Importance of embryo transfers in transgenic mouse facilities.

With the increasing demand for and production of transgenic and mutant mice for biomedical research, embryo transfer plays a paramount role. The purpose of performing embryo transfer in this species is to generate transgenic mice via blastocyst injection of embryonic stem cells or pronuclear injection of DNA constructs, to revitalize cryopreserved sperm and embryos, and to generate mouse lines that meet specific pathogen-free health standards for breeding in barrier areas (rederivation). We present results from two years of carrying out embryo transfers for rederivation purposes in the large mouse breeding facility of the GSF—National Research Center for Environment and Health, Neuherberg, Germany. Pathogens to be eradicated from inbred transgenic (C57BL/6 background) and mutant (C3H background) mouse lines included mouse hepatitis virus, mouse minute virus, and mouse parvovirus. In vitro- and in vivo-produced two-cell embryos were washed 3 times in M2 medium. A total of 20 embryos each were transferred to the oviduct of 8- to 12-week-old specific pathogen-free pseudopregnant (Day 0.5) Swiss recipients under aseptic conditions. Mice were then kept singly in individually ventilated cages and manipulated in a Class II laminar flow hood. From each transfer to one to five recipients with embryos originating from the same mouse line, one recipient was tested for the presence of microorganisms 6 to 12 weeks after embryo transfer, i.e. at 0 to 6 weeks after weaning, according to the FELASA (Federation of European Laboratory Animal Science Associations) Guidelines. A total of 290 embryo transfers were performed for revitalization of cryopreserved sperm from 52 mouse lines, cryopreserved two-cell embryos from 18 mouse lines and rederivation of 12 mouse lines using freshly collected two-cell embryos. From these 290 embryo transfers, 59 mouse lines were re-established (40 from cryopreserved sperm, 7 from cryopreserved embryos and 12 from in vivo-produced embryos). Health monitoring of 54 recipients showed that all mouse lines generated were free of all pathogens stated in the FELASA list. The results presented here show that all 12 (100%) mouse lines were re-established after transfer of freshly collected two-cell embryos whereas 77% and 39% success rates were observed for revitalization of cryopreserved sperm and embryos, respectively. The success of embryo transfer in eradicating pathogens depends on the inability of these pathogens to transverse the zona pellucida and enter and/or infect embryonic cells. In our mouse facility, embryo transfer provided an efficient method to successfully revitalize cells of the mouse germ line as well as to eradicate prevalent murine pathogens. Furthermore, the results demonstrate the efficiency of transferring embryos of different origins and thereby obtaining and maintaining specific pathogen-free health standards in our mouse colonies