THE CHARACTERISTICS OF stem cells are self—renewal (ability to divide indefinitely without differentiating), maintaining a full diploid karyotype, generating any tissue when introduced into an embryo, and colonizing the germ lines of recipient embryos. It is difficult to grow stem cells in culture because of the spontaneity of differentiation. Stem cells must be cultured on a medium that provides signals for maintaining the undifferentiated state or they will proliferate and differentiate without control. The culture, manipulation, and characterization of embryonic stem cells, embryonic germ cells, and adult stem cells in vitro require a mix of nutrients, hormones, growth factors, and blood serum. Human embryonic stem cells derived from an inner cell mass of blastocysts have the pluripotency to form all three embryonic germ layers; some scientists prefer the term totipotency, because it means the cell can produce any cell in the body, though the suggestion of totality could also be ...