Human embryonic stem cells: the battle between self-renewal and differentiation.

Human embryonic stem cells are pluripotent cells derived from the inner cell mass of blastocyst-stage embryos. These cells possess two unique properties: an indefinite self-renewal capacity and pluripotency, the ability to differentiate to cells from the three germ layers. The pathways governing self-renewal and pluripotency are currently under intensive research. Much effort is devoted to the establishment of feeder-free cultures by elucidation of the cytokines and growth factors required for cell propagation. These seem thus far, to be distinct from those required by mouse embryonic stem cells. In addition, transcriptional regulators unique to embryonic stem cells seem to govern the pluripotent state. These transcriptional regulators determine cell fate, and decide whether the cell will remain pluripotent or differentiate. Together, the understanding of the exogenous and endogenous factors determining cell fate will facilitate the use of these cells in cell-based therapies and will allow understanding of early developmental processes.