110 
Monitoring Stem Cell Research 
general observations and specific examples that might help 
non-scientist readers understand the overall state of present 
human stem cell research, its therapeutic promise, and some of 
the problems that need to be solved if the research is to yield 
sound knowledge and clinical benefit. To that end, we high- 
light the importance of well-characterized, stable preparations 
of stem cells for obtaining reproducible experimental results 
and we identify several problems that must be solved before 
these requirements can be fully met. This chapter then de- 
scribes, by way of illustration and example, some of the better- 
characterized adult and embryonic stem cells. It also indicates 
some of the specific investigations that are being conducted 
with their aid. Finally, it considers how human stem cells are 
being used to explore their potential for treating disease, using 
experiments in animal models of Tvpe-1 diabetes as an exam- 
ple, and it points out some of the difficulties that must be over- 
come before stem cell-based remedies may be available to 
treat human diseases. 
We confine our attention here to newly identified types of 
human stem cells and their potential use in research and fu- 
ture medical treatment. Accordingly, we do not consider those 
stem cell types that are already well established in medical 
practice and research. Specifically, we v\hll not examine those 
preparations of bone marrow cells that have been clinically 
used for some years to treat various forms of anemia and can- 
cer.® Neither will we deal with hematopoietic (blood-forming) 
stem cells that have been isolated and purified from bone mar- 
row and are now being intensively studied.^® Although these 
developments lie beyond the scope of this report, the demon- 
strated usefulness of these cells for research and therapy en- 
courages many researchers to expect similar benefits from the 
newer stem cells that we shall consider here. 
I. STEM CELLS AND THEIR DERIVATIVES 
The adult human body, and all its differentiated cells, tis- 
sues, and organs, arise from a small group of cells contained 
within the early embryo at the blastocyst stage of its develop- 
ment. During in vivo embryonic development, these cells, con- 
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