Cloning of bovine embryos from vitrified donor blastomeres

The use of cryopreserved in vitro produced bovine embryos as nuclear transf er donors was assessed. Day 4 or 5 morulae were vitrified and warmed using the open pulled straw method and used as donors for nuclear transfer. Altho ugh the proportion of morulae and blastocysts that developed from nuclear t ransfer embryos derived from day 5 vitrified embryos did not differ from th at derived from fresh embryos (16.7 and 24.3%, respectively), development t o blastocysts was reduced when vitrified donor cells were used (8.3 and 19. 1%, respectively). Likewise, development to morulae and blastocysts was not different between nuclear transfer embryos derived from day 4 vitrified em bryos allowed to recover for 24 h, and day 5 vitrified embryos allowed to r ecover for 1-2 h (27.7 and 15.6%, respectively), but the development to bla stocysts was reduced when day 5 vitrified donor cells were used (23.2 and 1 0.0%, respectively). However, in nuclear transfer embryos derived from eith er day 4 vitrified or day 5 fresh donors, no differences were observed in d evelopment rates to morulae and blastocysts (34.3 and 36.3%, respectively) or to blastocysts alone (20.2 and 18.1%, respectively). Nor were there diff erences in the development rates of fresh or day 4 or day 5 vitrified in vi tro produced (non-nuclear transfer) embryos (47.9, 51.0 and 35.5% developin g to blastocysts at day 7, respectively). In vitro produced embryos and nuc lear transfer embryos derived from day 4 vitrified or day 5 fresh donors we re transferred to recipients at morula or blastocyst stage at day 6 or 7. T he pregnancy rates were similar in both groups of nuclear transfer embryos, but higher in the control group consisting of in vitro produced embryos (4 7, 42 and 67%, respectively). In conclusion, if vitrified donor embryos are allowed to recover for 24 h after warming, their use in nuclear transfer r esults in similar efficiencies to those achieved with fresh embryos.