**1.1.1 Human embryonic stem cells**

The pioneering work on mESCs, and later advances in culturing techniques that were developed to culture nonhuman primate embryonic stem cell lines eventually led to the first successful generation of hESC lines by Thompson and coworkers and two years later by Reubinoff and coworkers (Evans & Kaufman, 1981; Martin, 1981; Reubinoff et al., 2000; Thomson et al., 1995; Thomson et al., 1996; Thomson et al., 1998). These hESCs were derived from human embryos that were produced by *in vitro* fertilization for clinical purposes. HESC lines were karyotypically normal and maintained the developmental potential to contribute to derivatives of all three germ layers, even after clonal derivation and prolonged undifferentiated proliferation (Amit et al., 2000). Since then, hundreds of stem cell lines have been derived world-wide from morula, later blastocyst stage embryos, fresh and cryopreserved supernumerary embryos, single blastomeres and parthenogenetic embryos (Klimanskaya et al., 2006; Lin et al., 2007; Mai et al., 2007; Revazova et al., 2007; Stojkovic et al., 2004; Strelchenko et al., 2004).

HESCs grow in tightly packed colonies and maintain defined borders at the periphery of colonies. High nucleus to cytoplasm ratio and prominent nucleoli are typical features of individual hESCs within colonies. HESCs are also characterized by high telomerase activity and expression of a number of cell surface markers and transcription factors including stage-specific embryonic antigen (SSEA)-4, SSEA-3, TRA antigens, Oct3/4, Nanog and absence of hESCs negative markers such as SSEA-1 (Carpenter et al., 2003; Chambers et al., 2003; Draper et al., 2004; Heins et al., 2004; Nichols et al., 1998). Functional confirmation of the multipotent nature of hESCs is generally achieved by examining their potential to differentiate into all three germ layers (ectoderm, mesoderm and endoderm) both *in vitro* and *in vivo. In vitro*, hESCs are allowed to randomly differentiate as embryoid bodies (EBs), which are aggregates of cells grown in suspension culture, followed by immunocytochemical analysis, or measurement of expression of genes associated with the three germ layers by RT-PCR (Reubinoff et al., 2000). The *in vivo* test for pluripotency of hESCs is normally teratoma formation in immunocompromised mice (Bosma et al., 1983).
