Although organ cultures maintain a relatively highly differentiated state of the cells for limited periods of time, until recently most cell lines, even after explantation in vitro for a few weeks, lost many differentiated characteristics that the tissue may have had in the whole animal. While there are exceptions to this statement, such as keratinocyte cultures (cf. Linge, 1996) and the growth of cells on specific substrata or “scaffolds” (Freed and Vunjak-Novakovic, 1998), the majority of cell lines and strains maintained in culture show relatively few differentiated charac- teristics, morphologically or molecularly. This problem has now been surmounted by the dem- onstration of the differentiation of cultures of embryonic stem cells (ES cells; Chapter 5) into a variety of different tissue types in vitro and in vivo (O’Shea, 1999). Stem cells may be defined as cells exhibiting extensive self-renewal properties extending throughout the life of the organism while at the same time retaining the ability to generate differentiated progeny (cf. Hollands,1997; Morrison et al., 1997). It has now become apparent, within the last decade, that such ES cells exhibit the potential to differentiate along different tissue lines as a result of the presence of a variety of different factors, usually polypeptides, that regulate differentiation into various lin- eages (cf. Pedersen, 1999). Thus, with appropriate manipulation it is possible to generate cells of the hematopoietic (Dieterlen-Lièvre, 1997), immune (Chen and Mok, 1995), and neural (Brüstle et al., 1999) systems. Stem cells of the epidermis and gastrointestinal tract derived from adult tissues have also been isolated (Jones, 1997). A diagram of the procedure used for generating stem cells from the early embryo with their potential for transplantation in vivo is seen in Figure
14.2. While in the past it has been possible to maintain certain differentiated characteristics by careful control of a variety of culture conditions (Wolffe and Tata, 1984; Watt, 1991), the ability to characterize, isolate, and culture both ES cells and stem cells from adult tissues from which differentiated tissues arise offers great potential not only for a study of the mechanisms of differ- entiation but also for the practical application of organ transplantation and replacement (Peder- sen, 1999; Thomson et al., 1998).