Embryonic stem cells are derived from blastocyst cells obtained during in vitro fertilization procedure. The sperm and egg cells are fused to form a zygote, which is the initial cell of the embryo. The zygote undergoes multiple divisions and after five days in culture the blastocyst is formed. The blastocyst is comprised of an inner cell mass, which subsequently forms the embryo, and a surrounding outer trophoblast layer, which later forms the outer chorionic sac and the fetal component of the placenta. The inner cell mass is isolated and cultured under specific growth conditions to support pluripotency and self-renewal. The resulting cells are now termed embryonic stem cells – ESCs. ESCs can be further differentiated into cells of any of the three germ layers: endoderm, mesoderm and ectoderm.