restrict their therapeutic applications. Moreover, no human parthenogenetic or androgenetic stem cell lines have been established, and more research is needed to determine whether these techniques can be applied to human oocytes for production of stem cell lines.
At current rates of success of generation of NT blastocysts and ES cells, one major limitation of expansion of this approach will be the availability of oocytes for NT. Current and possible future sources of such oocytes include excess oocytes and unfertilized oocytes from IVF procedures, oocytes matured from ovariectomies or fetal ovaries from pregnancy terminations, oocyte donation, derivation of oocytes from nonreproductive material, and use of nonhuman oocytes.
Excess oocytes and unfertilized eggs from IVF procedures. During IVF, hormonal induction is used to generate oocytes for fertilization in vitro. Often, more oocytes are generated than are needed for reproductive purposes, and some oocytes may be available for research donation. In addition, after IVF, not all oocytes are successfully fertilized, and unfertilized oocytes would otherwise be discarded if not donated for research. Experiments to explore use of such oocytes for NT derivation of hES cells have been approved and initiated in the United Kingdom. However, this source of oocytes is limited, and the unfertilized oocytes may be of lower quality for cell line production. It is ethically problematic to consider alteration of the IVF clinical procedure to deliberately induce more oocytes than needed for reproduction, even with the consent of the participants. Thus, this source of oocytes is likely to be limited and unreliable for any major NT ES cell program.
Oocytes matured from ovariectomies or fetal ovaries from pregnancy terminations. Adult as well as fetal ovaries contain a large supply of immature oocytes, which in principle could be harvested from adult ovaries donated after removal for clinical reasons or from fetal ovaries that are obtained from legal pregnancy terminations. In the case of other mammals, it is possible to mature such oocytes in culture and achieve fertilization and normal development, although the process is not efficient (O’Brien et al., 2003). In humans, success has been limited and requires an intermediate xenograft (transplantation into an animal) of the ovarian tissue for oocyte maturation. Research on how to expand the supply and how to mature human oocytes in vitro could make this a reasonable source of donated material.
Oocyte donation. The most reliable source of oocytes for NT ES cells today seems to be direct donation of oocytes by female donors after hormonal induction and oocyte recovery. Such third-party donation has much in common with organ donation and already occurs in some IVF programs for