the body, stem cells are available for scientific investigation. 
Unlike more differentiated cells, stem cells can be propagated 
in vitro for many generations — perhaps an unlimited number — 
of cell-doublings. 
Stem cells are of interest for two major reasons, the one sci- 
entific, the other medical. First, stem cells provide a wonderful 
tool for the study of cellular and developmental processes, 
both normal and abnormal. With them, scientists hope to be 
able to figure out the molecular mechanisms of differentiation 
through which cells become specialized and organized into 
tissues and organs. They hope to imderstand how these 
mechanisms work when they work well, and what goes wrong 
when they work badly. Second, stem cells and their deriva- 
tives may prove a valuable source of transplantable cells and 
tissues for repair and regeneration. If these healing powers 
could be harnessed, the medical benefits for humankind would 
be immense, perhaps ushering in an era of truly regenerative 
medicine. No wonder that scientists around the world are ac- 
tively pursuing research with stem cells. 
Why, then, is there public contention about stem cell re- 
search? Not because anyone questions the goals of such re- 
search, but primarily because there are, for many people, ethi- 
cal issues connected to the means of obtaining some of the 
cells. The main source of contention arises because some es- 
pecially useful stem cells can be derived from early-stage hu- 
man embryos, which must be destroyed in the process of ob- 
taining the cells. Arguments about the ethics of using human 
embryos in research are not new. They date back to the mid- 
1970s, beginning not long after in vitro fertilization (IVF) was 
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