A human embryo — precisely because it is a complete member of the human species — can develop towards maturity, given a suitable environment and adequate nutrition. The embryo possesses the genetic and epigenetic primordia and the active disposition for self-directed growth towards the next more-mature stage. But this is not true of a stem cell or even of a mass of stem cells. Like somatic cells that might be used in cloning, they possess merely a passive capacity to be subjected to various techniques of asexual reproduction and so become parts of a new human organism.
Silver bases his claim that “embryonic stem cells are equivalent to embryos” on the fact that mouse embryos can be generated from embryonic mouse stem cells and have all of their genetic makeup, and cell lineage, derived from those initial stem cells. A tetraploid embryo-like entity known, though controversially, as a tetraploid “embryo” (“tetraploid” meaning that the entity has four sets of chromosomes rather than the normal two sets) is developmentally defective, so it can give rise only to trophoblastic cells (precursors of the placenta and associated tissues) and not to the cells of the “embryo proper.” When combined with mouse ES cells (ones that have a normal number of chromosomes), these can produce a chimeric mouse in which the cell lineage of its placenta and associated tissues is derived from the tetraploid entity (or “embryo”), and the cell lineage of the mature embryo (the “embryo proper”) is derived from the ES cells. From this, Silver infers that ES cells can by themselves develop into the mature stage of the animal (see his book, p. 140) — “by themselves” in the sense that the DNA in all of the mature embryo’s cells is identical to that in the ES cells.
From National Review article by Patrick Lee & Robert P. George.
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