Pluripotent, i.e. undifferentiated cells are naturally generated (in vivo) in the blastocysts, an early embryonic stage. They form the inner cell mass. These stem cells form all cells of the embryo (A) during development, but not the extra-embryonic cells of the placenta. In cell culture (in vitro), the cells of the inner cell mass can divide almost indefinitely, and make a culture of embryonic stem cells (ES cells) (B). Through research by Shinya Yamanaka, it is now possible to reprogram body cells into pluripotent stem cells (induced pluripotent stem cells (iPS cells)). This induction is achieved by means of four transcription factors, i.e. factors that bind to the DNA in the nucleus and regulate genes (C).