Strains used for Microinjection
B6SJL (C57BL/6 X SJL)F1
The B6SJLF1 hybrid strain is the TTML's strain of choice as young females consistently generate large numbers of injectable embryos. Consequently, fewer mice are needed for each microinjection and projects can move more quickly through the facility. Pigmented B6 females are crossed with albino SJL to generate B6SJLF1 mice. B6SJLF1 females are mated with B6SJLF1 males to generate the embryos for microinjection and therefore, the mixed B6;SJL transgenic founders can be any number of colors depending on which coat color genes are inherited.
FVB/N is an inbred albino strain derived from outbred CD-1/ICR mice. More mice are required for microinjections as these females do not routinely yield consistent numbers of fertilized embryos. Consequently, FVB projects are more costly and take longer to complete.
Although not considered a standard strain for microinjection in our laboratory, due to limited space and resources, inbred C57BL/6 mice are commonly used to generate transgenic founders and are available for microinjections. There are additional costs and projects take longer to complete.
Other strains can be used if necessary for specific experiments, however, not all strains are suitable for microinjections. Many strains, such as SJL or 129, have poor reproductive capabilities while others, like BALB/c, do not incorporate transgenic DNA efficiently.
Strains used for Blastocyst injections
129-derived ES cell-C57BL/6 blastocyst strain combinations
The classical scheme for generating chimeric mice involves the microinjection of 129 strain-derived ES cells into C57BL/6 strain blastocysts. 129 mouse substrains are chinchilla or agouti in color whereas C57BL/6 mice are black. Therefore, the resulting chimeric pups have multicolored coats with any black coloration arising from genes inherited from the cells of the host blastocyst and all other colors arising from tissues derived from the microinjected ES cells. The percent chimerism of an animal is then determined by estimating the overall degree of coat color contribution derived from the ES cell (ie, any coloration that is not pure black). For example, high percentage HM1 129P2/OlaHsd chimeras (Aw/Aw, pTyrc-ch/ pTyrc-ch) have mosaic coat colors of chinchilla, white, agouti and very little black coloration (picture). Other 129 derived chimeras, such as those derived from the R1 ES cell line (ref), have a characteristic agouti-black stripping coat color pattern (picture).
C57BL/6-derived ES cell-albino B6 blastocyst strain combinations
C57BL/6 (a/a, C/C) ES cell clones can be microinjected into blastocysts from an albino B6(Cg)-Tyrc-2J/J (a/a; c/c), creating black and white animals (picture). Germline transmission of ES cell derived genes can be identified at birth by the presence of pups with black eyes, due to the inheritance of the C allele at the albino locus from the C57BL/6 ES cell, which is dominant to the recessive c allele of the albino blastocyst strain. Highly chimeric animals will be mostly black in color with black eyes. Unlike the 129 chimeras, percent chimerism of B6 ES cell-derived animals is not necessary correlative with the inheritance of ES cell-derived germ cells, although this seems to be more particular to the parental ES cell line. Chimeras are then mated back to B6(Cg)-Tyrc-2J/J mice and any resulting black pups are genotyped for inheritance of the mutated allele.