**2.1 Animals from pluripotent cells**

ESCs are the prototypical pluripotent stem cell and thus the most thoroughly characterized. They can self-renew indefinitely and are effectively immortal in cell culture. Although they lack the self-organizing capabilities of the fertilized egg, they can form any tissue in the adult organism as demonstrated by two key studies: injecting ESCs into blastocysts gives rise to chimaeric animals with tissues contributed by the injected ESCs as well as the original ICM (Moustafa and Brinster, 1972); injection of ESCs into blastocysts that have been rendered tetraploid (four genome copies, and therefore genomically incapable of forming a complete organism) produces animals wholly derived from the injected ESCs (Eggan et al., 2002). The latter technique is possible because tetraploid blastocysts retain the structural organization of a normal blastocyst, and although the tetraploid ICM will inevitably die out or senesce (and be replaced by the injected cells), the trophoblast component retains its function despite tetraploidy since trophoblasts eventually fuse and become polyploid anyway upon embryonic implantation into the maternal uterus. These properties are shared with all pluripotent cells, EC cells injected into blastocysts can also give rise to chimaeric animals (Mintz and Illmensee, 1975). Because of this potential, ESCs very quickly became a focus of applied research.
