Having elaborated the functional definition of stem cells, and reviewed some of their properties, it should now be clearer why it is that many scientists insist that human adult and ES cell research should be pursued in tandem. In the province of basic research, human ES lines provide a surrogate to the human embryo, allowing investigation of early developmental processes that otherwise can only be studied in other mammals. As mentioned above, for the purposes of pre-clinical or clinical research and applications, each area has its advantages and disadvantages. Proliferative capacity, broad range of differentiation, and ready availability, are major advantages offered by ES cells.Control over differentiation, possible autol-ogous sourcing, a better safety profile, and universal ethical acceptability constitute the advantages of adult stem cells. It is unclear which approach will offer the most rapid clinical solutions. The likelihood is that neither will have a monopoly over all clinical indications. The only resounding therapeutic success using adult stem cells is bone marrow transplantation. Whilst this demonstration confirms the potential of adult stem cell use, the fact that this solitary success was first achieved over 40 years ago highlights the need for new thinking as well as alternative approaches.
Taken jointly, the reasons given above support the claim that pursuing adult and ES cell research in tandem is less speculative than it is common-sensical. This in-tandem recommendation also extends to embryonic germ (EG) cells.8 EG cells possess similar characteristics to ES cells. For example, they can be propagated in culture over a long period (though less readily than ES cells — 200 population doublings over two years for EG cells9 versus 700 for ES cells), and can, with the correct prompts, spontaneously differentiate into derivatives of all three primary germ layers — endoderm, mesoderm, and ectoderm.10 It has been established that EG cell lines could serve as reliable sources for cell-based interventions.11 There has been so little work carried out on EG cells in comparison to both ES and adults stem cells, that it is impossible to pronounce on their relative clinical applicability with any authority — only more research shall determine whether and for which indications EG cells might be suited.
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