Chimeric mouse production by microinjection.

The culmination of the creation of a mutation in mouse embryonic stem (ES) cells is, commonly, the generation of a mouse line that can propagate the mutation. The ability to combine methods of homologous recombination in ES cells with blastocyst-mediated transgenesis has resulted in an explosion of tailored mutant mouse strains. These animals provide research tools that are virtually impossible to create using other methodologies. This unit describes the methods necessary to generate chimeras, from injection of the ES cells into the blastocoel cavity of 3.5-day-old embryos through the implantation of the injected embryos into the foster mother. The resultant pups are true chimeras: their tissues are derived from both the host embryo and from the ES cells. If the ES cells are able to populate the germ line, the chimera can pass an altered gene to offspring, resulting in a new mouse strain in which all cells contain an altered gene.