embryonic stem cells (escs) are derived from the inner cell mass of the preimplantation stage embryo and are capable of prolonged symmetrical self-renewal (both daughter cells remain escs) as well as differentiation into derivatives of all three embryonic germ layers. escs therefore have the potential to provide an unlimited supply of transplantable cells to replace or regenerate damaged or diseased tissues. however, several barriers must be overcome before successful clinical trials are possible: for example, pure populations of the desired cell type need to be selected and expanded in clinically relevant numbers, and a method for preventing immunological rejection of the transplanted cells without long-term immunosuppressive therapy is also required. in this review, we highlight recent developments in human esc derivation and expansion, outline current understanding of the signalling pathways underlying stem cell renewal, and discuss challenging problems related to the selective differentiation and immune properties of human escs.
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