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Does
it help us ethically to distinguish scientifically between totipotency and
pluripotency? Is it accurate to attribute the status of embryo to totipotent
stem cells and deny it to pluripotent stem cells? Even if we are successful at
removing pluripotent stem cells from the category of embryos--and, thereby,
exempting them from legal restrictions on embryo research--have we really dealt
sufficiently with the ethical issues?
Let
us review the proposed scientific distinction. Why is it that pluripotent stem
cells are one step down the embryonic staircase? Totipotent
stem cells are capable of producing all tissue, including the trophoblast
necessary for implantation. Totipotent stem cells have the potential of
becoming embryos. Pluripotent stem cells have every capability of the
totipotent stem cells minus one, namely, they have no trophoblast. Minus the
trophoblast, pluripotent cells cannot for this reason develop into a full human
being. In this limited sense, pluripotent cells are not potential embryos.
It
may turn out that the difference between totipotency and pluripotency is little
more than a verbal distinction. The genetic code remains the same. The potency
for making all bodily tissues remains the same. When using the blastocyst as
source, the trophechtoderm is removed; and this denies to the internal hES
cells access to what is necessary for implantation. What distinguishes the
pluripotent stem cells is their loss of the blastocyst environment and the
potential for sharing in the benefits of implantation. In sum, it is not nature
but rather the laboratory procedure that demotes cells from totipotency to
pluripotency.
Relevant
here is the question: would a pluripotent stem cell under optimum conditions be
able to produce a trophoblast and eventually a placenta for implantation? Why
not, if the genetic code is complete and if it has the potential for making any
tissue? Already experiments with mice have successfully shown that it is
possible to form a fetus from a stem cell. It is the
pre-differentiated state of the ES cell that, in principle, makes it capable of
producing not only any bodily tissue but also of becoming an embryo.
This
forces us to ask: would all ethical concerns previously pertaining to the use
of embryos in research now apply to pluirpotent hES cells? It would seem that
this is the case. But, there is more. We now need to ask just what is being
covered up by the totipotent-pluirpotent distinction and also ask about the
implications of possible totipotency in normal somatic cells.
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| Contributed by: Dr. Ted Peters
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