Research and Therapy Developments 
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stituting the inner cell mass (ICM) . will divide and differentiate 
in concert with each other and with the whole of which they 
are a part, eventually producing the specialized and integrated 
tissues and organs of the body. But when embryos are grown 
[using in vitro fertilization dVF )] in a laboratory setting, these 
ICM cells may be removed and isolated, and under appropriate 
conditions some will proliferate in vitro and become embryonic 
stem cell lines. 
These embryonic stem cells are capable of becoming many 
different types of differentiated cells if stimulated to do so in 
vitro [see endnote 2 for references]. However, it is not yet clear 
that the cells that survive the in vitro selection process to be- 
come embryonic stem cells have all of the same biological 
properties and potentials as the ICM cells of the blastocyst."^ In 
particular, it is not known for certain that human embryonic 
stem cells in vitro can give rise to all the different cell types of 
the adult body.’ 
As noted in the Introduction to this report, stem cells are a 
diverse class of cells, which can now be isolated from a variety 
of embryonic, fetal, and adult tissues. Stem cells share two 
characteristic properties: (1) unlimited or prolonged self- 
renewal (that is, the capacity to maintain a pool of stem cells 
like themselves), and (2) potency for differentiation, the poten- 
tial to produce more differentiated cell types — usually more 
than one and, in some cases, many.'*' When stem cells head 
down the pathway toward differentiation, they usually pro- 
ceed by first giving rise to a more specialized kind of stem cell 
(sometimes called "precursor cells” or "progenitor cells”), 
which can in turn either proliferate through self-renewal or 
produce fully specialized or differentiated cells (see Figure 1). 
It is also not known whether stem cells, either human or animal, when cul- 
tured in vitro apart from the embryonic whole from which they were origi- 
nally derived, v\hll function in all respects like cells do when they act as parts 
of a developing organic whole. 
^ Some stem cells, however, give rise to only one tyiDe of specialized cell For 
example, one type of stem cell found in the epidermis (skin) apparently gives 
rise only to keratinoctyes (cells that produce the protein keratin, found in hair 
and nails). 
PRE -PUBLICATION VERSION 
