Embryo transfer and related technologies in sheep reproduction.

This paper reviews the status of embryo transfer and the major technologies applied to preimplantation of embryos in sheep. Embryo production from superovulated ewes is hindered by an unpredictable response to hormonal treatment. Progress in this area should be expected by an appropriated control of follicular development with gonadotropin-releasing hormone (GnRH) agonist or antagonist prior to gonadotrophin administration. Simple protocols for the cryopreservation of sheep embryos by vitrification are already available and the development of frozen-thawed blastocysts to term is close to the fresh ones. Further research is required to identify factors able to promote the maturation in vitro of oocytes, namely those obtained from prepubertal animals. Semen and embryo sexing procedures are available in cattle although much less attention was paid to their application to sheep. Among all the reproductive technologies, cloning with embryonic and foetal cells has progressed dramatically in sheep and nuclear transfer has been used to produce transgenic animals as an alternative to pronuclear injection. The production of the first lamb cloned from a somatic cell opened new opportunities in animal breeding as well as exciting lines of basic research. The overall conclusions are that, apart from superovulation, the application of in vitro technologies is likely to evolve rapidly and once applied, a great impact on traditional and new animal productions should be expected. However, a better understanding of the changes in gene expression, induced in embryos by different in vitro manipulation procedures, is necessary to prevent abnormal foetal development.