Embryo technology in cattle: Brief review

Aspects of embryo technology are discussed with the main emphasis on cattle . In vivo embryo production with recovery of embryos from superovulated ani mals followed by freezing and transfer of embryos is performed routinely. T he average yield of transferable embryos obtained in the Danish MOET projec t is six originating from approximately 80% of the superovulated donors. Th e pregnancy rate following non-surgical transfer of fresh or frozen embryos is around 50% and the fetal loss 5%. In vitro production of embryos based on oocytes recovered from slaughterhouse material or from live animals foll owing ultrasound guided transvaginal oocyte recovery (OPU or TVOR) is used in many laboratories for experimentation and also in practice as an alterna tive or supplement to in vivo embryo production. The expected rate of blast ocyst formation on day 8 after insemination reaches 40-50% in several labor atories. The main problem encountered with in vitro production is the reduc ed viability of embryos compared with their in vivo counterparts. This is p articularly apparent following freezing and thawing. Vitrification of embry os seems to be a valuable alternative to conventional freezing of embryos w hich have a more fragile constitution, e.g. early in vitro produced or clon ed embryos. Sexing of embryos through PCR analysis of embryo biopsies is us ed in embryo transfer practice in several countries. Sexing of semen is sti ll at the experimental level and is used in conjunction with in vitro produ ction of calves. Cloning of embryos is in progress in many laboratories, bu t with limited success. Current blastocyst and pregnancy rates are 30% and 20%, respectively, but the rate of fetal loss is high. Factors such as cell cycle synchrony, remodelling and reprogramming of the introduced nucleus s till need further elucidation before a more widespread practical use of thi s technology. Genetically modified animals are used for production of certa in valuable pharmaceuticals in their milk and to improve certain growth cha rateristics. The actual success is still limited, but transgenic pigs will be used in biomedical research in the future. Production of embryonic stem cell-like cell lines has been successful in certain laboratories.