Sources, derivation, and culture of human embryonic stem cells.

Human embryonic stem cells (hESCs) are immortal cells capable of perpetual self-renewal in culture while maintaining their undifferentiated state, high telomerase activity, normal karyotype, and specific pattern expression of embryonic surface markers and pluripotent transcription factors such as Oct-4 and Nanog. Since their first derivation in 1998, hundreds of hESC lines have been derived and characterized. Normal surplus embryos from IVF programs are the main source for the derivation of hESC lines but cell lines from poor-quality discarded embryos or embryos carrying genetic defects following preimplantation genetic diagnosis were also isolated. Such isolation is usually accomplished by either mechanical or immunosurgical removal of the trophectoderm and culture of the inner cell mass on inactivated feeder cells. In light of the future need for clinical-grade cells, the subject of defining specific culture conditions has been addressed widely. Indeed, derivation and maintenance of hESCs without feeder cells and in media free of animal products have been attained recently. This well-defined culture system may facilitate research and clinical applications, and use the remarkable potential of these exceptional cells to its fullest in both the laboratory and the clinic.