He suggests that cell-culture heterogeneity may be a factor in such results. EGCs appear later in embryogenesis. Derived from primordial germ cells, EGCs are the ancestors of sperm and egg cells. The growth requirements, surface antigens, and morphologies of ESCs and EGCs differ. Embryonic stem cells and germ cells, nevertheless, are both pluripotent.
Papers are now in preparation.
Some ethicists argue that EGC research is less objectionable because the cells are obtained at a later stage of development from already aborted material. ESC work, in contrast, requires the destruction of an embryo. Yet a cloud hovers over the future of EGCs, at least as therapeutic tools. When EGCs develop soon afterward, imprinting might not yet be fully reestablished. Even subtle abnormalities weren't apparent, much less the gross deficits Stewart has seen in cell lines and chimeras derived from androgenetic and parthenogenetic ESCs.