Recycling bovine embryos for nuclear transfer

The terms recycling and serial transfer refer to the use of cells from nuclear transfer embryos as a source of donor nuclei for a subsequent round of nuclear transfer. This approach has two benefits: improved reprogramming of the donor nucleus and an increase in absolute numbers of identical offspring. The beneficial effect of sequential exposure of donor nuclei to the early cytoplasm has been proven in several species, including amphibia, mice and cattle. Transferable embryos and live offspring have also been obtained from cattle and goat embryos produced by three and five rounds of nuclear transfer respectively. Although the limits to recycling have not been determined, embryonic development has been obtained from up to 10 cycles of nuclear transfer. Our results do not indicate differences in the embryonic developmental competence between clones from three generations in bovine embryo recycling. Although one live offspring from the second generation and pregnancies from third generation clones have been obtained, overall lower pregnancy rates and higher fetal losses than from first generation clones have been observed. Application of a novel vitrification method to nuclear transfer recycling removes many of the practical limitations of the technology.