Comparison of BALB/c and B6-albino mouse strain blastocysts as hosts for the injection of C57BL6/N-derived C2 embryonic stem cells

3R measures in facilities for the production of genetically modified rodents

Sociocultural changes in the human-animal relationship have led to increasing demands for animal welfare in biomedical research. The 3R concept is the basis for bringing this demand into practice: Replace animal experiments with alternatives where possible, Reduce the number of animals used to a scientifically justified minimum and Refine the procedure to minimize animal harm. The generation of gene-modified sentient animals such as mice and rats involves many steps that include various forms of manipulation. So far, no coherent analysis of the application of the 3Rs to gene manipulation has been performed. Here we provide guidelines from the Committee on Genetics and Breeding of Laboratory Animals of the German Society for Laboratory Animal Science to implement the 3Rs in every step during the generation of genetically modified animals. We provide recommendations for applying the 3Rs as well as success/intervention parameters for each step of the process, from experiment planning to choice of technology, harm-benefit analysis, husbandry conditions, management of genetically modified lines and actual procedures. We also discuss future challenges for animal welfare in the context of developing technologies. Taken together, we expect that our comprehensive analysis and our recommendations for the appropriate implementation of the 3Rs to technologies for genetic modifications of rodents will benefit scientists from a wide range of disciplines and will help to improve the welfare of a large number of laboratory animals worldwide.

Efficiency comparison of B6(Cg)-Tyrc-2j /J and C57BL/6NTac embryos as hosts for the generation of knockout mice

Careful selection of the host embryo is critical to the efficient production of knockout (KO) mice when injecting mouse embryonic stem (mES) cells into blastocysts. B6(Cg)-Tyrc-2j/J (B6 albino) and C57BL/6NTac (B6NTac) strains of mice are widely used to produce host blastocysts for such procedures. Here, we tested these two strains to identify an appropriate match for modified agouti C57BL/6N (JM8A3.N1) mES cells. When comparing blastocyst yield, super-ovulated B6NTac mice produced more injectable blastocysts per female than B6 albino mice (8.2 vs. 5.4). There was no significant difference in birth rate when injected embryos were transferred to the same pseudopregnant recipient strain. However, the live birth rate was significantly higher for B6NTac blastocysts than B6 albino blastocysts (62.7% vs. 50.2%). In addition, the proportion of pups exhibiting high-level and complete chimerism, as identified by coat color, was also significantly higher in the B6NTac strain. There was no obvious difference in the efficiency of germline transmission (GLT) when compared between B6NTac and B6 albino host embryos (61.5% vs. 63.3% for mES clones; 64.5% vs. 67.9% for genes, respectively), thus suggesting that an equivalent GLT rate could be obtained with only a few blastocyst injections for B6NTac embryos. In conclusion, our data indicate that B6NTac blastocysts are a better choice for the microinjection of JM8A3.N1 mES cells than B6 albino blastocysts.

Selection against BALB/c strain cells in mouse chimaeras

It has been shown previously that BALB/c strain embryos tend to contribute poorly to mouse aggregation chimaeras. In the present study we showed that BALB/c cells were not preferentially allocated to any extraembryonic lineages of mouse aggregation chimaeras, but their contribution decreased during the early postimplantation period and they were significantly depleted by E8.5. The development of BALB/c strain preimplantation embryos lagged behind embryos from some other strains and the contribution that BALB/c and other embryos made to chimaeras correlated with their developmental stage at E2.5. This relationship suggests that the poor contribution of BALB/c embryos to aggregation chimaeras is at least partly a consequence of generalised selection related to slow or delayed preimplantation development. The suitability of BALB/c embryos for maximising the ES cell contribution to mouse ES cell chimaeras is also discussed.

Summary: BALB/c strain embryos contributed poorly to mouse aggregation chimaeras by E8.5. Selection appears linked to slow BALB/c development and might also explain the good ES cell contribution in BALB/c↔ES-cell chimaeras.