In Vitro Isolation of Murine Embryonic Stem Cells

Embryonic stem (ES) cells are derived directly from those undifferentiated progenitor cells of early mouse embryos that have the developmental potential to subsequently form all the tissues of the fetus itself. With appropriate culture conditions, these embryonic cells can be maintained continuously in an undifferentiated state in vitro and form permanent ES cell lines (1 ,2 ) (see Note 1 ). When introduced into mouse blastocysts or aggregated with morulae, however, the ES cells are capable of responding to in vivo developmental signals and participate in normal embryogenesis. The stem cells differentiate and contribute to all the tissues of the fetus (3 ), and occasionally to the trophectodermal and primitive endodermal lineages of the extraembryonic membranes (4 ), leading to the formation of chimeric offspring. The full demonstration of their pluripotent potential is seen in the capacity of ES cells also to colonize the germline in chimeras, and form fully functional gametes (3 ). More remarkably, some ES cell lines are capable of supporting complete fetal development, following aggregation with tetraploid embryos, and generate fertile adult mice that are entirely ES cell derived (5 ). One of the major applications of ES cells is in providing a powerful approach for the introduction of novel genetic change into the mouse genome. While the ES cells are in culture, specific genes of interest can be modified in a desired manner with homologous recombination (gene targeting) technology, and by utilizing their pluripotent potential, the ES cells can be used as carriers of this genetic change through the germline and into subsequent generations of mice (6 ) (see Chapters 7 and 8 ).