8 
Monitoring Stem Cell Research 
of stem cells, we think the following introduction to the “cast 
of characters" would be useful at the start.* 
A. Embryonic Stem (ES) Cells 
As noted above, ES cells are derived from the inner cell 
mass of embryos at the blastocyst stage, roughly five to nine 
days after fertilization — after the zygote has divided enough 
times to result in about 200 cells, but before it has undergone 
gastrulation and differentiation into the three primary germ 
layers (see Appendix A)."^ The inner cell mass is the part of the 
blastocyst-stage embryo whose cells normally go on to be- 
come the body of the new individual. The outer cells of the 
blastocyst-stage embryo (the trophoblast cells) normally (that 
is, in vivo) go on to become the fetal contribution to the pla- 
centa and other structures that connect the developing indi- 
vidual to the mother’s bloodstream and that otherwise support 
the embryo’s further development. Collecting the cells of the 
inner cell mass results in the destruction of the developing or- 
ganism. The embryos from which human stem cells can be de- 
rived are available (so far) only from in vitro fertilization (IVF): 
they have been conceived by a combination of egg and sperm, 
occurring outside the body.* 
* The remarks about embryo-derived cells presented in the next two sections 
apply to human embryonic stem cells, as opposed to, say, mouse embryonic 
stem cells (which will be referred to in several places because they have 
provided the basis for much of what we now know about embryonic stem 
cells). 
^ In this report, we will not call the cells contained in the inner-cell-mass 
“stem cells," so long as they remain inside the intact embryo. We reserve the 
term “stem cells" for those cells that are successfully cultured outside the 
embryo, following artful derivation, and that demonstrate the characteristic 
capacities of “sternness": a capacity for self-renewal and a capacity for dif- 
ferentiation. Inner-cell-mass cells may or may not be identical to ES cells, 
though in an intact embryo the inner-cell-mass cells are still part of a nascent 
organic whole. Indeed, it is important to remember that the developmental 
fate of all cells inside the body is in part a function of their location within the 
larger whole and of the influences of the local embryonic environments to 
which they are subject. 
* As of this writing, experiments in asexual methods of conceiving a human 
embryo, such as parthenogenesis or cloning, have not, to our knowledge, 
been successful beyond the very early stages of development. Embryos, fer- 
tilized in vivo, covild also be procured for use in research by flushing them 
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