In vitro production of cattle embryos both from slaughterhouse material and live animals has progressed immensely during the past 5-10 years. It is now possible to achieve a blastocyst rate of 25% of the proportion of oocytes submitted to culture and when transferred these embryos have the same capability to develop into normal offspring as embryos produced in vivo. Improvement of the system is still needed, such as reduction of the oxygen tension and concommitant abolition of coculture, in order to minimize the risk for the formation of oxygen free radicals, which are known to cause damage to the early embryos. This would be of further advantage for avoiding transmission of contagious diseases and thus place less health hazard on the in vitro conceived embryos. Modifications of certain energy (glucose) - and protein substrates in the media may also improve the systems. The detrimental effect of freezing and thawing on in vitro produced embryos is of great concern, since it precludes a more widespread use of this technology. Although pregnanciees have been established also in our laboratory more research is needed in this essential part of the in vitro production procedure. Oocytes and in vitro produced embryos may be used for cloning by nuclear transfer, and although some progress has been made in this field the results are far from satisfactory. A simple calculation shows that considerable manpower must be involved just for production of a small number of cloned embryos per week and with the current freezing success they may not tolerated deep freezing and/or recloning. In vitro produced embryos can be used for microinjection and as a mean of either substituting or supplementing superovulation e.g., in animals where this procedure has not been successful or in animals with sterility problems. The advance in ultrasound guided aspiration of oocytes from unstimulated animals has facilitated the use of in vitro embryo production as an alternative to normal embryo recovery. In the future cattle breeders may use these new technologies along with PCR on preimplantation embryos not only to determine their sex but also to select for other traits.
