Scope of Embryonic Stem Cells in Research

Embryonic stem cells (ES cells) are pluripotent stem cells derived from the inner cell mass of the blastocyst, an earlystage embryo. Human embryos reach the blastocyst stage 4-5 days post fertilization, at which time they consist of 50- 150 cells. Isolating the embryoblast orinner cell mass (ICM) results in destruction of the fertilized human embryo. They can be propagated as homogeneous stem cell cultures and expanded without apparent limit. ES cells retain the character of embryo founder cells, even after prolonged culture and extensive manipulation. Thus, they are able to reintegrate fully into embryogenesis when returned to the early embryo. The ability to propagate pluripotent ES cells presents unique opportunities for experimental analysis of gene regulation and function during self-renewal, cell commitment, and differentiation. The combination of intrinsic and extrinsic factors that maintain developmental identity and potency is beginning to be defined. Progress is also being made toward understanding and controlling lineage- and/or cell-type-specific differentiation of ES cells in vitro.