Re-establishment of breeding stocks of mutant and inbred strains of mice from embryos stored at –196 °C for prolonged periods

Breeding stocks were re-established from embryos of various mutant and inbred strains of mice after prolonged storage in liquid nitrogen at the 8-cell stage even with strains where only a proportion of the progeny were of the desired type, i.e. in the Mo dP and XO strains. Gonadotrophin treatment failed to produce superovulation consistently in any of the strains tested. Although the initial survival of embryos after thawing and culture to the morula and blastocyst stage was highest for embryos from XO mothers (61%), these embryos suffered the heaviest early postimplantation loss after transfer (61%). The proportion of embryos, originally frozen, developing into foetuses and offspring was variable (13% HT, 14% PT, 20% Mo dP , 14% XO, 21% CBA and CBA-T6) and lower than previously reported for hybrid 3H1 embryos (20–30%). The sex ratio of the liveborn young was within the normal expected limits except for the Mo dp strain, where it differed significantly from the exected 2♂:1♀ ratio. The proportion of young of the desired type from the frozen embryos of Mo dp and XO females was less than expected (17 and 3% respectively). In all cases a normal breeding stock was reestablished whose performance was within normal limits for each strain. Even without further improvements in embryo collection and the freezing technique per se , the storage of embryos in liquid nitrogen is an extremely economic way of preserving mouse genetic stocks.