Comparison between in vivo-derived and in vitro-produced pre-elongation embryos from domestic ruminants.

In vitro production of ruminant embryos has become routine and is increasingly available as a commercial service to dairy, meat and wool producers. However, the efficiency of producing viable embryos and the development of such embryos after transfer to recipients are perceived to be inferior to that which occurs in vivo. The present review outlines the biochemical and morphological similarities and differences between embryos produced in vitro and those produced in vivo. Some measures of metabolism are not markedly different between in vitro- and in vivo-derived blastocysts. However, at a cellular and subcellular level, differences in metabolism, morphology and ultrastructure have been described, as has susceptibility to manipulation and cryopreservation. Most importantly are the differences in lambing and calving rates and the reports of abnormal fetal development from embryos produced in vitro. These latter observations are of major concern, as they suggest that the in vitro environment may affect subsequent developmental physiology. At the extreme, these effects may not be expressed until adult life. Further efforts to improve the efficiency of in vitro embryo production must be accompanied by a commitment to assess the long-term consequences of these procedures.