INVITRO CULTURE OF SHEEP EMBRYOS WITHOUT COCULTURE - SUCCESSES AND PERSPECTIVES

The finding that zygotes (one-cell embryos) from several livestock species can be routinely cultured in relatively simple media to the blastocyst stage questions the nature of the environment provided by the oviducts for the development of early stage embryos. In this paper, we review recent findings on the development of sheep embryos in simple media without co-culture. Evidence indicates that embryos are relatively insensitive to changes in the composition of media and that zygotes can develop to blastocysts at rates equal to or higher than those obtained in vivo. However, in vitro culture is associated with several developmental abnormalities. These include cytoplasmic fragmentation, early time of blastocoele formation and a reduced number of nuclei per blastocyst. Viability of embryos (to Day 50 of pregnancy) after 5 days of culture was reduced compared with embryos cultured in vivo (48.2% vs. 59.4%, P < 0.1). Similarly, the viability of micromanipulated (pronuclear gene injected) embryos was significantly reduced after a comparable in vitro culture period (17.0% vs. 26.8%, P < 0.05). Preliminary observations also indicate that an association exists between in vitro culture and an increase in mean gestation length, mean lamb birth weight and lamb mortality. It is possible that these abnormalities are a consequence of the developmental deficiencies incurred by the preimplantation embryo during in vitro culture. The significance of these results and the potential benefit of including somatic cell support in the culture system are discussed.