Rederivation

The "status" of the animals used in experiments is determined by a combination of 3 parameters: genetic characteristics, health status and housing conditions.

In the vast majority of cases, genetically modified animals are not immunodeficient organisms and therefore do not require special treatment different from that of unmodified mice, but the microorganisms and parasites present in their bodies and environment modify physiological processes and behaviour, so the hygienic status of the experimental animals is an important factor for the reproducibility of experiments. The animals used for experiments are not sterile because mice need normal gut flora for normal digestion, but for long-term maintenance and breeding they must be free of the most common infectious agents, as recommended by FELASA, the Federation of European Laboratory Animal Science Associations. Depending on the pathogens present in the animal's environment, a distinction is made between conventional, specified pathogen-free (SPF) and opportunistic pathogen-free (SOPF) animal houses.

Animals from infected colonies can only be introduced into clean animal houses by means of rederivation. The classic method is caesarean section, where viable foetuses are removed in a clean environment before calving and the pups are placed with a non-infected wet-nurse. In the case of genetically modified mice, the typical procedure is rederivation via embryo transfer. This is based on the fact that no infectious virus or bacteria can be detected in the ovaries, oviductus, testes and epididymides even if the colony and the animals are otherwise infected.

Pre-implantation stage embryos are obtained either by conventional mating or by assisted reproduction, implanted in pseudo-breeding females in the clear room. After weaning, the health status of the mock pregnant mother is checked and the pups are placed in the clean rooms of the animal house.